Pascal Bigarre

Karim Ben Slimane

Jean-Pierre Josien

Marion Guillaume

' Bigarre

Ben Slimane

Guillaume M " Ineris

Methodology of research in the fight against rockbursts at the Provence colliery

Keywords: rockbursts, three-dimensional modeling, mine-induced seismity, in-situ measurements, prevention

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INTRODUCTION AND GENERAL SETTING

At the Provence colliery, coal is mined at a depth reaching 1100 m. The seam thickness is around 3 meters, while strata dips westward around 10°. The longwall face method with caving process is used, involving high mechanization and self-advancing support for face lengths of 200 rn. Rate of production has increased steadily in the past, reaching the current level of 11 tonnes per shift and per day, with an average, daily rate of advance of 6 meters per day per working face.

Nowadays, the coal mine experiences a daily average of 25 seismic events of magnitude 1.5 and greater, 15% of which are magnitude 2 and more. Most of these events are attributed to the goafing process associated with the longwall mining operation. However, on an annual basis, many of these events result in serious rockburst damage at the advancing face and along haulage gateways. As regards the southern part of the colliery and the mining of Estaque-sud district,which began in 1987, many major tectonic faults have been suspected to play a major part in dynamic loading of the coal seam through fault-slip induced by mining.

The general geological setting of the basin is quite simple. After Caviglio & al [1988], the structure is dominated by a major thrust sheet overthrusting northward, with a average dip of 25° (figure 1). Strikeslip, sub-vertical faulting is present all over the area, with lengths of several hundreds of meters. Two major zones have been distinguished in the coal field, with regard to the direction of the strata and stress measurements. In the zone we are concerned with, there is a strong anisotropy of the principal stress components, characterized by high horizontal tectonic stresses and a sublithostatic vertical stress. Both have been explained by the regional, geological history (Piguet & Georges [1981], [START_REF] Revalor | stress measurement by hydraulic fracturing: Etoile district[END_REF], Gaviglio [ 1985]). Associated strata are made of limestones, qualified as hard and brittle [START_REF] Josien | Lutte contre les coups de couche[END_REF]).

ROCKBURSTMECHANISMS ATTHE PROVENCE COLLIERY

For the last fifteen years, stimulated by the steady increase in the daily mine tremors and annual rockburst occurences, a research program has been undertaken at INERIS, aiming first to understand the mechanisms involved and then to improve prevention. Classified with regard to rockburst locations and effects at the Provence coal mine, three main types of bursts have been recognized, as [START_REF] Gaviglio | Tectonic struture, strata properties and rockburst occurence in a french coal mine[END_REF]) (figure 2):

-type 1: ends of the faces, especially on the old panel side. [START_REF] Slimane | Sismicite et exploitation miniere[END_REF]lasted until January, 1992. Seven rockbursts were recorded, two of them accompanied by quite severe damages.

A schematic description of the larger 1) We include here all significiant dynamic events recorded, ranging from dynamic spalling to large, underground damages described here-after damaging events is suggested in figure 3, with the following characteristics:

-violent expulsion of the coal in the gateway, -no significant fracturation or convergence of the immediate hangingwall, -quite often accompanied by floor heavage reaching 1 meter (whether due to buckling mechanism or deeper shear failure is still not clear [START_REF] Mathieu | Apport de l'ecoute sismoacoustique pour la surveillance des chantiers miniers affectes de coups de terrains[END_REF])). It is noteworthy that this kind of damage has been controlled for the last year by floor slotting ahead of the face, although the efficiency of this method has not been accurately estimated, due to the lack of data since its implementation.

The main hypothesis for type 2 events -analysing with all avalaible data the major, suspected faults respectively with mining geometry and different scenarios to get a better understanding of potential fault-slip behaviour. This has been undertaken last year and use of numerical methods is presented later [START_REF] Bigarre | 3-dimensional modeling of fault-slip rockbursting[END_REF]). -developing a mine-scale seismic network, able to give location of each mine tremor with good accuracy as well as its focus parameters. This should permit to relate the located focus of the mine tremor and the underground damaged areas and thus assert which mechanism may be considered. This has been undertaken two years ago (Ben [START_REF] Slimane | Sismicite et exploitation miniere[END_REF]). The mine is currently improving the network to get accurate locations and better focus parameters. Seismic monitoring of a recently mined panel of the Eguilles district is also presented later.

-in-situ measurements as stress measurements or deformations are increasingly undertaken in order to try to determine areas prone to rockbursts.

NUMERICAL MODELING

Numerical modeling has been carried out aiming to quantify rockburst potential for the seismic triggering mechanism from fault-slip along major, pre-existing geologic structures. Because of both the mining configuration and orientation of the faults of the Estaque-sud area to be modeled, it was decidedto undertake three-dimensional numerical analysis. Eventually, the strongly discontinuous nature of the problem led to the choice of the distinct element method.

Due to the lack of seismic data over the period of mining of the Estaque-sud district and the insitu conditions for mined areas of such extents, the aim of this study was to:

-examine the ability of the three dimensional distinct element method (3DEC, Itasca) to study fault-slip assessment for a complex system of discrete, deformable blocks, -examine the fault-slip potential for large-scale faults lying in the mined area and correlate in space the modeled mining process and the incremental plastic deformations with the insitu recorded rockburst sequence, -to develop a methodology of modeling closely associated with geological survey and above all data made avalaible by the newly established seismic network available.

Modeling of rockburst mechanisms with 3DEC has been undertaken before, to simulate fault-slip behaviour of discontinuous medium, applied to fault and dyke slip at the Strathcona mine, Canada [START_REF] Hart | Examination of Fault-slip induced Rockbursting at the strathcona Mine, in Key questions in Rock Mechanics[END_REF]), (Tinucci & al, [1990]). The numerical analyses were able to point out the consistency of fault-slip assessment in mine-induced seismicity and rockbursting.

NUMERICAL PARAMETERS

Our 2800mxl600mx2400m model consited of 188 convex blocks formed by 7 structural features, comprising 5 large-scale faults (500 to 2000 m of extension in space) and two bedding planes (dipping east-west 10°) located at 150 m above the seam, in the upper roof, and below the seam, in the lower footwall, respectively (figures 4-5). Four longwalls (T13, T14, T25 and T15) were simulated, with a mining scenario reproducing the in-situ excavation geometry. The four longwalls were then excavated in 15 incremental steps, by deleting blocks, (figure 5) with equilibrium reached at each step, providing a quasi-static analysis able to put forward the influence of the incremental mined areas on the plastic deformations along the modeled faults. Each step represented an excavated volume equivalent to two months of mining at a rate of 100 meters per month. finite-difference zones, were assumed to behave elastically, while all structural features followed a perfect, elastoplastic behaviour, based on a Mohr-Coulomb yield condition (table 1). Two parameters were quantified in order to relate plastic deformation along each feature to each sequential excavation (a code was written to do this):

S D X

where A s is the SubC.

subcontact area and D x its tangential displacement. M may be interpreted coarsely as the seismic moment of the fault divided by its shear modulus. It characterizes the mechanical moment acting on the structure while new equilibrium is reached.

where F s is the SubC.

tangential force acting at the subcontact location. DE is the non-recoverable, released energy dissipated by the excess shear force induced at each step. These two parameters are related, in such a multi-step, quasi-static analysis by the relation: M =where t is the tangential stress acting at the subcontact location.

RESULTS

Figures 6 and7 indicate the energy dissipated through plastic strain and the parameter M at each step, with the rockburst sequence plotted on the right, vertical axis. Results show that only the simulated overthrusting fault (fault A) and upper bedding plane (feature I) show large plastic deformations. These deformations appear above all from starting of longwall 14 (mining step 8-9). Summing up briefly, we can do comment as follows : calculated on a daily basis, provide a value of 3.10 9 J, which corresponds to an event with an order of magnitude of 3. failure mechanisms along bedding plane I were of two types: shear and tensile due to flexural behaviour. The relatively small energy dissipated was due to a low induced shear stress on the fault, parallel to the seam, and dipping 10° with regard to original principal stresses. In fact, tensile failure took place essentially at steps 10, 11 and 15 with the widening of the mining area.

Slip along fault A was essentially oblique, i.e. with a mixed offset of reverse and right-lateral strike faulting. This motion corresponded to the natural, thrust faulting of the structure. Interaction of the two structures A and I close to their intersection was difficult to estimate. It is worth noting that they most likely amplified each other because of the compatibility of the kinematics of the blocks.

Along other features, there was no noticeable plastic deformation although induced stresses were unfavourable for most of them, i.e. ratio x/a increases, excepted significant deformation along fault C. However, if large strain was to be obtained on this modeled joint, it would led to reduce initial friction to unrealistic values (20-22°).

SEISMIC MONITORING

DESCRIPTION OF THE NETWORK AND THE DATA

In July 1990, a local seismic network was installed by INERIS at the Provence Coal Mine, while the monitoring started in August 1990, with at first five geophones (one tridirectionnal station). In May 1991, four stations were added underground at the mine level (figure 9). All receivers are short period ones (1 Hz). Data are radiotransmitted (band of 400-450 Hz) for PC based computer digital conversion and data acquisition is made with a sampling rate of 155 Hz. Processing of data files gives the usual characteristics of each event as: date, coordinates, errors and magnitude, as well as a map of the calculated locations superimposed on maps of the mined areas.

Magnitude is calculated on the base of the time duration of the signal over a certain threshold, from the tridirectionnal station recording. Agreement between the magnitudes obtained and those calculated by the regional seismic networks is very good, after close calibration was established.

During the mining of longwall 06 of the Eguilles district (figure 10), 3420 seismic events were recorded, amongst which 651 are of magnitude 2 or greater (maximum magnitude recorded was 2.9 while minimum was 1.5). Eventually, around 50 events of magnitude 2.5 or greater have been Data analysis was undertaken by INERIS with, at its disposal, the following additional informations:

-rate of mining per month of the longwall -total sei smic released energy per month -distributions of the events per magnitude

ANALYSIS OF THE DISTRIBUTION OF THE SEISMIC RELEASED ENERGY IN SPACE AND TIME

As regards both the rate of advance of the longwall and its development in the district, the distribution of the seismic activity can be split up into several stages:

-starting of the longwall (August & September 1990): the mining was done without the presence of old workings on any side of the panel. The level of seismic released energy per meter of advance was rather low (figure 12), -the panel reached the previous old panel 05 (from October 1990): significant increase of the seismic released energy per meter of advance was observed, tremors of magnitude 2.5 or greater appeared, with an increasing occurence all along -December was marked by low values in number of events as well as released sesimic energy. It is worth noting that during the following month, two rockbursts were recorded, -the panel moved beyond the influence of panel 05 (from April 1991): number of tremors of magnitude 2 or greater as well as released seismic energy per meter of advance dropped dramatically. Values of released seismic energy in April and May 91 were similar to those recorded in November and October respectively, showing a symmetrical behaviour before and after old workings were passed -a new increase in June 1991 showed a clear influence of previous panel T41, although it was separated from the working face by an 200 m wide, stiff pillar. Thispillar is however intersected all along by major, subvertical faults, and might be suspected not to have fullfilled its function -in August 1991, a low value of the released seismic energy and number of events of magnitude greater than 2 characterized a zone in which, two months later, a major rockburst occured. Both facts related to each other, it seems therefore that an unfavorable caving process took place during this period. -from September 1991 onwards, seismic activity increased again, particularly the number of tremors of magnitude greater than 2. It is noteworthy that from this period on, large events (M>2.8) appeared (October and November). It seems that "delay" in the caving process was recovered, with an average seismic released energy per tremor equivalent This analysis shows that the seismic "behaviour" of longwall 06 was discontinuous. To some periodsof its mining, characterized by low seismic released energy, corresponded dynamic phenomena occurring either shortly after or later. To these periods succeed very active seismic phases.

Except for events with magnitude recorded greater than 2.3, which were located manually, locations of the whole set of tremors were processed automatically, involving a certain lack of accuracy. The following comments and analysis are limited to the first set (M>2.3):

-a concentration of events occurred over the zone between the respective limits of panel 05 and 41

-this remarkable distribution around this particular zone is manifested in depth too, -a zone with few events recorded occurred in April and May, -from September on, an important seismic activity characterized by a dispersion in locations in space took place.

IN-SITU MEASUREMENTS

The in-situ measurements undertaken at the Provence colliery, they are of different kinds. To develop a survey of this field is beyond the scope of this paper, and we will just emphasize this kind of investigation through results concerning stress measurements by hydraulic fracturing, flat jacks and overcoring.

Results of previous campaigns showed that the main directions of the stresses which occur are usually sub-vertical and subhorizontal ; the major principal stress, in all workings in the vicinity of zones which have given rise to dynamic phenomena occurences, is horizontal and, on average, twice as great as the vertical stress (Etoile sud district, Eguilles district). Eventually, the vertical stress is found to be sublithostatic [START_REF] Slimane | Amelioration des methodes de reconnaissance et de traitement des zones a risques de coups de terrains[END_REF]) (figure 13).

In 1992, two field stress measurements by the hydraulic fracturing method were conducted at the Estaque sud district, in order to infer dynamic phenomena potential of this very sensitive part of the basin as well as to obtain better indication of input farfield stresses for numerical modeling [START_REF] Bigarre | 3-dimensional modeling of fault-slip rockbursting[END_REF]). Results are briefly discussed as:

-data analysis was conducted by INERIS with Surfract software (Lizeur & al, [1991]), available for any kind of stress measurements (flat jacks, overcoring, borehole slotting, hydraulic fracturing, etc) and yielding all statistical parameters and distributions through numerical simulations,

-best results were obtained with assumption of a vertical principal stress in both cases, -thevertical principal stress for both measurements was found again to be slighty sublithostatic, dip directions of the horizontal principal stresses were found to be in good agreement with previous results, considering the deepening of the seam westwards, -strong anisotropy of the stress tensor appeared to be largely reduced in the very south-west part (measure 1). This zone might be less disturbed tectonically since no trace of the overthrusting fault has been observed recently.

SUMMARY AND CONCLUSION

Over the last fifteen years, considerable efforts have been made to understand and preventrockburstoccurences. Many aspects of the research conducted by INERIS at the fig. 14 -In-situ stresses: directions and values (MPa)

Provence colliery have been presented here. Concerning modeling, the aim of the study was to evaluate whether using modeling of typical, discontinuous problems in prediction of fault-slip rockbursting might be useful. This study shows a good agreement between the rockburst sequence and the response of some of the modeled discontinuous features. Mechanisms involved in the response of the system are clearly identified, critical geometry and spans are pointed out, fault-rupture locations can be calculated, fundamental parameter such as seismic energy and seismic moment are estimated and seem realistic. The three-dimensional aspect in modeling is pointed out as very critical. However, at the present time, no calibration can be demonstrated. As well, impact of failure along the structures on mining areas are impossible to estimate from a modeling results. Three-dimensional modeling by the distinct element method appear as a method able to quantify numerous potential faultslip problems.

Large scale seismic monitoring should provide fundamental support in understanding of the rock mass behaviour influenced by mining. It permits identification of different phases in the rock mass response. Both seismic energy release rate and distribution of tremors per magnitude are related to the geometry of multiple panels, and seem to be potentially correlated either in space or time with the recorded dynamic phenomenae.

Association of numerical dynamic modeling of the discontinuous rock mass correlated as closely as possible with seismic analysis is expected to come out as very promising concerning many kinds of mechanisms of rupture. The study of the dynamic response of the coal seam, at the working face and gateways, to the dynamic loading from the numerous, daily tremors, as well as local failure mechanisms, must integrate in-situ measurements in order to qualify fundamental characteristics such as the stress-field and mechanical properties of

  fig. 1-Geological cross-section of the basin

  fig. 4 -Map of the most recent mined districts N

fig. 6 -

 6 fig. 6 -Energy dissipated at each step for joints A & I

  fig. 9 -The seismic network

h

  fig. 10 a & b : View of the Eguilles district and configuration of T 06 (not to exact scale)

  fig. 11 -Time distribution of events 03/91 : no recording

  he rock matrix and thus, optimize prevention.

table I stiffnesses M.C. parameters structural features knjcs=10000 MPa /m Fric=35°,Coh=0, Rt=O MPi

 I

				rock matrix MPt
				K.O=13333,8000
	table 1 -elasto-plastic parameters of the model
	Initial pre-mining state of stress
	waschosen to be very close to available field
	measurements obtained in the Etoile-sud
	district, closest to the one modeled (table 2).
	table 2		O2	C3
	value MPa	-40	-20	-17
	dip"		120	30	0
	dip dir.	°al	0	0	90
		table 2 -input stresses


Christian Tauzffide

Yvon Mouilleau

Remy Bouet

Modelling of gas

published or not. The documents may come

INTRODUCTION

Caved-in longwall mining poses the particular problem of dilution of firedamp at the junction of the longwall and the retum airway. A large number of measuring runs carried out in the Lorraine Coalfield have shown that 50 to 70 % of the firedamp which enters the Ventilation network does so in this zone, despite high air flow rates (30 to 40 m^/s) and a high drainage efficiency (äs much äs 70 % in certain cases). Various methods can be used to control this phenomenon and either prevent the formation of zones where the firedamp concentration in the air is dangerous, or push them back towards the goaf. These methods include the exhausting Ventilation System developed in France (Lorraine Basin Collieries, 1990) and the Back Retum System used in Great Britain (British Coal, 1993). However, these methods are only effective under certain favourable conditions and can therefore only provide a limited, partial solution to the problem.

For this reason, it seemed essential to improve our knowledge of gas circulation in the goaf. Various experimental studies have been successfully carried out in the past, aimed in particular at providing details about firedamp and air exchanges between the face and the goaf. However, relatively little is known about gas flows in the goaf itself, both äs concems circuits and flow rates.

In the Lorraine Collieries, there is also the risk of spontaneous combustion in the goaf. The method used to control this is injecting nitrogen into the goaf. However, this practice remains relatively empirical in terms of the choice of injection points and the flow rates used.

A better understanding of these flows would therefore allow the following : most likely to prevent firedamp heterogeneities at the face/retum gate junction,

• improved specifications of firedamp drainage means,

• finally, the use of more efficient and perhaps more economical, nitrogen injections to control spontaneous combustion.

Carrying out experiments in the goaf is both very difficult and costly because of the inaccessibility of this area. Thus, the modelling of gas flows (air, methane and nitrogen) is an investigation method which is fairly well adapted to the Situation. This approach is made possible by the development of Computers and the existence on the market of general codes to solve fluid mechanics equations (Computational Fluid Dynamics or CFD). Work is being carried out in Great Britain on the basis of these considerations (British Coal, 1993). In France, we have used a set of programmes called PHOENICS W to produce a gas flow model for the entire geometry of a working : the face and the goaf area. The second chapter describes this model, while the third describes the qualitative results of the first simulations carried out.

DESCRIPTION OF THE FLOW MODEL

The approach used to produce the flow model consists in writing all the equations needed to describe the phenomena involved, digitize the System and then solve it using conventional numerical methods such äs finite differences or finite elements techniques. The most populär computational algorithms have been extensively described (particularly [START_REF] Patenkar | Numerical Heat Transfer and Fluid Flow[END_REF].

In the case we are interested in, the equations to be solved are those conceming mass conservation, linear momentum, and the transport of gases for which we need to know the breakdown in the geometry being considered i.e. methane and nitrogen if we wish to simulate the injection of this last gas.

Initially, for reasons of simplification and reduction of the caiculation time, heat transfers will not be taken into account. The energy conservation equation is therefore not included in the System.

Once again with the aim of reducing the caiculation time, the "gates/face/goaf" set has been reduced to a two-dimensional geometry.

Finally, because of the air spteeds, which are less man ten meters per second, the fluids are considered to be Newtonian and incompressible.

The flows which need to be modelled occur within two media of an entirely different nature -the goaf can be considered to be a porous medium while, in the face and roadways, the flows are "free". Consequently, the forces which are exerted on the fluids are different from one medium to the other. Likewise, the diffusion of one or several gases occurs differently from one case to the other. Thus, to model all the phenomena, two "sets" of formulae must be used, one to determine the free flow (in the roadways and face) and the other to determine the flow in a porous medium. These two formulae are described in the following paragraphs.

Modelling free flow

Conventionally, using two-dimensional Cartesian co-ordinates, the continuity equation for an incompressible fluid is written äs follows:

(l) (2) (3) Sp Su Sv " H + p + p = 0 öt öx ö y
where:

-is the density -u, v are the average speeds of the fluid in the x and y directions.

Based on the steady state hypothesis, equation (l) is reduced to :

5u öv _ PT-+PT-=°ö x öy
Likewise, conservation of the linear momentum alone the x-axis is written :

r 8 4r -öu -5v pu-+pv- öx öy ÖP Sx l" ^l §x 2 + Öy 2 J 8 [ +PSX-P.-L" 7 ^ + u/2
5y "where :

-P is the pressure (Pa) -|i is the dynamic viscosity (kg/ffl. s) -gx is the acceleration x-component due to gravity (m/s 2 ) -u' and v' are the speed fluctuations along the x-and y-axes.

The term (3a) represents the forces of inertia, Hb) expresses the effect of pressure, (3c) represents the effect of viscous Stresses, (3d) that of gravity, and (3e) expresses the effect of fluctuations due to turbulence.

Conservation of linear momentum along the y-axis is written in the same way.

Introduction of the term (3e), and thus the new variables u' and v', make it impossible to resolve the System formed by equations

(2) and (3). This is the well known problem of "closing the system".

Most CFD codes use the Boussinesq hypothesis which consists in considering turbulent agitation in a similar way to viscosity. The influence of turbulence is expressed using an equivalent "turbulent viscosity". To determine this new valüe, a so-called turbulence model must be developed. Such modeis and the theories they are based on are described fairly completely [START_REF] Hinze | Turbulence[END_REF][START_REF] Launder | Mathematical modeis of turbulence[END_REF].

In the present case, the flows are extremely turbulent. They result in very high pressure drops throughout the Ventilation circuit. The effect of turbulence must.therefore be expressed in order for the flow dynamics to be represented correctiy.

When using a turbulence model, experience shows that, among other things, the quality of the caiculations is related to the quality of the geometric description of the medium being considered. This means that the grid must be close-meshed enough to describe the shape and size of the roadways. In practice, this invariably leads to a number of meshes which makes the caiculation time prohibitive. This type of approach is therefore not practicable.

A large number of measuring runs carried out in the past [START_REF] Simode | Valeurs pratiques des resistances[END_REF] have enabled the values of typical linear head losses in roadways to be realised. Rather man setting up a turbulence model to ultimately obtain head losses which correspond to actual experience, it is far simpler in the long run and requires much less caiculation time to introduce a flow resisting force into equations (2) and (3) which leads directiy to the required head losses. Head losses can be expressed by equivalent heights, for example.

The equation for transport of a gas in the steady state is written äs follows :

-ÖY -5Y _ r^Y S'YI ,,, pu-+pv-=pD, LT-+^TJ (4) 5x öy .öx öy-

where :

-Y is the mass fraction of the gas considered, -D{ is the turbulent diffusion coefficient.

Conventionally, the turbulent diffusion coefficient is obtained from a turbulence model. In the light of the previous remarks, it is obvious that this coefficient cannot be caiculated. It must be remembered however, that the main aim with regards to free flow is to obtain the flow dynamics i.e. to caiculate the pressure and average speed. The aim is not so much to follow a gas through the atmosphere of the working, but to ascertain its route in the goaf. A value between 10 and 100 ümes the order of magnitude of Fick diffusion coefficient D used when the flow is laminar was therefore taken for D^.

Modelling the flow in a porous medium Once again, we are faced with the question of solving a System based on the laws of conservation. In the present case, however, they must be modified in order to express the twofold effect of the porous medium: first, only a fraction of the total volume is available for the flow and second, an additional resisting force is exerted on the fluid. Thus, the System of equations ( 2) and ( 3) is modified to give the following : Su 5v -._.

Ri

JL

k" u where:

-kp is the permeability of the porous medium -ß is a function of the permeability and Reynolds number.

This approach, already proposed by a large number of authors (Vafai and Kim, 1990;[START_REF] Nield | The limitations of the Brinkman -Forchheimer equation in modelling flow in a saturated porous medium and at interface[END_REF]Kim and Rüssel, 1985;Bradley et al., 1989), consists in expressing the forces exerted by the porous medium using a term resulting from the Darcy law (7a) and a term expressing the effect of the forces ofinertia (7b). S One difficulty lies in determining the permeability kp. Many formulations are to be found in the literature which connect the permeability to the geometrical characteristics of the medium, such äs the porosity ((), the size of the grains in the medium, and the diameter of the pores. Dullien has given a relatively complete list of these formulations [START_REF] Dullien | Single phase flow through porous media and pore structure[END_REF].

The most widely used formulae in practice are no doubt those of Ergun and Kozeny-Carman (Carman, 1961). For example, the Kozeny-Carman law gives the following formula:

k,, = --for (| ) < 0,8 (8)
p (l-(j)) 2 k.S
where So is the specific surfäce of the grains in the medium, k is the Kozeny-Carman constant, between 5 and 6.

For the transport equation of a gas, the influence of the porous medium on the diffusion coefficient needs to be determined. To do this, correlations were used in order to express an "equivalent" diffusion coefficient äs a function of the For Ri, the following expression has been chosen :

Reynolds number, which is characteristic of the porous medium (Sinclair and Potter, 1965;[START_REF] Cussler | Mass transfer in fluid System[END_REF].

Finally, the model builtfrom the System of equations ( 2) to ( 8) describes the flows in the roadways and in the goaf.

In the case of goaf, the spatial distribution for the following must be defined :

-the permeability, -the methane inlet flows towards the working.

Spatial distribution of the permeability

Several authors (Brunner, 1985;Stokes, 1985;[START_REF] Ediz | Numerical Simulation of time dependent methane flow[END_REF] relate the permeability to the mechanical stress exerted on the medium.

It can be imagined that when the stresses exerted on the solid rock are high, the blocks of rock are relatively compact, which results in low permeability. On the other hand, when the ground pressure is lower, there is less consolidation and greater permeability. The areas with the greatest permeability therefore correspond to the areas with the lowest stress level.

Given what we know about the mechanics of the ground around a caved-in longwall, it can be said that:

• the permeability will be higher in the area immediately behind the face,

• further back, the caved-in area is compressed again and the permeability gradually decreases and no doubt becomes stable after a certain distance.

It should also be noted that the old intake and retum roadways and the face Start plays a special role. With the arches left Standing and the adjacent coal pülar playing a supporting role, recompaction of the caved-in ground does not occur in the same way äs it does in the general area of the goaf. A Strip of higher permeability therefore exists on the three sides of the rectangle formed by the goaf. This distribution is based on fairly extensive knowledge of firedamp emission mechanisms. In particular:

• on a line perpendicular to the face, the firedamp flow increases • fi'o m 0 to a maximum value located at a certain distance from the longwall and then decreases to a value in the vicinity of 0 next to the departure of the longwall (Tauziede and Pokryszka, 1993).

• on a line parallel to the face, the firedamp flow is higher near the edges of the panel than in its centre. This is due to the existence of cracks, over and under these particular zones, resulting from the movement of strata which occurs there after caving-in and relaxation. These cracks are ideal paths for firedamp flowing out from distant beds towards the working. In the middle zone, the beds in the roof, except for the dose beds, subside more regularly. The firedamp has a greater tendency to flow in the direction of the beds and particularly the coal seams -which are relaxedtowards these cracks [START_REF] Jeger C | Will firedamp emission stop progress in longwall production ?[END_REF].

In the following simulations, firedamp from mined coal has not been taken into account since, in the Lorraine Coalfield, it represents a very small proportion of the whole (5 to 10% in most cases).

In any event, this simplification has little or no influence on gas flow in the goaf area. The grid representing the face and goaf and the air intake and retum roadways sections, consists of about 14 000 meshes.

The ahn of the simulations was to check the capacity of the model to reproduce effects such äs that of the face dip on the methane concentration distribution in the goaf, due to differences in density between methane and air. For this reason, different caiculations were made, using the following dips : 0°, +10°, +20° (upcast Ventilation) and -10° (downcast Ventilation).

Looking at the plots of the CH4 concentrations in Figure 3, pressures and speeds obtained, the following points can be noted:

• in all four cases: the caiculation indicates a high methane concentration area (with respect to the statutory limit concentrations or the lower explosiveness limit) in the "blind angle" at the junction between the face and retum gate. From a qualitative point of view, this is entirely in accordance with observations made in the workings [START_REF] Cwiklinski | Influence of descensional Ventilation in a face on environmental conditions and distributions of gas and airbome dust[END_REF]. Flows in the goaf are more intense in the old start roadway and gates than in the rest of the goaf area. This, of course, is a result of the greater value given to permeability in diese zones. The observations made in the workings also confirm this point.

• in the three cases of a 0°dip or upcast Ventilation: flows observed in the goaf are always parallel to that of the face. With regard to methane concentrations, the zones with the highest concentration are situated in proximity to the retum gate. This corresponds to the density effect. It is observed that the greater the slope, the less extensive the zones with a high CH4 concentration, that is, the lower the average concentration in the goaf.

• in the case of downcast Ventilation, flows are observed in the goaf in the opposite direction to that of the face, Firedamp in the distant zones in goaf flows upwards. Very high CH4 concentrations therefore exist in virtually all of the goaf. The fringe of higher concentrations (for example > 5% CH4) is found in the goaf at a much smaller distance from the face in this last case than in the case of upcast Ventilation, and this is confirmed by the measurements.

ORIZONTAL VENTILATION 0°F

igure 3 (cont'd): Distribution ofconcentrations in the goaf(%)

CONCLUSIONS

The results of the first simulations carried out are in perfect agreement with observations and measurements made in the past. Although these results are only purely qualitative, they nevertheless provide an initial verification of the capacity of the model proposed to reproduce effects such äs those which result from the density of CH4.

Por a better validation of the model, a complete adjustment has to be carried out on one or more real cases. The caiculation Parameters have to be adjusted (such äs the relative permeability values between the different zones, for example, or the Parameters of the turbulent diffusion model) until the best correspondence is obtained between the measuring results and the caiculation results.

Anyway, the previous results indicate that CFD codes, such PHOENICS, are very promising tools for improving our knowledge of flows in the goaf. As a result, they will enable mine operators to perfect the different processes and techniques used to control firedamp and spontaneous combustion.

  • identification, for example, of the geometrical mining configurations 25eme Conference Internationale des Instituts de Recherches sur la Securite dans les Mines, Pretoria, 13-17 septembre 1993, vol. 5, p. 41-50.

Figure 2

 2 Figure2shows the distribuüon of methane surfacic flows coming into the medium which forms the goaf.

Figure l :

 l Figure l: Distribution ofpermeabilities in the goaf

ACKNOWLEDGEMENTS

The work whose results are presented here received financial backing from the Ministry of Industry and from Charbonnages de France.

The authors would like to thank the engineers and technicians from the Lorraine Coalfield for their efficient assistance in carrying out this work. BIBLIOGRAPHY BRADLEY C.P., PROCARIONE J.A. McCARTER M.K, 1989, "Prediction of air flow through broken rock by finite difference grids", University ofUtah. BRITISH COAL, 1993, "Methane control for retreat faces", CEC Research Project 7220-AC/838. BRUNNER M.J" 1985, "Ventilation modeis for longwall gob leakage Simulation", 2nd US Mine Ventilation Symposium, Reno. CARMAN P.C., 1961, "L'ecoulement des gaz ä travers les milieux poreux, traduit de l'anglais par Jacques MACHEFER", Institut National des Sciences et Techniques Nucleaires, Presse Universitaire de France.

UPCAST VENTILATION + 10°U

PCAST VENTILATION +20°F igure 3: Distribution ofCH4 concentrations in the goaf(%) Fluid How,vol 11,n°3,


Christian Tauzffide

Zbigniew Pokryszka

DYNAMIC PREDICTION OF CH4 EMISSION IN LONGWALLS

Methods for predicting mean CH4 emission in longwalls have been developped in France and other countries for many years. These methods have been widely validated but remain limited in their use because of the continuous increase in productivity of such working.

Therefore, development of dynamic prediction methods -e.g. allowing prediction of emission on a daily or weekly basis -is strongly necessary.

Aiming to that, two ways have been investigated :

-a statistical approach. Analysis of CH4 emission of 14 faces has allowed building the expression of the emission during a given week äs a function of mean predicted emission and of advance (or production) of the three previous weeks.

-a mathematical modelling of CH4 emission äs a function of stratigraphy and of gas content of the strata surrounding the mined seam. Analytical expression contains numerous Parameters that must be adjusted using a reference face, so äs to be able to carry out a real prediction for later workings.

INTRODUCTION

Prediction methods have been largely tried and tested for a number of years but remain limited due to the increase in the productivity of workings. The advance rate of longwalls can be very high, which has two consequences:

• the time required for the emission to build up at the beginning of the panel is sometimes relatively high,

• the variations in emission with regard to the average are also important.

The methods used to predict the average emission are not sufficient to account for these two phenomena (see Figure l). Dynamic prediction methods therefore need to be developed which will enable the day-by-day or week-by-week emission to be ascertained äs a function of the activity of the previous periods. Most of the authors who have examined the problem use a statistical approach to predict firedamp emission. They propose taking the average, the dispersion and the frequency of the emission values äs the characteristic parameters of this phenomenon (Bruyet, 1967;Borowski, 1969;Winter, 1972). This approach enables the "normal" emission level to be defined, äs well äs a safety factor resulting from the 25eme Conference Internationale des Instituts de Recherches sur la Securite dans les Mines, Pretoria, 13-17 septembre 1993, vol. 5, p. 27-40.

probability of deviating from this level. However, it does not enable a real prediction of the emission to be made äs a function of the activity.

Another approach which contains a certain amount of dynamic prediction has already been proposed (Kaffanke, 1980). Kaffanke expresses the firedamp emission for each day of the week äs a certain combination of several factors (production of the day in question, cumulated production of the previous days). The functions used to caiculate the daily emission are different for each day of the week. The factors proposed äs having an influence on the emission of firedamp are not related to the physical mechanisms. The success of the Kaffanke model therefore rests, to a large extent, on the low variability of the weekly production cycles.

An approach has also been proposed based on the hypothesis that the gas emission during a given day depends on the production of that day and on that of a certain number of previous days (British Coal, 1988). In this model, the function expressing the emission is applied in the same way to all the days of the week.

The constants of this function are caiculated for a given working according to the emission values measured in this very working. Once the constants have been determined, the firedamp emission is predicted for the remaining part of the working.

METHOD USED IN FRANCE TO PREDIGT THE AVERAGE EMISSION IN LONGWALLS

The method widely used in France for predicting firedamp emission in faces was developed more than 25 ye'ars ago (Günther, 1965). It has been improved on several occasions, and particularly in recent times (Jeger, 1980;Ineris, 1992). This model is based on several fundamental observations :

• The emission of firedamp is the result of partial degassing of the coal seams and rock beds situated in a volume of influence at the roof and the floor öf the seam being mined. After a certain advance of the face, corresponding to the extension of the volume of influence, it becomes stabilized (see Figure 2). The limits of the volume degassed by a face are in the order of 170 meters for the roof and 60 meters for the floor of the mined seam.

• In the volume of influence, the desorbable gas content of a seam decreases from its initial content to a final residual content. This residual content depends on the distance of the seam being considered from the seam being mined. Figure 3 indicates the degassing rates of the affected seams.

For the mined seam, a degassing rate of 50 % is usually taken.

• In the volume of influence, the rock beds also release part of the firedamp initially compressed in their intergranular voids. The amount of gas released is caiculated according to the porosity of the rocks, the initial in situ pressure and the residual pressure (Ineris, 1992).

The specific emission is caiculated by dividing the sum of the basic volumes of gas released in the volume of influence by the number of tonnes of coal extracted -(or by the advance of the face). This ratio J therefore represents an estimate of the volume of CH4 likely to be released per tonne of extracted coal (or per meter of advance).

Under the geological and operating conditions of the Lorraine Coalfield, this method gives a satisfactory degree of precision. The discrepancy between the predicted emission and the emission actually observed is in the order of ± 10 to 20% and only goes outside this ränge in very special cases (see Figure 3). However, it must be remembered that this caiculation to predict the specific emission only gives an average value for the lifetime of the face, after its initial starting period.

DYNAMIC PREDICTION USING A STATISTICAL APPROACH

Observation of the firedamp balance in a large number of faces has shown the pertinence of the hypothesis already made (British Coal, 1988) by which the gas emission over a week depends not only on the week's advance, but also on the advance of a certam number of previous weeks.

In order to carry out investigations aimed at confirming this idea, a data base for the firedamp balance of a representative set of coal mines in the Lorraine Basin has been created. The faces selected met the following conditions: dip of 15° to 30°, opening of 2.5 to 4.0 meters, caving, length of face 200 to 300 meters, advance rate 10 to 40m/week. In order to determine the relationship between the emission for a given week (n) and the advance of a previous week (n-x), linear regressions were carried out. The results, which are based on all the data in the life of the faces since their starting, are presented in Figure 4. Since they did not prove conclusive, it was noted that one factor in particular strongly influenced the statistical relationship between these variables. This factor is the increase in the firedamp emission after the face begins, corresponding to the expansion of the volume of influence.

An analysis was then carried out of the correlation between the specific emission and the cumulated advance of the face. This was used to determine the average value of a "critical length" for the Lorraine Basin faces i.e. the cumulated advance for which the face is in steady state with regard to firedamp. The value found is about 220 m (see Figure 5).

Elimination of part of the data corresponding to cumulated advances of less man 220 meters considerably improves the results of the previous linear regressions (see Figure 6).

After carrying out a statistical Student-Fischer test to check that the correlations are significant, it can be said that the volume of firedamp released during the week depends in practice on the advance during that week and those during the previous two weeks. In general, this dependence is much greater for the advance of the week in question and decreases with time, which is in accordance with the physical firedamp emission modeis (Günther, 1965;Ettinger, 1966;Airey, 1971). The emission DQ for week n can thus be expressed äs follows :

D n=^s[. a n A n+ :i n-l A n-l+^n-2 A n-2+ c aŵ here:
-Dn is the emission for week n, in m^, -D § is the specific CH4 emission of the face, in m3 per meter of advance, -A^, AQ_^, Aß.2 are the advances of week n and the two previous weeks, -a^, a^.i, a^.^, C^are constants.

Using a multiple regression analysis between the weekly advances An, A^.i, Aii-2 (which are independent variables) and the weekly emission D^ (which is the dependent variable), the values of the constants in equation (l) were found for a set of 14 faces.

Significant correlations with a physical meaning (positive values of constants) were obtained for 7 faces. The results are presented in Table l. It should be noted that the majority of the faces for which non-significant correlations or nonsensical constants were obtained are either faces affected by other workings or faces for which very intensive drainage has occurred.

Given the relatively low Variation in the values of the parameters obtained, average values were taken (see Table l). Thus, the following formula can be used to predict the firedamp emission according to the advance of the face being considered:

Dn=Ds[306An+150An-l+75An-2+5470) (m3) (2)
where:

Dn is the expected volume of firedamp for week n, in m^, D § is the specific emission, obtained using the method described previously (chapter 2), in m3 of CI-U. per meter of advance, AQ is the planned advance for week n, in m, AQ.I and A^-2 are the real advances for weeks n-1 and n-2 in meters.

A similar formula can be obtained by expressing the emission for week n äs a fünction of the tonnage produced during weeks n, n-1 and n-2.

The use of this method for several faces has produced very promising results, even for faces affected by other workings or those with intensive drainage. Figures 7 and8 give a comparison, by way of example, of the weekly emissions actually observed for two faces and those obtained using the prediction method.

However, this method still needs further examination. The values of the parameters obtained are average values for all of the Lorraine Basin. It should be possible to find the values of these parameters for a specific area or mine, using the regionalized variables method, for example.

Finally, the values of the constants obtained need to be related to physical data, such äs stratigraphic data.

PHYSICAL MODELLING PROPOSAL FOR FIREDAMP EMISSION IN A FACE.

It is quite obvious that, although the statistical approach proposed above gives interesting results, it only applies to the period after the face starting phase.

This limitation of the method has led to the search for another model which will enable a dynamic prediction of the firedamp emission to be made throughout the life of a face.

The model proposed below is based on experience acquired in the field of gas emission. It is therefore a physical model based on experimental results.

If we observe the CH4 emission of a coal or rock bed located at a certain distance in the roof or the floor of the face äs a fünction of the displacement of the face, a curve similar to that given in Figure 9 is obtained (CEC, 1980;Airuni, 1981). The emission rate here is plotted against time which, in the case of a regulär advance, is the same thing.

The following can be noted :

• a delay in the beginning of emission, corresponding to extension of the volume of influence up or down to the bed considered, • a very rapid increase in intensity, corresponding to relaxation of the bed, • then, after reaching a maximum, a much slower decrease.

For a given period of time, the volume of CH4 released by the bed will be that represented by the shaded area in Figure 9.

This physical phenomenon can be easily modelled by the following mathematical expression (for the roof):

for t<t^: ^(0=0 for t > to :

r R R i 2 i ^.-[Log(t-tQ )-m J ------^_^_______________J R 2 2 (o ) ^2)t o (t-Iß )
the delay t^, the shorter the time required to reach maximum intensity t^, and the higher the maximum emission speed. In other words, a bed in dose proximity releases its gas earlier, faster and at a higher flow rate. A distant bed releases it later, for a longer period and with a lower flow rate.

Figure 10 represents the emission curves for 3 beds located at different distances.

In order to represent these phenomena, it can be considered that the values of tâ nd t^ obey parabolic laws äs a function of the distance between the bed i being considered and the mined seam, (^ (i): t^a-^d)] 2 (4) (parabolas l and l' on the Figure 10)

1^(1)= ß^d^i)] 2
(5) (parabolas 2 and 2' on the Figure 10) and that the maximum value of the function P 1 obeys a hyperbolic law :

(3) This is a log-normal distribution where :

-P^ is the gas release rate for that part of the bed being considered (roof) in m^/day or m^/week, -t is the time elapsed after the face has passed at the vertical of the point being considered, -t^, m 11 and ö 11 are parameters.

It can also be said that the closer the bed is to the seam being mined, the shorter

-[^[t^i)]-!!! 1 ^)] ^Pl----------r R ,2-------J= r K ^2 [o (i)] l (6) 1^(1)= ^(D-tod)] 2 (characteristic of the log-normal distribution) (7)
where ö^, ^R and y 11 are parameters.

In these last expressions, all the parameters tg , t, , m and o are D expressed äs a function of i, the index representing the different beds contained in the volume of influence. These expressions are the same for the roof and floor beds, but with different parameter values (F Indexes for the latter).

We know that the face starting phase corresponds to an expression of the volume of influence while the subsequent phase corresponds to its translation. During the first phase, there are longer delays before emission begins from that part of the bed being considered than in the second phase. This phenomenon can be modelled by parameterizing parabolas l, 2, l' and 2' in the following way (for the roof):

(T <+£ (8)
where :

-0.0 is a constant, -L is the cumulated advance of the face from the starting, -Lc is the critical advance length, -e = 0. l or 0.05 for example.

The parabolas 3, 4, 3' and 4' corresponding to the starting period tend respectively towards parabolas l, 2, l' and 2', which correspond to the subsequent phase and are steady. The Parameters ß and 7 are taken to be constant (ßp and 7o)-Having mathematically expressed all the physical phenomena, the total CH4 emission during a given period, on a given day, j, for example, can be expressed äs follows :

100 R i + SP"(i)£ A(k) J : k+l f.^Odt J-k l k=l +fpF(i)£A(k) JI^f^Odt] (9) S PF (i) i i=l k=l
where:

-Dg is the specific emission caiculated (see chapter 2) in m^/m of advance, -PMS is the share of this specific emission produced by the mined seam, in %, -PR (i) is the share of this specific emission produced by bed number i at the roof, in %, S -Pp (i) is the share ^of this specific emission produced by bed number i at the floor, in %, -R is the number of beds at the roof, -F is the number of beds at the floor, -A(j) and A(k) are the face advances on days j and k respectively, in m, -^(t) is given, for bed i, by expression

(3) and f^t), by a similar one for the floor. with: -0^(1), t^(i) and m^(i) obtained by resolving the System of equations ( 4) to(8), -(^(i^t^andm^i) obtained by resolving a similar System for the floor.

The CH4 emission, G(j) is expressed äs a function of six parameters, a^,ß^,y^ for the roof and o^, ß^, y^ for the floor.

The practical method consists in adjusting the parameter values on a reference face by minimizing the discrepancies between the caiculations and measurements for the working. The least error squares method can be used for this, for example. In practice, if the CI-L). emissions are caleulated on a day-to-day basis, they can be marred by errors. For this reason, it is preferable initially to consider a weekly CH4 emission.

After obtaining the Optimum values for the six parameters and checking their validity, the same formulae can subsequently be used to make real predictions for other faces.

CONCLUSIONS

Here, two methods are presented for a dynamic prediction of firedamp emission in longwalls :

• a statistical method has been defined and has already given interesting results for several workings, but normally cannot correctiy represent the starting period for the face, .-'

• the mathematical bases have been laid for an empirical method closer to the physical reality of the phenomenon. This method will need to be validated during implementation.

The emission of firedamp around a mining structure is an eminently complex problem. It is very difficult to equate the problem äs a whole, given the number of parameters involved and, in particular, the practical difficulty of apprehending the values of these parameters. Thus, what is proposed here is a pragmatic approach to the problem.

These methods can provide the operator with a prediction of the firedamp emission. He needs tools to optimize the mining work, that is, to adjust the human and matenal resources available to the real production capacities of a working.

The prediction methods described here are an improvement in this respect and finally should result in better productivity for workings äs well äs greater safety.

  Figure l

Figure 2

 2 Figure 2

  Model for predicting the average CH4 emission in a face

						Parameter values
	Longwall	Mine	Correlation	Degrees of	Weekn	Week n-1	Weekn-2
			coefißcient	freedom
	Louise 11 bis	Reumaux	r2 0.62	31	223	124	«n-2 76
	Irma II sud	Reumaux	0.47	40	143	41	50
	Louise I bis	Reumaux	0.84	18	399	219	56
	Kpan.2	Forbach	0.64	48	344	167	127
	H2pan.l	Forbach	0.93	10	413	268	66
	Albert 9.6.0	La Houve	0.77	12	451	116	46
	Frieda 5 Sud Degassing volume Reumaux	0.50	36	168 Intensity of degassing 116	112
	pan.1
				TABLE1
				-9-

ACKNOWLEDGEMENTS

The work whose results are presented here received financial backing from the Coal Directorate of the Commission of European Comunities, from the French Ministry of Industry and from Charbonnages de France.

We would like to thank the engineers and technicians from the Lorraine Coalfields for their efficient assistance in carrying out this work.

BIBLIOGRAPHY

'v AIREY E.M., 1971, "A theory of gas emission in mining operations". International Conference on Safety in Mines Research Institutes, Donetzk. AIRUNI A.T., 1981, "Theorie et pratique de la lütte contre le grisou dans les mines profondes", Editions Nedra -Moscou. BOROWSKI J., 1969, "Les relations entre la production et le degagement en taille", Travaux du GIG, Publication n° 472, Katowice. BRITISH COAL CORPORATION, 1988, "Short -term forecasting of methane emission levels using continuously monitored data", CEC Research Project 7220, AC/832. BRUYET B., 1967, "Les variations de teneurs dans les retours d'air de tailles -Influence des facteurs d'exploitation", Publication CERCHAR n° 1815 -Verneuil-en-Halatte. CEC (Coal Directorate of the Commission of the European Communities), 1980, "Firedamp Drainage", Verlag Glückauf, Essen. ETTINGER I.L., 1966, "Capacite d'adsorption des gaz par le charbon", Edition Nedra, Moscou. GÜNTHER, 1965, "Etüde de la liaison gazcharbon", Revue de l'Industrie Minerale, volume 47, n° 10, Saint-Etienne.

Comparison between specific emissions emissions predicted and actually observed Week to which regression applies Figure 4 -ll -

-A---Albert 3.1.2.

-x---Alben 9.5.1.

-• -• • Theodore 2.9.6.P.1

Variation of specific emission äs a function of the cumulated advance Theodor« 2.9.6.P.1

Variation of the CH4 emission rate äs a function of time Figure 9 Shape of the emission curve äs a function of the vertical distance of the beds from the mined seam.

Figure 10 -13-(l) and (2): maximum parabolas (period after the starting phase)

(3) and ( 4): parabolas during the starting phase.

Prediction ofweekly CH4 emission äs a function ofthe advance


93-41

DIESEL ENGINES FOR FIREDAMP MINES

INTRODÜCTION

The diese) engine protection principles used in firedamp mines have remained practically unchangeci since they were onginaily introduced into mines. The protection used is aimed at preventing excessive temperatures, flames, and sparks from appearing and preventing the transmission of combustion inside the engine into the atmosphere. All the regulations in force in most mining countries require the limiting of surface temperatures and exhaust gases, a more or less flameproof design for the engine and safety equipment. and the installation of flame arresting devices in the intake circuit and exhaust circuit.

The old French mining regulations defined the requirements for the approval of liquid fuel locomotives in the ministerial order of 30th October 1961. A minimum length öf 25 mm was required for the flameproof joints on the enclosures formed by the intake and exhaust Systems.

The tests defined in this order had to prove that the endosures could withstand a pressure equal to 1.5 times the maximum pressure developed by a mixture of air and 9.8 % of methane and prevent transmission of the intemal combustion of ä mixture of air and methane to the outside,

NEW FRENCH REGULATIONS

The introducüon of foreign diesel engines (mainly German engines with 12.5 mm flameproof joints) into French coal mines, the need to make extensive modifications to the cylinder heads, the increase in our knowledge of the reliability of flameproof endosures and the appearance of European Standards for electrical equipment led the French administration to replace the order of October 1961 (Chapter lll) by the order of 5th August 1987.

The main requirements of this order are äs follows: • flame arresters consisting ofstacked piates with set gaps between them. at the intake and exhaust; • flameproof chambers between the flame arresters and the cylinder head. and a flameproof joint with a minimum length of 12.5 mm; -testing identical to that of European Standard EN 50 018 of March 1977, for the flameproof endosures of group l electrical equipment i.e. * measurement of the pressure developed by the intemal explosion of a mixture of air and 9.8 % methane; * a chamber resistance test under1.5 times the maximum explosion pressure measured; * tests for non-transmission of the intemal combustion of a mixture consisting of air. 7.25 % hydrogen and 5.25 % methane. This type of mixture has a safety coeffident of 1.42 with respect to the air and methane mixture. These new regulations have been in force since July 1988. Their application has not posed any Problems in the case of dassical diesel engines i.e. with natural aspiration. On the otherhand, in the case ofturbocharged diesel engines, we experienced the several difficulties, which are described beiow.

APPLICATION TO TURBOCHARGED DIESEL ENGINES

In a turbocharged diesel engine, the engine exhaust gases drive a turbine which, by 25eme Conference Internationale des Instituts de Recherches sur la Securite dans les Mines, Pretoria, 13-17 septembre 1993, vol. l, p. 73-75.

compressing the air in the engine intake. increases the amount of oxygen for combustion. The gain in power for the same size engine can be 70 %. This is of considerable interest to French coal mines who need more and more powerfui engines to drive their rubber tyred loaders. However. this type of engine is not deariy provided for in the French regulations which creates problems for the construction and certification of conformity of these engines.

We examined a CATERPILLAR 3306PCTA turbocharged engine which was designed for a WAGNER transport loader used in firedamp mines.

This engine had been adapted by PYROBAN to meet French regulations. It consisted ofthe following :

• at the intake. between the air filter and actual engine, a shutter, a flame arrester with stacked plates at an interval of 0.65 mm over a length of 50 mm, a connection pipe. the turbocompressor, a second connection pipe and the intake cooler; -at the exhaust, the cooled turbine, a cooled connection pipe and a flame arrester with stacked plates at an interval of 0.7 mm over a length of 50 mm in a gas scrubbing and cooling pan. The dimensions of the flameproofjoints were correct. The non-transmission tests gave satisfactory results for the exhaust part but not for the intake part.

In order to take the effect of the turbocompressor into account. we precompressed the flammable mixture to the theoretical value given by the turbocompressor manufacturer (100 kPa) since it was impossible to reproduce the functioning of the turbocompressor in our lest device. There was transmission of combustion äs soon äs the first explosion occurred in the intake chamber.

This led PYROBAN to change the type of flame airester.

• A flame arrester with an interval of 0.5 mm between plates proved ineffective. Redudng the intervals even further was not possible since it would prevent the engine from working properly. -A flame arresting device of a different design was tested. It consisted of a crimped band and a flat band. These two cupro-nickel bands were wound into a Spiral and placed inside a collar. The intake airtherefore-had to pass through a number of conduits with a triangulär section of 0.18 mm* over a length of 38 mm. The intake device. with this flame arresting device, came through all the tests for nontransmission of combustion with explosive mixtures pre-compressed to 100 kPa. The French administration has accepted the equivalence ofthis flame arresting device with the stacked plate devices stipulated in the order of August 1987, which means the engine can be used in mines.

ä t

Air Inlat valve and flaae arrestar coBbinatlon fitted on 3306PCTA engine (constructed by PIROBAK)

COMMENTS

The European Standardizatlon Committee (CEN) is currently studying a Standard for the construction and testing of intemal combusüon engines used in areas where there are risks of explosion. This study, in which we are partidpating. is based on the experience and regulations In force in the different European Community countries and takes Into account the essential safety and heatth requirements relating to the design and construction of machinery given in a directive of the CounciI of European Communities (89/392/EEC).

A survey of diese! engine manufacturers has enabled us to realistically define those situations which can produce combustion under normal operating cond'rtions and under reasonable defect conditions.

The results of this the survey show the following:

-The main risks seems to be related to ovemeating of the engine. -For a correctiy, regulariy maintained englne operating in an atmosphere free of inflammable gases, the appearance of flames at the intake and exhaust is highiy improbable. This pcobability increases in the event of inflammable mixtures being ingested and under certain faulty conditions. -Bursting of housing is very improbable for the types of engines used in mines. It may therefore be asked whether the regutations that we apply are not too stringent, since anomalies are very rare.

CONCLÜSION

A crimped band flame arresting device proved more effective than the stacked plate devices. This must be taken into account in the standaro being prepared by the CEN.


V Petit' 11

R P J Swannelp

R S Sokhi' 31

R Cabridenc

REVIEW OF STRATEGIES FOR MODELLING THE ENVIRONMENTAL FATE OF PESTICIDES DISCHARGED INTO RIVERINE SYSTEMS

Pesticides are often produced and stored in large quantities near rivers posing a potential hazard for the aquatic environment. Accidental incidents such as storage facility fires are of major concern as significant amounts of pesticide chemicals can enter the nearby riverine system possibly causing considerable environmental damage (for example the Sandoz fire in Basel in 1986 which polluted the Rhine river). In order to assess the potential impact on the riverine environment of such an incident it is necessary to identify the controlling environmental fate processes that affect pesticides in freshwater systems. A strategy for mathematically modelling these processes to predict the fate of the chemical contaminants then needs to be established. This paper discuss and reviews the major environmental controlling fate processes of selected herbicides in freshwater. Strategies on how to mathematical model the environmental fate of pesticides in rivers are also considered.

1.

INTRODUCTION

Pesticides are introduced in aquatic environment through intentional application (control of aquatic weeds, algae, fish, or unwanted invertebrates), aerial drift, runoff from agricultural applications or runoff from accidental release. Accidents involving fires of pesticide storage or factories situated near rivers or estuaries have proved to be of particular concern. A well known example is the Sandoz accident that occurred in Basel in 1986, where 1350 tons of chemicals were combusted, and between 10000 and 15000 m 3 of contaminated fire-fighting water were discharged into the Rhine river. Many of those chemicals were pesticides, and the impact on the environment was catastrophic [START_REF] Capel | Accidental input of pesticides into the Rhine river. Environmental impact and behavior of pollutants discharge during a chemical storehouse fire[END_REF].

As mentioned above it is common to find pesticide factories located near rivers or estuaries in European countries. Figure 1 shows the location of major pesticide plants within the UK mainland. The rivers form complex ecological systems in the natural environment, as illustrated in Figure 2. Rivers are directly linked to estuaries, coasts, and open seas where the pollutants are transported by the flow, and are in permanent contact with the atmosphere to which pollutants can be transferred through volatilization. The river system is also connected to the groundwater either because the groundwater feeds the river when the piezometrical level of the groundwater is higher than the level of water in the river, or because the river feeds the groundwater when the level of water in the river is higher than the piezometrical level of the groundwater. In the second case, the pollutants present in the river are transported into the groundwater and which may pollute both the ground and the drinking water. The pesticides present in the water can also pollute directly the surrounding land by adsorption through the groundwater or by the rain after volatilization into the atmosphere. Concern thus arises because of the location of many of the factories, the storage plants and the risk they potentially pose to the aquatic environment after an accidental fire.

Although numerous studies of fate of pesticides have been conducted relatively few deal with their fate in water (see for example [START_REF] Crossland | Evaluation of models used to assess the fate of chemicals in aquatic systems[END_REF]. Moreover, these studies usually evaluate the ecological impact of pure products, not of mixtures of pesticides.

Herbicides form a major group of pesticides and in France, United Kingdom, and Denmark, they represent more than 50% of the production of pesticides every year. Such products enter the aquatic environment through the soil, directly when they are used to control the development of plants in rivers, or when an accident occurs in a storage plant and thus represent a potential threat to the aquatic environment.

The work presented in this paper is part of an overall programme to investigate the ecological impact on freshwater systems of pesticide warehouse fires. This paper reviews and discusses the role of the major fate processes of herbicides in water. The discussions focus on three herbicides; glyphosate, diquat and paraquat. Mathematical models to simulate and predict the fate processes of pesticides are also discussed in relation to pesticide storehouse fires. A possible strategy for mathematically modelling these fate processes is suggested.

FATE PROCESSES OF SELECTED HERBICIDES

Once pesticides enter the water environment they are subject to various physical, chemical and microbiological fate processes. Figure 3 shows the major fate processes effecting pesticides in water and thus illustrates the complex nature of task to trying to understand such processes. This paper focuses on three herbicides that are widely used in Europe whose properties are summarises in Table 1. The choice of these herbicide was determined primarily by the fact that they degraded predominantly by defined fate processes. Glyphosate is primarily biodegraded, diquat and paraquat are photodecomposed, adsorbed, and biodegraded.

Glyphosate [N-(Phosphonomethyl)glycine] is a non selective organophosphorus aquatic herbicide, slightly toxic to aquatic fauna (Table 1), also used for vegetation control in forestry (Ghassemi & Quinlivan, 1982 ;[START_REF] Hutson | Progress in pesticides biochemistry and toxicology[END_REF]. It is an organophosphorus compound which possesses a carbon-phosphorus (C-P) bond. This C-P bond is resistant to chemical hydrolysis, thermal decomposition, and photolysis [START_REF] Moore | Isolation of aPseudomonas sp. which utilizes the phosphanate herbicide glyphosate[END_REF]. On the other hand, glyphosate is biodegraded by bacteria and adsorbed to clay.

Diquat [Dipyrido (1,2-a:2',1'-c)pyrazinediium,6,7,-dihydro-, dibromide monohydrate] and paraquat (4,4'-bipyridinium, 1,1 '-dimethyl-, dichloride) are pyridine herbicides. Both are more toxic to aquatic animals and invertebrates than glyphosate (Table 1). Diquat is used for aquatic weed control, potato haulm, and seed crop desiccation. Paraquat is used to clean the stubble, for inter-row weed control, crop desiccation, and elimination of old swards [START_REF] Hutson | Progress in pesticides biochemistry and toxicology[END_REF]. Diquat and paraquat are degraded by photolysis, biodegradation, and are strongly adsorbed to clay.

Sorption

Sorption is a physicochemical dynamic process of pesticide-sediment-water interactions, correlated with the nature of the solids in the system (particle size distribution, type of clay mineral, clay content, organic matter content, cation exchange capacity) and the characteristics of the pesticide (water solubility, octanol-water partition coefficient, fugacity). The ability of a pesticide to be adsorbed on sediments depends on its polarity; a pesticide is adsorbed more when its polarity is low [START_REF] Hutson | Progress in pesticides biochemistry and toxicology[END_REF].

There are two sorption coefficients to evaluate the rate of sorption of a pesticide: (1) K,, the sorption partition coefficient, used for low concentrations of pesticide (K d is expressed in I/kg): Kd = concentration in sediment concentration in water and (2) K^ the adsorption coefficient, defined as the K d corrected for the organic content of the sediment :

K d oc (% organic carbon )
The higher Kj and K^ are, the greater the role of sorption assumes in the removal of the herbicide from the water [START_REF] Reinert | Fate and persistence of aquatic herbicides[END_REF].

Once they are adsorbed to sediment, pesticides can be desorbed and be submitted to subsequent diffusion in the river. The effect of adsorption-desorption on transport of pesticides in rivers is to retard their solute diffusion. The process of adsorption-desorption has been studied by [START_REF] Corwin | An assessment of the significant physicochemical interactions involved in pesticide diffusion within a pesticide-sediment-water system[END_REF] on the base of numerical models from Freudlich and Langmuir. These authors showed that in the case of strongly adsorbed cationic pesticides such as diquat and paraquat, the adsorbed phase degradation rate may be more significant than the degradation rate of pesticide in solution. A sediment with a high adsorptive capacity and a high strength of adsorption would release pesticide to the water at a slow rate and in low amounts, presenting less hazard of toxicology for aquatic organisms but a higher threat to filter-feeding organisms which rely upon organic matter at the sediment-water interface as a food source.

Glyphosate is readily adsorbed onto clay. Its adsorption increases when the pH of the clayglyphosate system decreases. Glyphosate is also strongly adsorbed to Al 3+ and Fe 3+ , and is not adsorbed to Na + [START_REF] Sprankle | Adsorption, mobility and microbial degradation of glyphosate in soil[END_REF][START_REF] Grossbard | The herbicide glyphosate[END_REF]Me Connell & Hossner, 1985). The consequence of this adsorption is that glyphosate is inactivated in soils that contain clay [START_REF] Hance | Adsorption of glyphosate by soils[END_REF][START_REF] Grossbard | The herbicide glyphosate[END_REF]. [START_REF] Hartman | Effect of suspended bentonite clay on the acute toxicity of Rondup (glyphosate) to Daphnia pulex and Lemma minor[END_REF] studied the effect of suspended bentonite clay on acute toxicity (0 -100 mg/l) of the herbicide glyphosate. The acute toxicity tests have been made on an aquatic invertebrate (Daphnia pulex) and an aquatic plant (Lemna minor). Other experiments have been conducted on glyphosate-sediment interactions and phytotoxicity on safflower roots in turbid water for low concentrations of glyphosate (< 6 mg/l) (Bowmeref al., 1986). All these experiments showed that the effect of glyphosate adsorption to suspended clay on its toxicity depends on the way how the organisms utilise the media where they grow, and that the adsorption of glyphosate to clay is negligible for low concentration of the herbicide in water. For example, glyphosate gets unavailable to Lemma minor, a plant that removes dissolved constituents from the aqueous phase of the medium in which it grows, when there is no apparent lasting effect on Daphnia pulex, which ingests glyphosate with particulate matter by the filter-feeding.

Diquat and paraquat, because of their cationic properties, are strongly adsorbed to sediments and suspended materials, especially to clays that have a high cation-exchange capacity. The adsorption of these herbicides depends on the surface charge density of the clay and increases when pH decreases, but seems to be independent of the temperature and of the water solubility [START_REF] Gunther | The triazine herbicides[END_REF]. As paraquat and diquat are very strongly adsorbed to soils and sediments, they are biologically unavailable in that form, reducing their rate of degradation by microorganisms [START_REF] Eisler | Paraquat hazards to fish, wildlife, and invertebrates : a synoptic review[END_REF]. Addition of clay on soil also decreases diquat and paraquat phytotoxicity because, after adsorption on the clay, the herbicide was less available for plant roots [START_REF] Gunther | The triazine herbicides[END_REF].

Photolysis

Much of the energy required to drive the environmental transformations of organic compounds comes directly from the solar radiation. Pesticides, dissolved or suspended in water or in gaseous phase, can absorb light and decompose spontaneously or by reaction with reagents generated by light (e.g. oxygen or ozone).Pesticide photodegradation, or photolysis, depends on different factors such as light quality and quantity, water turbidity and pH [START_REF] Hill | Pesticide microbiology[END_REF].

The overall rate of pesticide photolysis can be expressed as follows [START_REF] Schnoor | Fate of pesticides and chemicals in the environment[END_REF] : UV light rapidly degrades both diquat and paraquat: 50% of the 14 C from both compounds is lost after 48 hours, and more than 75% is lost after 96 hours of exposure to UV light [START_REF] Kearney | Degradation of herbicides[END_REF][START_REF] Reinert | Fate and persistence of aquatic herbicides[END_REF]. Kearney and Kaufman proposed that the first compound of degradation of paraquat could be 1 -methyl-4-carboxypyridinium ion. This compound could then be transformed to methylamine hydrochloride. It has been shown experimentally that paraquat can be photodegraded in soils by UV radiation in the presence of oxygen or ozone with 1-methyl-4-carboxypyridinium ion, 4-picolinic acid, hydroxy-4-picolinic acid, N-formylglycine, malic acid, and oxalic acid as reaction products [START_REF] Eisler | Paraquat hazards to fish, wildlife, and invertebrates : a synoptic review[END_REF]. Glyphosate on the other hand is stable to photolysis.

Dc = -k p C dt

Biological transformation processes

Pesticides in the aquatic environment are taken up by organisms. The biota deals with pesticides in four different ways: (1) pesticides are accumulated in the body, eventually having a toxic effect on the organism or causing the death : this is bioconcentration or bioaccumulation, (2) they are transmitted to other organisms through the food chain and are accumulated by different organisms, particularly consumers at the upper end of the foodchain (e.g. birds of prey and man) : this is biomagnification, (3) they are transformed by organisms and excreted : this is biotransformation and is known to occur in some aquatic animals and even fungi, (4) they are decomposed significantly possibly even to mineral products such as CO 2 , CH* and H 2 O : this is biodegradation and is .

Bioconcentration can be evaluated by two factors :

(1) the bioconcentration factor (BCF), that determines the potential of a compound to bioconcentrate within an organism, is defined as follows:

BCF = concentration in living tissue concentration in water the higher the BCF, the greater the potential for bioconcentration and long-term harm to aquatic organisms and aquatic environments [START_REF] Reinert | Fate and persistence of aquatic herbicides[END_REF].

(2) the octanol-water partition coefficient (K ow ) measures the ease of the transfer of a chemical from the aqueous to the lipid phase. It is inversely proportional to the water solubility (W s ) of the herbicide [START_REF] Mackay | Solubility, partition coefficients, volatility, and evaporation rates[END_REF][START_REF] Hutson | Progress in pesticides biochemistry and toxicology[END_REF]. K ow is defined by the following equation : K ow = concentration in octanol concentration in water the larger the K ow is, the higher is the rate of bioconcentration of the herbicide [START_REF] Reinert | Fate and persistence of aquatic herbicides[END_REF]. However, none of the selected herbicides is concentrated in biota.

Biotransformation occurs when the original compound is modified by organisms usually to reduce its toxicity. Biodegradation is the breakdown of a compound by microorganisms (HMSO, 1981). This may be : (1) primary (the alteration of the chemical structure of a substance resulting in loss of specific property of that substance) or ( 2) ultimate (the complete breakdown of a compound to either fully oxidised or reduced, simple molecules (e.g. CO 2 , CH 4 , H 2 O, NO 3 -, NH 4 + ) and the formation of new cells).

The total decay rate coefficient (K T ) can be calculated, if the degradation is first order, for each herbicide by adding the K values (K^, K d , K^J. Then, the half-life time (t-, /2 ) can be calculated as follows [START_REF] Reinert | Fate and persistence of aquatic herbicides[END_REF] :

t 1/2 = 0.693 KT
Herbicides with small K T values will persist longer in the aquatic environment (large t 1/2 ) than herbicides with larger K T [START_REF] Reinert | Fate and persistence of aquatic herbicides[END_REF]. The results obtained by this model should be verified experimentally.

Glyphosate is completely degraded in soil by micro-organisms. One of the major metabolites of this degradation is aminomethylphosphonic acid which is non toxic and also degraded in soils but at a slower rate than glyphosate (Rueppel eta/., 1977). Glyphosate decomposition has been shown under both aerobic and anaerobic conditions. This decomposition is closely correlated with microbial activity in soil [START_REF] Muller | Fate of glyphosate and its influence on nitrogen cycling in two Finnish agricultural soil[END_REF] [START_REF] Kearney | Degradation of herbicides[END_REF]. Actinomycetes, including species of Streptomyces and Nocardia, have also been shown to degrade paraquat forming 1 -methyl-4-carboxypyridinium ion [START_REF] Hill | Pesticide microbiology[END_REF]. The formation of 1-methyl-4carboxypyridinium ion (N-methylisonicotinic acid) was first proposed as the product of paraquat biodegradation by an unidentified bacterium in 1967 [START_REF] Hill | Pesticide microbiology[END_REF]. The proposed pathway of degradation was in two steps : (1) initial demethylation that transforms paraquat into 1 -methyl-4,4'-dipyridinium ion, and (2) cleavage of one of the pyridine rings that transforms 1 -methyl-4,4'-dipyridinium ion into 1-methyl-4carboxypyridinium ion.

MATHEMATICAL MODELLING STRATEGIES

Mathematical Models

A common way to model the transport of pesticides in rivers is based on compartmentalization of the river in sections (Neely eta/., 1976 ;[START_REF] Chen | Water and sediment routing in rivers[END_REF][START_REF] Thomas | Computer modelling of rivers : HEC 1-6[END_REF]Yoshiba eta/., 1983 ;Schnoor eta/., 1987). Modelling of transport can often be simplified by making the assumptions that the bed stream is uniform, and that the velocity and density of sediment are constant. As these assumptions can be justified only over a short length of the river and it is necessary to model the stream in sections. The transport of pollutants and sediments can then be calculated in each section and aggregated to obtain the transport characteristics through the whole river.

Biodegradation of pesticides in soil has been modelled by Soulas (1981), taking into account four populations of micro-organisms: (1) metabolizing micro-organisms which degrade the pesticide and use it as a source of nutrient ;

(2) co-metabolism microorganisms which degrade or transform the pesticide without using it as a source of nutrient ;

(3) non-degrading sensitive micro-organisms which are sensitive to the lethal action of the chemical; and (4) non-degrading non-sensitive micro-organisms which are indifferent to the lethal action of the chemical. On the other hand, biodegradation of pesticides in water has not been studied or modelled in detail.

The strategy for modelling adsorption and desorption processes employed by [START_REF] Corwin | An assessment of the significant physicochemical interactions involved in pesticide diffusion within a pesticide-sediment-water system[END_REF] has been tested on diquat and has shown that this herbicide is strongly adsorbed to clay and that its rate of desorption was very low. Direct photolysis has been modelled by [START_REF] Zepp | Rates of direct photolysis in aquatic environment[END_REF] under the assumption that the pollutant is isotropically distributed in the water body and the amount of light is constant during a given period of time. These assumptions can be verified if the river is treated in sections along the length of the bed of the river and in subsections along the depth of the river where the rate of light decreases. Crossland et a/(1986) used the mathematical model SOLAR to predict the rate of phototransformation of pentachlorophenol and dichloroaniline in ponds. This model was based on the following parameters: light absorbance of the pond water, its depth, the season, and the latitude.

A range of aquatic chemical fate models have been reviewed in [START_REF] Samiullah | Prediction of the Environmental Fate of Chemicals[END_REF]. In particular the application of fugacity models to study environmental partioning have been discussed. A feature of fugacity modelling is that they can be applied at various complexity levels [START_REF] Mackay | Application of fugacity models to the estimation of chemical distribution and persistence in the environment[END_REF]. Such a model employs a compartmental approach where each compartment is a volume in which the chemical is in a selected environment and is subject to selected processes. The chemical is assumed to move from one compartment to another according to the laws of chemical kinetics.

Discussion on Mathematical Modelling Strategies

When a pesticide spill occurs into a river it is important to model the transport and the fate of pesticides along the whole stream so that a complete assessment of the ecological impact can be made. The relative contributions of the major interacting fate process have to be taken into account by the adopted modelling strategy. The previous section described mathematical models that compute the transport and the fate processes of the selected herbicides in water, as there are many complicated biogeochemical processes taking place it is usual to make simplifying assumptions. Such as, parameters such as velocity, light, or concentration of sediments may have to be assumed to be constant over a specific section of the river. As these assumptions can be justified only over in a limited space and time period it may be necessary to treat the river in sections of length and subsections of depth. The transport and the fate processes can then be modelled in each section. Whichever strategy is adopted it has to be formulated inlight of the overall objectives set for the model. These may include:

(1) How fast will the pesticides be degraded in the river ?

(2) Where and how fast will the pesticides be transported in the river ?

(3) What concentrations will the pesticides attain in each riverine compartment? (4) What implications do these levels have in terms of ecotoxicity ?

The first stage involves deciding which are the most important fate processes. For example, in the case of degradation of glyphosate, two major fate processes are involved ; biodegradation and sorption. Biodegradation and adsorption to clay in benthic sediment or on suspended sediment have negative influences on each other. The more glyphosate is degraded, the less it will be adsorbed to clay, however the more it is adsorbed, the less it is available for microorganisms and hence it will be less biodegraded. The process is opposite with desorption. Biodegradation increases with desorption because as glyphosate becomes available for microorganisms. The situation is similar with diquat and paraquat except that these herbicides are very rapidly degraded by photolysis. Biodegradation and photolysis may have negative influences on each other, although the product of photolysis may well be biodegradable.

In the second stage the influence of the fate processes on transport has to ascertained, that is, whether they increase the rate of transport or decrease it. Cases of glyphosate, diquat and paraquat are similar, the degradation processes (biodegradation for glyphosate, photolysis and biodegradation for diquat and paraquat) decrease the quantity of the parent pesticide transported along the river. Adsorption to clay in benthic sediment also decreases the rate of transport. Adsorption to suspended solids and desorption would tend to increase the rate of transport.

As an example the multiple pathways that can result of the different fate interactions are illustrated in the case of glyphosate, see Figure 4. It is convenient to consider the quantity of product degraded and that adsorbed and how the fate processes affects the transport of the pesticides through the river. When glyphosate enters the river, a certain quantity (Qb) will be directly biodegraded. This quantity is equal to the total amount of pesticide discharged into the river (Qt) minus the quantity adsorbed to benthic sediment (Qa) and to suspended sediment (Qs). During the time it takes to degraded it, glyphosate will be transported along the river (time Tb). With regard to the amount of glyphosate adsorbed onto clay particles, glyphosate can either be desorbed and biodegraded (quantity Qad and Qsd) or directly biodegraded (quantity Qab and Qsb). If glyphosate is adsorbed to clay in benthic sediment and then directly biodegraded, it will not be transported ; if it is then desorbed and biodegraded, the time of transport will be the time needed by glyphosate to be biodegraded (time Tad). If glyphosate is adsorbed onto suspended solids and then biodegraded, the time of transport of glyphosate will be the time needed by glyphosate to be biodegraded (Tsb) ; if it is desorbed and then biodegraded, it will be transported when it is adsorbed and until it is totally biodegraded (Tsd). The system would be similar for diquat and paraquat but more complex because of the importance of photolysis.

In such a compartmentalized approach the river is considered in terms of short sections of a defined length. Each block of the river consists of several parts that can be defined as suspended solids, benthos, plants, and animals. The transport can be modelled between each compartment and the aqueous phase. In each compartment, several fate processes can be modelled. The main fate processes that govern the transport can be then modelled to and fro each block of the river. Such an approach offers the advantage of the model being able to be validated with mesocosms which would be the physical counterparts of the section of the river. As the river is sectioned a mesocosm can then be setup to reproduce the conditions in the particular section.

CONCLUSION

The present paper describes the major fate processes affecting herbicides in riverine systems. Mathematical modelling strategies are reviewed and discussed and a compartmental modelling approach is suggested as a possible approach. Glyphosate, diquat, and paraquat have been considered in this paper because of their wide use in Europe and because of their mode of degradation. Three major processes that affect the fate of these herbicides in rivers are: biodegradation, sorption, and photolysis. Glyphosate has been shown to be biodegraded in soils and to be adsorbed to clay with a rate depending on the unoccupied phosphate sorption capacity, but it is poorly adsorbed to suspended clay in rivers. In the aquatic environment, glyphosate might be degraded by microorganisms that are on vegetation surface, on suspended sediments, or in the sediment after adsorption of glyphosate. Both diquat and paraquat are very rapidly photodegraded in freshwater by UV light. They are also biodegraded in water by microorganisms that use these herbicides as a sole source of carbon or nitrogen. Diquat and paraquat are very strongly adsorbed to sediments and suspended materials, reducing their biodegradation rate and their phytotoxicity because of their unavailability when they are adsorbed. Although many studies have been reported on the adsorption of these three herbicides to benthic sediment, their biodegradation in soil, and photodecomposition of diquat and paraquat in water relatively less work has been performed on their biodegradation behaviour in water.

In many cases, mathematical models are based on the rate of degradation and sorption of pesticides in water and relevant data, in the case of glyphosate, diquat and paraquat, is not always available. Further work is thus proposed in this area. It would be particularly interesting to compare the results obtained from tests on combustion products of these herbicides with identical tests performed on pure products. The importance of such studies goes beyond scientific interest as regulatory pressures continually grow to not only prevent storehouse fire accidents but also to understand the full ecological impact should such incidents occur.

ACKNOWLEDGEMENTS

INEERIS and WSL are thanked for supporting this work. (Worthing, 1987 ;[START_REF] Reinert | Fate and persistence of aquatic herbicides[END_REF]Grossbard etai, 1985 ;Kenaga & Goring, 1980 ;[START_REF] Eisler | Paraquat hazards to fish, wildlife, and invertebrates : a synoptic review[END_REF]

  The photolysis rate constant, k p , is can be calculated with the following first order equation k p = 2.303 f S I, e, or molar extinction coefficient at wavelength 1 There are two photochemical processes : (1) direct photolysis of pesticides proceeds via direct absorption of sunlight energy causing chemical reactions (its kinetics are composed of two factors ; light absorption and quantum yield), (2) indirect photolysis which requires a sensitizer to initiate light absorption (sensitizer include O 3 ).

Figure 1 :Figure 2 :

 12 Figure 1 : Location of pesticides plants in UK

  Figure 3 :

Table 1 :

 1 Properties of Glyphosate, Diquat, and Paraquat