Corinne Hulot
email: corinne.hulot@ineris.fr

Amadou Thiam

TRANSFER OF VOLATILE COMPOUNDS FROM SOIL: COMPARISON BETWEEN PREDICTIONS AND FIELD MEASUREMENTS WITH DIFFERENT MODELS; DEVELOPMENT OF THE VOLASOIL MODEL FOR A DEPLETING SOURCE

Keywords: contaminated soil, vapour, indoor air, transfer, model, measure, finite source, attenuation. 1

Soil vapour migration into buildings, and subsequent inhalation, are often the main exposure pathway to humans at sites contaminated with Volatile Organic Compounds (VOCs). For such sites, the quantification of pollutant concentrations in the indoor air is essential while assessing risks for the human health. Modeling is often necessary, for example in the view of future situations, but also in combination with site measurements. The Johnson & Ettinger and VOLASOIL models are the two models mainly used in Europe to predict indoor air concentrations. The validation of these models is still incomplete, beyond some global verification of indoor air concentrations: step by step measurements for a detailed model calibration and "validation" hardly exist in the literature, and predictions of indoor air concentrations from different models sometimes vary by several orders of magnitude. Verifying and improving vapour intrusion models is necessary for a better risk estimation and subsequent site management. Besides, the VOLASOIL model does not represent the depletion of the source due to the volatilization, beyond an overall mass control; this may result in over-conservative estimates of indoor air concentrations, for example when a VOC source is near the soil surface.

This paper first presents a development of the VOLASOIL model (in its multilayer version) for a depleting source with a site application, and secondly two site studies where detailed site "validation" data were collected and compared to model results.

The site investigations (2003)(2004)(2005)(2006) concerned the pollutant concentrations in soil, soil gas, indoor air, and pollutant flux at the slab surface, and key model parameters: depth of the pollution, characteristics of the soil and of the building (slab, atmospheric / soil pressure difference, ventilation rates…). They were conducted at different seasons. The transfer modeling, from soil and soil gas to indoor air, was performed with two transport models, based on the equations respectively of Johnson & Ettinger, and of VOLASOIL (multilayer version). A comprehensive sensitivity analysis was undertaken.

The modeling of a depleting source led to downsize the vapour flux by one order of magnitude or more over one year at the application site. This illustrates the interest of this modeling. The mathematical developments of VOLASOIL presented here should allow a broader use of VOLASOIL, e.g. for a building with a concrete slab. The comparisons between predicted and measured concentrations in indoor air showed on the real site over-prediction and in some cases under-predictions of the models by one or two orders of magnitude. In spite of an extensive site characterisation, those differences, and the significant time and space variability in measured soil gas or indoor air concentrations could often not be explained. Significant impacts on the modelled concentrations could be obtained by modifying the parameter values within their respective possible ranges.

The study emphasizes the importance of a humble attitude towards the results of the modeling of vapour transfer, in the current state of model validation. Combining models and measurements is essential for a "robust" risk assessment. A comprehensive understanding of the model used, its concept and its equations, is essential. We recommend to re-code one's own tools so as to better test and understand the sensitivity of the model in different situations. Improvements in modeling and measuring vapour intrusion are still possible. When dealing with finite source modelling, the research should be extended to the impact of degradation in the vadose zone for benzene and e.g vinyl chloride as a source of variability and systematic differences. Detailed quantitative measurements and interpretation of the main site parameters appear essential when aiming at model verification. This is a difficult and time-consuming work, which has to be repeated with different types of sites (building designs, construction materials, soil) and at different periods under adequate conditions (heating, "normal" ventilation…). Collaborations are needed, at national and international scales, so as to gather strengths on data collection and interpretation. Such collaborations are initiated for example with the HERACLES initiative and with the French and European research projects FLUXOBAT and CITYCHLOR about chlorinated solvents pollution in soil and groundwater.

Introduction

Soil vapour migration into buildings, and subsequent inhalation, are often the main exposure pathway to humans at sites contaminated with Volatile Organic Compounds (VOCs). For such sites, the quantification of pollutant concentrations in the indoor air is essential while assessing risks for the human health. Modeling is often necessary, for example in the view of future situations, but also in combination with site measurements: both modeling and measuring present important uncertainties, their combination helps reaching a more "robust" site assessment. A variety of soil vapour intrusion algorithms, or « empirical » / dilution factors are available that calculate transfer of soil gas to the indoor air (not exhaustive list [1 to 11, 22, 23]). The Johnson & Ettinger and VOLASOIL models are the two models mainly used in Europe.

The validation of these models is still incomplete, beyond global verification of predicted indoor air concentrations and comparisons of model results in different situations [e.g. 23]: step by step measurements for a detailed model calibration and "validation" hardly exist in the literature, and predictions of indoor air concentrations from different models sometimes vary by several orders of magnitude. Verifying and improving vapour intrusion models is necessary for a better risk estimation and subsequent site management. Besides, the VOLASOIL model does not represent the depletion of the source due to the volatilization, beyond an overall mass control; this may result in over-conservative estimates of indoor air concentrations, for example when a VOC source is near the soil surface. This paper first presents a development of the VOLASOIL model (in its multilayer version) for a depleting (finite) source with a site application, and secondly a site study where detailed site "validation" data were collected and compared to model results.

The site investigations (2003)(2004)(2005)(2006) concerned the pollutant concentrations in soil, soil gas, indoor air, and pollutant flux at the slab surface, and key model parameters: depth of the pollution, characteristics of the soil and of the building (slab, atmospheric / soil pressure difference, ventilation rates…). They were conducted at different seasons. Following previous works presented at CONSOIL 2003 [START_REF] Hulot | Vapor emissions from contaminated soils into buildings : comparison between predictions from transport models and field measurements[END_REF] and 2005 [START_REF] Hazebrouck | Vapor emissions from contaminated soils into buildings : Developments of the volasoil model for multilayer soil[END_REF] (site n r 1), the site study included a new site (site n r 2) and new compounds (BTEX and naphthalene beside chlorinated solvents).

The transfer modeling, from soil and soil gas to indoor air, was performed with two transport models, based on the equations respectively of Johnson & Ettinger [START_REF] Johnson | Heuristic Model for Predicting the Intrusion Rate of Contaminant Vapors into Buildings[END_REF], and of VOLASOIL (multilayer version, [START_REF] Hazebrouck | Vapor emissions from contaminated soils into buildings : Developments of the volasoil model for multilayer soil[END_REF], [START_REF] Bakker | Site-specific humantoxicological risk assessment of soil contamination with volatile compounds[END_REF]). A comprehensive sensitivity analysis was undertaken.

The goal of this research is to help improve the site-specific selection and application of vapour transfer models, for better risk-based actions (remediation, action on current buildings or future construction plans,…).

Development of the VOLASOIL model for a depleting source

Conceptual model

We consider here n successive layers of homogeneous material (soil or concrete) in the vadose zone (Figure 1), indexed from the polluted layer (i=0) up to the surface (i=n for the layer in contact with the indoor air: crawl space, basement, or first floor).

The flux mechanisms supposed to occur are the diffusion in the soil water and in the soil gas, and the convection in the soil gas. Besides, the multiphase equilibrium between soil gas, soil water, and soil, is considered to take place (right side of Figure 1). The different fluxes are not independent: it is the same soil gas (or soil water in equilibrium with the soil gas) which is simultaneously submitted to the different mechanisms.

The conceptual model developed here for the depletion of the source is a progressive descent of the top of the source with time, as it looses its pollution through volatilisation. /s] which combines the convection and the diffusion (here neglecting the indoor air concentration), as in the new version of VOLASOIL [START_REF] Bakker | Site-specific humantoxicological risk assessment of soil contamination with volatile compounds[END_REF] 1 :

Indoor air

Convection

1 A further development integrating the evapotranspiration water flux and the transfer within the capillary fringe and the groundwater is also available [START_REF] Johnson | Heuristic Model for Predicting the Intrusion Rate of Contaminant Vapors into Buildings[END_REF].

Equation 1 o tot tot ci ci tot Csa DU L F - exp - 1 F = J         , with: Equation 2 tot is tot 3 ci L P K 10 F = ; ∑ = = n 1 i i tot L L ; ∑ = = n 1 i i i tot tot K L K L ; ∑ = i i i tot

Numerical conservative solution for a depleting source, taking into account both convection and diffusion

We consider here a progressive descent of the top of the source, starting from the depth Ltot at t=0, down to a depth ( ) ( )

t z L t L o tot tot + =
at time t, where z 0 (t) is the "cleaned" thickness in the source layer L 0 . The soil gas flux F ci from the top of the source to the indoor air, the mean multilayer and multiphase diffusion coefficient DUa, are now also time-dependent (they decrease with time).

The flux of VOC carried out, in mg/m 2 , between time t 0 =0 and time t is : Equation 4( )

∫ = t 0 tot out y y J (t) Q d ,
By mass conservation, this flux is connected to the "cleaned" thickness z 0 (t) in the source layer L 0 : Equation 5[ ] An analytical solution is reachable when we use the (slightly over-)conservative2 estimate of the total flux J tot (t) as the addition of two independent fluxes:

0) (t z (t) z C SD 10 (t) Q 0 0 s 3 out = - =
• The pollutant flux due to convection alone:

( ) ( ) ( ) t L t K P 10 Csa (t) F Csa t J tot tot is 3 o ci o conv ∆ = =
, and

• The pollutant flux due to diffusion alone:

( ) ( ) ( ) ( ) ( ) t L Csa t DU t L Csa Csa t DU t J tot o tot tot n o tot diff = - =
Then Equation 7 becomes : Equation 8( ) ( ) ( )

t z DU L DU DU DU Csa t z K L K K K P 10 Csa t ' z C SD 10 o tot tot o tot o o o tot tot o tot o is 3 o o s 3 + + + ∆ = or Equation 9 ( ) ( ) ( ) ( ) ( ) ( ) ( ) t ' z e t z d t z t ' z e t z d c t z b t z a SD 10 CT o o o o o o 2 o 3 sa s       + + + = + + + = → γ β α
with the following definitions:

( ) ( ) o o is 3 tot tot 2 tot o o o tot tot o tot tot tot DU K P 10 DU K d L K DU c DU K DU K L b K DU a + ∆ = = + = = ( ) ( )       - = - = = + ∆ = d e a b d e - c d d e a b d a DU K P 10 L DU K e tot tot is 3 tot o o γ β α
The solution of the differential equation 9 can be expressed as: Equation 10( ) ( ) ( )

      + = = → - t SD 10 CT 0 t z G G t z 3 sa s o 1 o
where the function G(x) is defined as following: Equation 11( )

      + + + = d e x ln d x x 2 x G 2 γ β α .
There is no analytical solution for z o (t) but it can be determined numerically by finding a root to the following equation: Equation 12(

) [ ] ( ) [ ] 0 t SD 10 CT 0 t z G t z G 3 sa s o o = - = - → .
With the numerical solution of z o (t), the mean flux of VOC carried out, in mg/m 2 /s, over the period of time t can be deduced from equation 5:

Equation 13 { } t 0) (t z (t) z ; L min C SD 10 t (t) Q 0 0 o s 3 out = - = ,
where the depletion of the source is limited to the thickness L o of the contaminated layer.

The ratio between this mean flux of VOC and the concentration Csa o in the soil gas at the source is then:

Equation 14 { } t 0) (t z (t) z ; L min CT SD 10 Csa t (t) Q CT 0 0 o sa s 3 o out em sao = - = = → → .

Analytical solution taking into account only the convection

As we could not get an analytical solution for the convection and diffusion equations together, we developed a VOLASOIL model for a finite source taking into account only the convection. In fact, a finite source modelling is needed only in cases of important fluxes, which are generally essentially convective. When using this modelling, it should then be checked after the modelling that the diffusion-alone flux remains low in comparison to the convective flux obtained (or, to be more surely conservative, it can be added to the only-convective flux obtained with depletion).

By taking into account only the convection, the equation 4 could be simplified : Equation 15( )

( ) ( ) ( ) ∫ ∫ ∫ ∫ ∆ = = = = t 0 t 0 tot tot is 3 o ci o t 0 conv t 0 tot out y y L y K P 10 Csa y (y) F Csa y y J y y J (t) Q d d d d
As equation 5 is still valid, equation 8 becomes : Equation 16( ) ( )

t z K L K K K P 10 Csa t ' z C SD 10 o tot tot o tot o is 3 o o s 3 + ∆ =
or Equation 17( ) ( )

t z K L K K SD P CT t ' z o tot tot o o is sa s o + ∆ = →
This differential equation can also be written : Equation 18( )

t SD P CT K 2 t z K L K is sa s o 2 o tot tot o d d ∆ =       + →
The solution of equation 18 is then:

Equation 19 tot tot o 2 tot tot o o is sa s 0 o K L K K L K 0) (t z t SD P CT K 2 (t) z -         + = + ∆ = →
Equation 13 and Equation 14 are still valid, and the analytical solution of z o (t) can be used in it to determine mean flux of VOC carried out, in mg/m 2 /s, over the period of time t.

If the pollution source occupies several soil layers, this solution can be applied successively to each of those layers, and the corresponding pollutant fluxes averaged over the total time. This requires the depletion time of each layer, which could be obtained by the numerical resolution of equation 19 with z 0 (t) = L 0 .

Application: modeled impact of the depletion

Method: A numerical application of the analytical solution was performed on a real case concerned with compounds ranging from less volatile (butanol, MethylEthylCetone) to more volatile (chlorinated solvents). We compared modeling results with depletion and without depletion in the hypothetical typical future conditions (closed and heated building). Indoor air measurements were also performed on the site but were not used (so far) for measure/model comparison. The conditions for that were not adequate: the building was largely open, the pressure difference between soil gas and indoor air was not measurable (< 1 Pa), and the source depletion would have been particularly slow to observe.

The soil consisted of two layers:

• A concrete slab with a thickness L 1 = 15 cm, and an air permeability K 1 = 1.1.10 -6 m 2 /Pa/s (medium quality: standard VOLASOIL value [START_REF] Waitz | The VOLASOIL risk assessment model based on CSOIL for soils contaminated with volatile compounds[END_REF]).

• Filling material with an air permeability K 0 = 5.6.10 -7 m 2 /Pa/s, contaminated from the top at t = 0 (and as an exercise for some pollutants from a depth of 2.5 m; no down limit).

The exposure duration considered was 1 year, the lowest duration relevant for chronic risk estimate, corresponding to the ATSDR toxicological reference values (MRL: Minimal Risk Levels).

The compound parameters and the other soil parameters are reported in the following tables.

,9E-3 1,9E-3 1,6E-2 2,1E+0 1,1E-1 1,1E-1 1,2E-1 1,2E-1 9,8E-2 9,8E-2 3,8E-1 6,4E-2 6,4E-2 "Cleaned" thickness at t Z 0 (t) m 3,5E-1 3,4E-2 1,1E+0 1,4E+1 3,2E+0 1,6E+0 3,3E+0 1,7E+0 3,0E+0 1,4E+0 5

No depletion of the source , diffusion only

With depletion of the source, diffusion and convection

No depletion of the source, convection only

As expected, the impact of the modelled depletion increases with volatility and with the height of the top of the source: over one year, it ranges from 1% for butanol 2.5 m under the slab, up to a factor 94 for vinyl chloride immediately under the slab. In that time, the top of the source descends deeper in the soil, from 3.5 cm up to 14 m deeper respectively (assuming a uniform source layer over this depth, which was not the case at the site). Due to this impact, the diffusive flux (without depletion of the source) becomes often significant (20% or more of the convective flux with depletion of the source).

Measure-model comparison on sites 4.1. Site n r 2 -field investigations: site, material and methods

The site N r 2 (Figure 1) was an industrial building located in an urban area, contaminated with aromatic hydrocarbons in unsaturated soils. This building had a concrete slab and was surrounded by other buildings, asphalt or concrete roads and bare ground, and grasses. The characteristics of the building such as its dimensions and the (visual) quality of the floor were registered. The building got demolished and the underlying ground was eventually excavated, which allowed us a better characterization of the soils (lithology, size distribution, etc.).

The field investigations were performed at different seasons (October, December, following June). The meteorological data were collected for each investigation. We could heat the building in winter, allowing conditions more favourable to the convective transfer of vapour (cf. « stack effects »). • The « accumulation chamber » [START_REF] Pokryszka | Method of measuring surface emissions of methane[END_REF] (Figure 1-A): gases emitted from the site within an elementary surface (50 cm x 50 cm) are pumped, sent to an analyzer and reinjected into the chamber (H = 20 cm), creating a recirculating flow and then an accumulation in the chamber. The surface flux can be easily derived from the measurement of the atmosphere enrichment as a function of time. The flow of contaminant is expressed relatively to the standard gas chosen [START_REF] Pokryszka | Method of measuring surface emissions of methane[END_REF]. This measurement system is easy and rapid to operate (One measurement every 5 to 10 minutes, but not very precise and rather indicative (FID). • The "dynamic scanning chamber" [START_REF] Pokryszka | Developpement and validation of a method for measuring biogas emissions using a dynamic chamber[END_REF] displayed in Figure 2-B is a chamber connected to an inert gas supply system and an exit with measurement points. The system gathers contaminant gas emitted from soil within the inert gas flow. The mixed gas sampled is analyzed. Therefore the measured parameters (concentrations and outgoing air-flows) allow to quantify the flow of emitted vapours outgoing from the chamber, and thus to deduce the flow released through the concrete slab. This approach allows a precise quantification of the various emitted substances. Because of the size of the chamber (L : 250 cm, l :100 cm, H : 15 cm) and the operating time required to reach low quantification limits, only 1 to 3 points can be measured in one day. About forty measurements of soil-to-air fluxes with the "accumulation chamber" were carried out at one field investigations (October), according to a regular grid (2 m x 4 m). During the campaigns of October and December, no flux was detected, and this even in the slab zones showing numerous cracks, so that no measure with this chamber was realized afterward but also no indications could be gained on the most suitable positions for the dynamic chamber. The "dynamic chamber" points were located on zones previously identified as polluted by hydrocarbons and presenting a fissured slab (2 points in October and December, only one point in June to be checked following an absence of measurable flux in December).

Soil and slab sampling at different depths and subsequent physical-chemical analyses (organic carbon fraction, size distribution, permeability, concentrations, etc.), Soil samples were analyzed for the organic compounds, like benzene, toluene, ethylbenzene, xylenes and naphthalene.

Based on the data gathered on the lithology and the soil contamination, thirteen soil gas wells were installed at various depths (sampling depth between 0,2-0,4 m and 2-3 m) in the unsaturated zone (source layer and above) in a selected area of the building (Figure 1-C). Soil gas was sampled during the 3 field investigations. The pressure difference between the indoor air and soil gas was measured in each gas well, using a differential pressure measuring device, with a detection limit and a precision of 1 Pa (Figure 1-E).

Indoor air and outdoor air were sampled respectively at 7 and 2 points (downstream and upstream of the source) during the 3 field investigations, at a height of about 1 m (Figure 1-D), and analysed. Air exchange rates were characterized using a tracer gas (SF6): after the injection of SF6 in the indoor air at several points and heights (6 points and 2 heights for the big hall, 1 point and 1 height for the office within the big hall), air samples were collected at two heights at different times and analyzed by a chromatograph. The air exchange rate was then calculated using the following formula: Ln(C/C0) = -x (t-t0), where C is the tracer-gas concentration at time t, C0 is the tracer gas concentration at time t0, x is the air exchange rate in the buildings (Figure 1-F),

Site n r 2 -field investigations: results

Significant soil concentrations of benzene and naphthalene were observed, generally in layers situated below 1,5 m depth, with values respectively in the order of 150 mg / kg and 2200 mg / kg (arithmetic average). Only benzene and naphthalene data are treated in the present paper, due to their volatility and their high concentrations measured in soils.

The measured soil gas and indoor air concentrations for benzene and naphthalene are summarized in the in Table 1: [minimum -maximum] Table 1 : Site n r 2 -Summary of soil gas and indoor air concentrations (mg/m 3 ) for benzene and naphthalene

Benzene
Those results show significant time and spatial variations in soil gas and indoor air concentrations. This was also observed with surface fluxes.

In a same soil gas well, for example, for naphthalene, the maximal time variation is two orders of magnitude, with a maximal concentration in June. Between different soil gas wells, but at the same time and same sampling depth, concentrations varied within a range of three orders of magnitude (for example benzene in June). The indoor air concentrations varied within a range of several orders of magnitude over time, for example, for benzene and naphthalene, one order of magnitude between December and June. The indoor air concentrations during the same field investigations were in the same order, with some exception (3 orders of magnitude for benzene in December (maximum concentration)). In all soil wells and for all campaigns, the indoor air/soil gas pressure difference was in the range of the detection limit of the material (1 Pa), except in December, when the building was heated (2 Pa).

The measured air exchange rates were respectively 0,6, 1,8, 0,8 hour -1 in October, December and June. Except for the higher benzene indoor concentrations in December (in spite of lower soil gas concentrations) possibly related to the heating and the subsequent higher pressure difference, the tremendous variations of concentrations could not be explained with the available data, especially on concentrations, fluxes, building characteristics or meteorological conditions.

Modeling

A steady-state modelling of the indoor air concentrations was based on the analytical equations of Johnson & Ettinger [START_REF] Johnson | Heuristic Model for Predicting the Intrusion Rate of Contaminant Vapors into Buildings[END_REF] and VOLASOIL [START_REF] Waitz | The VOLASOIL risk assessment model based on CSOIL for soils contaminated with volatile compounds[END_REF] adapted by INERIS [START_REF] Hazebrouck | Vapor emissions from contaminated soils into buildings : Developments of the volasoil model for multilayer soil[END_REF] (multi-layer model, named "VOLASOILa"). The main input data are reported in Table 2. The majority of those values were measured at the site. Some data estimated from expert judgment, modeling and/or literature [4 ; 8] were also used. For example, the concrete slab was parameterized separately according to each model, based on our understanding of the mechanisms at stake in each model and on literature data.

Value Origin of the data

Soil

Sand, sand with silt and gravel Organic carbon fraction 0,5

Soil gas concentrations

See Table 1 For each campaign, arithmetic average of the concentrations measured in soil gas between 1,8 and 3 m

Indoor air concentrations See Table 1 For each campaign, maximum concentrations or quantification limit (when not quantified).

Table 2 : Values of the main modeling parameters

Comparison of predicted and observed indoor air concentrations

With both models, the modelling of indoor air concentrations were performed starting from soil concentrations and from soil gas concentrations.

The comparisons were not limited to the concentrations in the indoor air, but also concerned intermediate data such as soil gas and vapour fluxes. Figure 2 plots measured and predicted indoor air values for benzene and naphthalene in the « soil source » case, regarding predicted/observed ratio for site n r 2. The results of the modeling from a soil gas source for the December campaign for benzene and the June campaign for the naphthalene are not illustrated in this document but only commented later in a general way. For comparison, some results of the study of the site n r 1 for trichlorethene [START_REF] Hulot | Vapor emissions from contaminated soils into buildings : comparison between predictions from transport models and field measurements[END_REF] are also plotted. The results of those simulations lead to the following comments:

• Modelled soil gas concentrations (triphase equilibrium starting from soil concentrations) differed significantly from measured soil gas concentrations, up to several orders of magnitude depending on the choice of input values of the models and on the field campaign; • For the « VOLASOIL-a » modeling : starting from soil concentrations, the predictions were conservative by up to one order of magnitude for benzene and naphthalene. The lowest overestimation was observed in December (with heating and measurable pressure difference) for benzene and in June for naphthalene. Starting from soil gas concentrations, the predicted indoor air concentrations are in the same order of magnitude as the measured concentrations for benzene (December), much lower (several orders of magnitudes) for naphthalene (June); • For the « J & E » modeling : starting from soil concentrations, the predictions also tend to be overconservative, globally with an order of magnitude (with an exception). In the case of naphthalene, in June the concentration predicted in indoor air is underconservative by an order of magnitude

Ratio modeled / measured concentration

(campaign with the highest measured indoor air concentrations). Starting from soil gas concentrations, the model predictions underestimated the indoor air concentrations by several orders of magnitude. • On site n r 1 [START_REF] Hulot | Vapor emissions from contaminated soils into buildings : comparison between predictions from transport models and field measurements[END_REF], the modeling for trichlorethene and perchlorethene, underestimated the observed concentrations, with one to four order of magnitude (starting from soil and soil gas concentrations). But on this site, there were not so many points of measures, and indoor concentrations were close to quantification limits.

Sensitivity and uncertainty analysis -Parametric study

A sensitivity and uncertainty analysis was realized. Various -including significant-impacts could be obtained by modifying the parameter values within their respective possible ranges, in spite of an extensive site characterisation. The most sensitive and uncertain parameters were certain physicochemical parameters of soil and building characteristics. But they strongly depend on the model, which can be related to the model conceptualization. Uncertainties on such parameters as soil gas porosity, soil gas permeability, pressure difference between the indoor air and the soil gas, impacted the modelled concentrations with up to an order of magnitude. Several reasons can be considered to explain the variations between observed and modeling data, connected either to the input parameters of the models, among others the results of the field investigations and their uncertainties, or to the specific conceptualization of the transfer phenomena of each model. But no trend could be outlined according to the model, the typology of the site or the characteristics of studied substances.

As an example: the measured pressure differences between the indoor air and the soil gas varied between the investigation campaigns: 2 Pa in December, otherwise measured at 1 Pa (detection limit and precision of the device). The impact of this parameter is minor in the VOLASOIL-a modelling: diffusion is the main transfer mechanism in this modeling, due to the low gas permeability of the soil (and to the depth of the source under the slab; the slab is 130 times more permeable) 3 . But the impact of the same parameter is major with "J*E" (allowed by a permeable slab and a different representation of the diffusive soil air flux). For example, for naphthalene, if we choose 0.1 Pa instead of 1 Pa, the indoor air concentration modelled with "J*E" decreases by a factor 10 about.

Conclusion and recommendation

The modeling of a depleting source led to downsizing the vapour flux by one order of magnitude or more over one year in the experimental site. This illustrates the importance of this modeling. The mathematical developments of VOLASOIL presented here should allow a broader use of VOLASOIL, especially for a source next to the concrete slab. The comparisons between predicted and measured concentrations in indoor air showed on the real site over-prediction by the models and in some cases under-predictions by the models with one or two orders of magnitude. In spite of an extensive site characterisation, those differences, and the significant time and space variability in measured soil gas or indoor air concentrations could often not be explained. Significant impacts on the modelled concentrations could be obtained by modifying the parameter values within their respective possible ranges.

The study emphasizes the importance of a humble attitude towards the results of the modeling of vapour transfer, in the current state of model validation. Combining models and measurements is essential for a "robust" risk assessment. A comprehensive understanding of the model used, its concept and its equations, is essential. We recommend to re-code one's own tools so as to better test and understand the sensitivity of the model in different situations. Improvements in modeling and measuring vapour intrusion are still possible. When dealing with finite source modelling, the research should be extended to the impact of degradation in the vadose zone for benzene and e.g vinyl chloride as a source of variability and systematic differences. Detailed quantitative measurements and interpretation of the main site parameters appear essential when aiming at model verification. This is a difficult and time-consuming work, which has to be repeated with different types of sites (building designs, construction materials, soil) and at different periods under adequate conditions (heating, "normal" ventilation…). Collaborations are needed, at national and international scales, so as to gather strengths on data collection and interpretation. Such collaborations are initiated for example with the HERACLES initiative and with the French and European research projects FLUXOBAT and CITYCHLOR about chlorinated solvents pollution in soil and groundwater.

Figure 1 : 3 . 2 .

 132 Figure 1: Multilayer model of vapour intrusion (see § 3.2 for parameter definition)

Figure 1 :

 1 Figure 1 : Field investigations Flux measurements were realized at the slab surface, using two devices developed by INERIS:• The « accumulation chamber »[START_REF] Pokryszka | Method of measuring surface emissions of methane[END_REF] (Figure1-A): gases emitted from the site within an elementary surface (50 cm x 50 cm) are pumped, sent to an analyzer and reinjected into the chamber (H = 20 cm), creating a recirculating flow and then an accumulation in the chamber. The surface flux can be easily derived from the measurement of the atmosphere enrichment as a function of time. The flow of contaminant is expressed relatively to the standard gas chosen[START_REF] Pokryszka | Method of measuring surface emissions of methane[END_REF]. This measurement system is easy and rapid to operate (One measurement every 5 to 10 minutes, but not very precise and rather indicative (FID). • The "dynamic scanning chamber"[START_REF] Pokryszka | Developpement and validation of a method for measuring biogas emissions using a dynamic chamber[END_REF] displayed in Figure2-B is a chamber connected to an inert gas supply system and an exit with measurement points. The system gathers contaminant gas emitted from soil within the inert gas flow. The mixed gas sampled is analyzed. Therefore the measured

  001 (LQ) [nd-<LQ] < 0,001 (LQ) [nd-<LQ] 0,009 (b) [nd-0,009 ] (a) : depth: 1,8-3 m ; (b) : same measurement point for max of Benzene in December 2004 and max of Naphthalene in June 2005 ; nd : not detected ; QL : quantification limit of 0,001 mg/m 3 for naphthalene ;

Figure 2 :

 2 Figure 2: Comparison of modelled and measured indoor air concentrations (starting from soil concentrations)

  and the soil concentration C s in the source layer is connected to the soil gas concentration Csa 0 :

	Combining equation 4 and equation 5, we obtain :
	Equation 7 Using ( ) t L tot	=	L	10 tot + 3	SD ( ) t z o	( ) t , and then: ' z C o s =	J	tot K ( t tot ) tot L	= ( ) ( ) t t	1 =	exp     tot tot K -L +	tot tot DU ( L ) t ( ) t and ) t ( t ( ci ci F F ( ) o o K t z -	)	o ( ) Csa ( ) tot tot t DU     t L	=	tot tot DU L	+	( ) o DU o t z	, and with
	the expression of F ci (t) given in Equation 2, Equation 7 becomes a differential equation with no analytical solution (to our knowledge).
	Equation 6			CT s sa = Csa o / C s =	Kd	+	Vw /	SD H	+	H	Va	/	SD	, with :
	Parameter Volumic fraction of water/air in the soil (here in the source layer) Dry soil bulk density Soil water partitioning coefficient (here in the source layer) Total mass concentration of pollutant in the soil (reported to dry soil) (here in the source layer) Soil / soil-air partitioning coefficient (here in the source layer)	Symbol Vw/Va SD Kd Cs CT s sa	Unit l pw /l soil and l sa /l soil kg dm /l soil (mg/kg dm )/(mg/l w ) mg/kg dm (mg/l sa )/(mg/kg ms )

Jtot < Jconv + Jdiff[START_REF] Hazebrouck | Vapor emissions from contaminated soils into buildings : Developments of the volasoil model for multilayer soil[END_REF]: the pollutant transfer caused by the combination of the convection and the diffusion is lower than the addition of the convection and the diffusion considered separately. The over-estimation is lower than a factor two, and particularly low when the convection is largely dominant over diffusion.

On the contrary the soil considered for the modelling of the depleting source has permeability 100 times higher, allowing the convection to predominate over the diffusion (even for a deep source).

Acknowledgements

This work is part of a research-project financially supported by the French Ministry in charge of the Environment and by the French public Agency ADEME. We are also grateful to the site owners for their collaboration. The opinions expressed in this paper are those of the authors and do not necessary represent those of the funding organizations. We thank Frank Swartjes and Johannes Lizjen, from RIVM, for the long term cooperation on this topic, and Johannes Lizjen for his valuable review of this paper.


Yves Benoit

Laure Chancerelle

Alain Dumestre

Joanna Falcao

Françoise Fayolle-Guichard

Geneviève Grundmann

Martina Kyselková

Jeanne Morinière

Pascal Pandard

Hans H Richnow

Mònica Rosell

Physico-chemical and biological characterization of an aquifer polluted with ETBE

Keywords: Aquifer, pollution, hydrocarbons, ether, bioremediation, ecotoxicity, biodiversity

Introduction

Petroleum compounds and among them, gasoline, is the most massively used chemicals worldwide and, as a consequence gasoline derives compounds are the most frequently found contaminants in groundwater. Apart from the petroleum derived components in gasoline, chemicals are added to in order to meet specific requirements. Oxygenates such as methyl tert-butyl ether (MTBE) or ethyl tertbutyl (ETBE) were chosen because of their high octane index. Such water soluble compounds are typically detected in groundwater after a contamination by gasoline. The environmental impact of ether fuels on groundwater was not estimated prior to their utilization and the actual level of MTBE and ETBE contamination is not known in Europe even after several years of systematical use. Moreover, the knowledge on the ecology of MTBE-and ETBE-biodegradation is quite poor. Two directives have been adopted in the EU to promote the use of biofuels through tax incentives, including the addition of bioethanol to gasoline. Some European MTBE producers have taken advantage of the subsidies by converting their plants to produce ethyl tertiary-butyl ether (ETBE) using bioethanol. ETBE is presently the main way of the bioethanol utilization as an alternative to straight ethanol. According to Lyondell, in 2008 there were 60 ETBE production units in Europe (installed production capacity 5.75 Mt). ETBE is produced mainly in France, Spain and Poland. In 2008, 4.5% of the whole gasoline sold in Europe was containing ETBE and half of the ETBE produced was synthesized using bioethanol raw material. In Europe, the maximal authorized ETBE concentration in gasoline has now reached 22% (v/v). More specifically, in France 1259 millions of liters of ETBE were incorporated in gasoline in 2008, i.e. 10.3% of gasoline (v/v). One of the main properties of MTBE and ETBE is their high water solubility (40 and 10 g.L -1 , respectively). In Europe, detectable levels of MTBE have been reported in samples from rivers in Germany [START_REF] Achten | Determination of methyl tert-butyl ether in surface water by use of solidphase microextraction[END_REF], 2002a, 2002b) and [START_REF] Rosell I Linares | Vigilància ambiental de metil tert-butil eter (MTBE), un additiu de la gasolina[END_REF] reported several cases of MTBE-polluted freshwater in different European countries. No specific study was carried out concerning ETBE. When an MTBE-supplemented gasoline is spilled, the plumes generated by MTBE are much larger than those generated by benzene, the most soluble compound of gasoline (water solubility = 1.755 g.L -1

). This persistence in groundwater of MTBE is the consequence of i) its high solubility ii) its low adsorption on organic matter and iii) its recalcitrance to biodegradation. No data are available on the environmental fate of ETBE but its physico-chemical properties, very similar to those of MTBE, should lead to a similar environmental behaviour. The European Commission has completed risk assessments on MTBE and ETBE and concluded that there were no health concerns with these products "in the case of non-repeated swallowing, contact with the skin and inhalation" but nothing is known about the possible risk following a long-lasting exposition to low concentrations of MTBE or ETBE when aquifers are polluted by such compounds. Moreover, these two compounds have very low odour and taste thresholds in water (ETBE: 13 and 47 µg.L -1

, respectively and MTBE: 10 and 24 µg.L -1

, respectively) rendering water undrinkable when present at very low concentrations. The EU legislation in preparation should propose a limit of 20 µg.L -1 for MTBE and ETBE in water to ensure quality.

Presentation of the project

The 3-year long project TISATIE is granted by the International Competitiveness Cluster Lyon & Rhône-Alpes (Thematic: Chemicals/Environment). It is focused on a French ETBE/MTBE polluted site (gas station) and aimed: (1) at improving our knowledge on the environmental fate of ETBE and (2) at defining a cleaning process of the site following its characterization.

An integrative strategy involving different biological tools was used to characterize and monitor the site. It included: -The choice of the site and its physico-chemical and geological characterization. This required implementing analytical methods dedicated to the trace analysis of ETBE and MTBE and also of their common biodegradation product, tert-butyl alcohol (TBA).

-The evaluation of the environmental impact of the pollution was determined by two means. (i) Estimation of the ecotoxicity of water sampled on the site by using standardized methods measuring the effects of water on different biological systems. (ii) Characterization of the microbial diversity on different points of the site using a phylogenic DNA microarray.

-The determination of the ETBE, MTBE and BTEXs biodegradation capacities of the indigenous microflora. This part was carried out by classical cultivation methods using as inoculums water sampled on the site. We also used the compound-specific stable isotope analysis (CSIA) to determine if ETBE biodegradation has occurred in the site and in microcosm studies to elucidate the predominant reaction mechanism. Moreover, efficiency of biodegradation has been evaluated by performing the same ecotoxicological tests after biological treatment. The results obtained from these different studies allowed us determining which remediation process could be the most efficient on this site (biostimulation and/or bioaugmentation). Following pilot tests performed at laboratory scale, a process has been recently implemented on the site.

The different partners of the project are SERPOL (project coordinator), IFP (Biotechnology Department), Université Lyon1 (Microbial Ecology Laboratory, LEM). The ecotoxicological study was carried out by INERIS. The CSIA experiments were carried out at Helmholtz -Centre for Environmental Research-UFZ (Department of Isotope Biogeochemistry, Germany).

Analytical methods. -Choice and characterization of the site

The choice of the pilot site, a working gas station, was initiated by a study to determine the vulnerability, the presence of pollution by gasoline, ETBE and MTBE being essential for the final choice.

To carry out this study, different analytical methods (GC and LC) were used to analyze both the aquifer and the soil and characterize the pollution levels on different part of the site. We determined:

• the total and solubilized organic carbon in the aquifer (TOC, DOC) • the water soluble fuel ethers (MTBE, ETBE) and associated alcohol (TBA) concentrations • the total soluble hydrocarbons (total HC and BTEXs) • the electron acceptors • the presence of metals, pesticides and chlorinated compounds.

-Physico-chemical characterization of the aquifer:

The hydrogeology of the site was determined (piezometers) and physico-chemical parameters were scanned: pH, temperature, redox potential, dissolved oxygen, concentrations.

-Determination of the ecotoxicity of the aquifer

In order to complete chemical analysis approach, an ecotoxicological survey of groundwater has been organized on several piezometers. The following commonly used tests have been carried out, according to ISO standards *Determination of the inhibition of the mobility of Daphnia magna Straus. *Freshwater algal growth inhibition test with unicellular green algae (Pseudokirchnierella subcapitata) *Determination of the inhibitory effect of water samples on the light emission of Vibrio fischeri (luminescent bacteria test) Due to samples specificity (i.e. low dissolved oxygen concentration, fine particles in suspension, volatile compounds like BTEX, ETBE, MTBE), the test conditions have been adapted to reoxygenate samples and reduce volatilization (solution prepared in refrigerated vessels, tests performed in closed vessels).

-Determination of the biodegradation potential of the indigenous microorganisms.

The biodegradation capacity of ETBE, MTBE and BTEXs was estimated by using the indigenous microflorae after filtration (0.22 µ) as the inoculum (10%, v/v) in a mineral medium (MM). ETBE, MTBE and a mixture of BTEXs were added as the carbon and energy source (100 mg.L -1

). Cultivation was carried out in closed flasks under aerobic conditions. The oxygen available in the flasks being nonlimiting regarding the Theoretical Oxygen demand (Th.O.D.) calculated for the different substrates. The biodegradation is monitored by measuring the residual substrate concentrations in culture samples (triplicates) When biodegradation of a substrate was observed, the culture was re-inoculated in fresh MM in the presence of the substrate to confirm the biodegradation capacities.

-Microbial diversity determination * DNA was extracted from 1 litre water sample with Ultraclean Water DNA kit (MoBio Laboratories, Carlsbad, CA, USA). The universal eubacterial primers T7-pA and pH [START_REF] Sanguin | Development of a 16S rRNA microarray approach for the monitoring of rhizosphere Pseudomonas populations associated with the decline of take-all disease of wheat[END_REF] were used to amplify rrs from DNA extracts obtained from environmental samples. The PCR reaction mixture, in vitro transcription, RNA purification and hybridization were as described in [START_REF] Sanguin | Development of a 16S rRNA microarray approach for the monitoring of rhizosphere Pseudomonas populations associated with the decline of take-all disease of wheat[END_REF]. The slides were scanned at 532 nm with a 10 µm resolution, using a GeneTac LS IV scanner (Genomic Solutions, Huntingdon, UK). Images were analyzed with the GenePix 4.01 software (Axon, Union City, CA). Finally, a given feature probe was considered positive when (i) hybridization signals were superior to the mean signal of the negative controls and (ii) at least 3 of 4 replicate spots were hybridized. * Purified PCR products were cloned into the plasmid vector pGEM-T (pGEMs-T Easy Vector System kit; Promega, Charbonnieres, France) according to the manufacturer's protocol. Fifty clones were sequenced on both strands (CoGenics, Meylan, France).

-CSIA experiments Groundwater samples were taken in November 2009 from 12 wells along the contamination plume in order to compare their ETBE carbon and hydrogen isotopic signatures to the ones close to the source. Depending on the ETBE concentration, the samples were preserved with approximately 70 g of NaCl in 240-mL serum bottles for manual headspace injection (HS) or 1-L bottles preserved with 1% (w/w) trisodium phosphate dodecahydrate (Na 3 PO 4 •12H 2 O or TSP) for purge and trap (P&T) preconcentration step. Just prior injection into the gas chromatography-combustion-isotope ratio monitoring mass spectrometry systems (GC-C-IRM-MS, for details check [START_REF] Rosell | Variations in 13C/12C and D/H enrichment factors of aerobic bacterial fuel oxygenate degradation[END_REF], each sample for P&T was diluted in distilled water to a final concentration of around 100 µg/L in three 120-mL serum bottles. These bottles were placed in a modified AquaTek 70 Autosampler coupled to a Velocity XPT Accelerated Purge and Trap System (both from Teledyne Tekmar, Mason, OH, USA). This P&T system was only connected to the instrument measuring carbon isotope composition. In all cases, a DB-MTBE column (60 m length x 0.32 mm I.D. x 1.8 µm film thickness from Agilent Technologies, Waldbronn, Germany) was used. Carbon delta values obtained by the two methodologies were standardized by aqueous standard injections in order to correct isotopic fractionations caused by the extraction techniques [START_REF] Zwank | Compound-specific carbon isotope analysis of volatile organic compounds in the low-microgram per liter range[END_REF]. In parallel, the isotope enrichment factors of an ETBE degrading enrichment culture obtained from well PZ1 were evaluated with resting cells (optical density at 700 nm was 0.1) under different oxygen concentrations (oxic versus hypoxic) and incubation temperatures (12ºC vs. 30ºC). All the cultures were incubated in a mineral salt medium containing vitamins and ETBE as sole source of carbon and energy. The monitoring of oxygen concentration was made through a non-invasive technique which requires oxygen sensitive optode spots (POF-PSt3) glued inside of the serum bottles and a fiber optic device (FIBOX 3 trace, both from PreSens, Germany) connected to a computer, see more details in [START_REF] Rosell | Evaluation of the Effects of Low Oxygen Concentration on Stable Isotope Fractionation during Aerobic MTBE Biodegradation[END_REF].

Results and discussion

The results presented here were obtained mainly during the first two years of the project.

-Site description and characterization of the pollution

The site of the study was finally chosen among three possible sites located in the Rhône-Alpes district in France. The site is situated in the valley of a river which is a tributary of the river Rhône and located on alluvia composed of sandy gravel laying on a rocky substratum mainly composed of gneiss and mica. The water table has been detected at 5 m depth. The aquifer is flowing on top of this substratum through slimy ground, developing high water permeability. As a hydraulic point of view, the aquifer detected right below the site is connected to the alluvium nappe of the river. This river has been considered as vulnerable due to its proximity to the surface and to the vulnerability of the hydraulic connection with the alluvium nappe. Around the site, this potentially vulnerable aquifer is used for sensitive usage (such as agricultural irrigation) or for non sensitive applications (industrial wells). The pollutant characterization revealed a hydrocarbon pollution source mainly composed of light weathered gasoline with ether additives and trace of gasoil. Impacted waste fills and polluted interstitial air-ground were also detected in many other areas of the site (upstream and downstream the source area). Concerning the groundwater impact, a floating organic phase composed of (i) weakly degraded gasoline (dissolved BTEXs and alkanes) and (ii) gasoil has been detected at different depth levels of the aquifer. This organic phase is mainly composed of organic pollutant such as ETBE and BTEXs. The contamination plume observed on the site before treatment regarding ETBE (Fig. 1). Compared to fuel oxygenate ether's, the length of the BTEXs plume is smaller and the pollutions concentrations also. In complement, the water composition also revealed the presence of heavy soluble metals especially arsenic (50 µg/l in a well). The highest permissible concentration for drinking water is set at max. 10 µg.L -1 (Directive 98/83/CE -November 1998). In P2, the aquifer is strictly anoxic with a total depletion of electron acceptor (oxygen, nitrates & sulphates). The anoxic conditions are closely linked to the biological activity of autochthonous aerobic and anaerobic microorganisms. The soluble fraction of BTEXs is easily biodegraded under aerobic conditions leading to the anoxic conditions observed near the area source. Despite MTBE traces, the major pollutant is ETBE with concentrations up to 200 mg.L -1 and TBA, a potential by-product of both fuel oxygenates biodegradation. ETBE biodegradation capacities of indigenous microflorae have been demonstrated for two different water samples upstream and in the middle of the site. ETBE accumulation in the aquifer is the consequence of (i) its high water solubility and (ii) the total oxygen depletion.

-Toxicity of samples of the aquifer at different locations on the site Ecotoxicity test results show high toxicity level of water sampled in the pollution plum (PZ10, P2 and PZ15 piezometers), compared to PZ1 and PZ9 samples, located upward of the pollution plum (Fig. 2).

Figure 2. Ecotoxicity of the core and upward pollution plum samples (undiluted)

It can be highlighted that a same toxicity level is measured for P2 and PZ10 samples. These two piezometers are located in the core of pollution plum. The downstream sample PZ15 shows a lower toxicity (see EC 50 values in Table 1). These results can be linked with a lower ETBE concentration, but it should be noticed that other compounds likes BTEXs and metals have been detected in the site that can also contribute to the observed toxicity. Globally, this combination of test allows to clearly discriminating between polluted and unpolluted samples. These results show also that Daphnia magna and Vibrio fischeri are the most sensitive organisms as monitoring tools for this type of pollution.

-Biodegradation capacity of indigenous microflorae

The biodegradation capacity of the indigenous microorganisms sampled at two locations of the site (see Fig. 1), PZ1 upward of the pollution plum and P2 located in the plum, under aerobic conditions was determined. The biodegradation of BTEXs was estimated on a mixture of BTEXs (concentration of each target compound was 18 mg.L -1

) and the biodegradation percentage was calculated on the residual concentration measured in the control (non-inoculated). The results are shown in Table 2 for both microflora (PZ1 and P2). ) were determined after inoculating MM medium (triplicates). When degradation occurred, the culture was used as the inoculum for a secondary cultivation on MTBE or ETBE. The MTBE and ETBE biodegradation rates were calculated from a first addition of the substrates (Table 3). The biodegradation of ETBE was observed on both cases at similar rates whereas no MTBE biodegradation by P2 microflorae was observed. The ETBE biodegradation rate was superior to that of MTBE using the microflora of PZ1 (x14). The transient production of TBA was observed in the case of PZ1 after 3 successive additions. In the case of P2, the third addition of ETBE leads to the accumulation of TBA that was not further degraded. This shows that it is very important to monitor also the TBA concentration on sites polluted by ETBE or MTBE.

Table 3. Biodegradation capacities of ETBE and MTBE by indigenous microflora from two locations on the site (PZ1 and P2)

Biodegradation capacity of: Origin of the indigenous microflorae ETBE (

) Well PZ1

19.6 1.4 Well P2 24.7 No -Microbial diversity on the site and in the enrichments Enrichment batches were set in the laboratory using water samples from a polluted well (P2). ETBE, TBA or BTEXs were added to the water sample. After 4 week incubation, communities were analysed using cloning and sequencing method. In the three cases, when degrading activity was detected, there was a shift toward Beta Proteobacteria although communities were specific of each pollutant. This indicates the large initial diversity in this water system. There was a prevalence of Hydrogenophaga in the case of ETBE and TBA and a prevalence of Acidovorax in the case of BTEXs. Among Hydrogenaphaga genus H. flava is known for its capacity to degrade MTBE but it has never been mentioned for ETBE degradation. Acidovorax has never been mentioned to degrade BTEXs.

PCA analysis microarray data on water sampled in 9 wells at three dates (Fig. 3), separates three zones on the site corresponding to the center of the main pollution, upstream nd down stream of the pollution. The bacterial community down stream of the main pollution zone was positioned in between the center and the upstream zone. This variability covered, including the time effect, 25% of the total variability. The complexity of a natural site with multiple pollutions might be responsible for the large unexplained variability (75%). Specific ethb gene PCR (targeting 4 sequences among 6 available) indicated the presence of this gene in water wells where ETBE was present. ) covered by the application of two different injection techniques (HS and P&T), no significant carbon ETBE isotopic fractionation was detected when comparing 13 C along the contamination plume (average -27.3 ± 0.3‰). The difference of 13 C respect the ones obtained closer to the source rarely exceeded the total analytical uncertainty ± 0.5‰ [START_REF] Sherwood Lollar | An approach for assessing total instrumental uncertainty in compound-specific carbon isotope analysis: Implications for environmental remediation studies[END_REF]). The same was observed for hydrogen (mean 2 H = -200 ± 7‰), although the applied HS technique was only able to analysed samples with ETBE concentration higher than 1 mg.L -1 . However, the fact that ETBE isotope fractionation in the field was not detected is not necessarily related to a lack of biodegradation. As observed in the microcosms studies performed with a concentrate of cells obtained originally from well PZ1, the related ETBE degrading community produces a very low carbon and almost non detectable hydrogen fractionation of ETBE during its complete biodegradation (see Table 4). The ETBE carbon enrichment factors ( C ) obtained under different oxygen and temperature conditions did not vary significantly. Therefore, the ETBE degraders from this enrichment seem to follow the same enzymatic reaction mechanism under the studied conditions. Comparing to previous studies, the obtained C are similar to the ones observed for Aquincola tertiaricarbonis L108 and Rhodococcus ruber IFP2001. In both strains, the initial monooxygenase reaction attacking the ethoxy group of ETBE is catalyzed by a cytochrome P450-type enzyme (CYP249) encoded by the ethABCD genes which has been found to be highly conserved in other strains able to grow on ETBE [START_REF] Malandain | Cytochromes P450-mediated degradation of fuel oxygenates by environmental isolates[END_REF]. The low exhibited fractionation pattern discarded any implication of Pseudonocardia tetrahydrofuranoxydans K1 ( C = -1.7 and H = -73‰) attributed to the expression of a tetrahydrofuran monooxygenase. These results fitted perfectly with the detection of the ethB gene in the field site.

Conclusions

-Concerning the tools used to characterize the site, the results obtained since the beginning of the project revealed their complementarity: physico-chemical tools (pollutants quantification), microbial tools (evaluation of the biodegradation capacities), stable isotope tools (predominant enzymatic reaction mechanism), molecular biological tools (microbial ecology), and ecotoxicological tools (environmental impact evaluation).

-Concerning the site, its characterization revealed the high potential of water transfer upstream the site with the presence of BTEXs (37 mg.L -1 at PZ10) and huge ETBE concentrations (196 mg.L -1 at PZ10), demonstrating the great water mobility and the persistence of ethers in the aquifer. Aerobic biodegradation of pollutants near the hot spot creates local anoxic conditions, a depletion of the other electron acceptors and an acidification leading to a change of metals speciation and more specifically of arsenic (As 3+ ) solubility (50 µg.L -1

). -Our studies have shown the aerobic biodegradation capacities of indigenous microflora. The microflora present in the water sampled upstream the site or in the site had both the potential to degrade a mixture of BTEXs and ETBE. MTBE was degraded only by the microflora present in PZ1 (upstream the site).

-The microbial approach delineated communities corresponding to meaningful situations on the site. This could serve as a reference to follow community modifications after remediation or to identify similar polluted situations. Furthermore, using an enrichment procedure, a natural ETBE degradation potential was shown. The various dominant taxa encountered at close distances suggest the existence of gene transfer.

-The results also revealed ecotoxicological effects near the hot spot (P2 and PZ10) where the different pollutants are present (BTEXs, ETBE, MTBE, TBA and also As 3+ ) but also in the plume (at PZ15) where ETBE is the only organic compound detected emphasizing the possible ecotoxicological effect of this oxygenate.

Perspectives

Experimental laboratory biodegradation tests (6 litres bioreactors) will be done on polluted water samples withdrawn from an impacted area of the site (PZ10). Biostimulation (O 2 , nitrate, and Fe) of intrinsic biodegradation capacities of ethers and BTEXs will be compared to bioaugmentation with enriched autochthonous microorganisms. The objective is to recommend the best available biodegradation strategy and extrapolate a bio treatment at site scale during the second half of 2010. After complete biodegradation at pilot scale, the effect of the treatment on pollutants concentrations (EtBE, MtBE and BTEXs) will be quantified as well as the water ecotoxicity in order to evaluate the environmental impact reduction. The evolution of the microbial consortia will be also tested with the DNA array described previously.

Figure 1 .

 1 Figure 1. ETBE concentrations on the site Downstream, the environmental impact is focused in the direction of identified targets (agricultural irrigation wells). The concentrations curves of dissolved pollutants in the aquifer revealed a greater mobility of ethers (Figure 1) compared to BTEXs. (results non presented). The analysis performed on water samples pumped in the aquifer near the source (P2) revealed the following organic pollutants: Total dissolved organic content of 200 mg.L -1 , ETBE: 197 mg.L -1 , MTBE: 0.9 mg.L -1 , TBA: 4.2 mg.L -1 , BTEX: 3.2 mg.L -1 and Total soluble hydrocarbons: 11 mg.L -1

Figure 3 .

 3 Figure 3. PCA analysis of microarray data of water samples from 9 wells, at three sampling dates November 08, March09, April 09. Center (C), Downstream (D), Upstream (U).

Table 1 . EC 50 values on P2, PZ10 and PZ15 water samples

 1

	Ecotoxicity tests	P2	PZ10	PZ15
	Inhibition of light emission of	EC 50 : 17.3%	EC 50 : 17.8%	EC 50 > 80%
	V. fischeri (30 min)
	Inhibition of D.magna mobility	EC 50 : 16.0%	EC 50 : 12.2%	EC 50 > 100%
	(48 h)
	Inhibition of algal growth (72 h)	EC 50 : 54.7%	EC 50 : 70.8%	EC 50 > 98.7%

Table 2 . Biodegradation capacities of BTEXs by indigenous microflora from two locations on the site (PZ1 and P2) Biodegradation of BTEXs (%) BTEX provided in the mixture * Water sampled in well P2 Water sampled in well PZ1

 2

	Benzene	100%	100%
	Toluene	99.4%	98.9%
	Ethylbenzene	98.9%	97.2%
	m-Xylene	98.3%	98.3%
	p-Xylene	98.3%	95.5%
	o-Xylene	98.4%	95.6%
	Total BTEXs	98.9%	96.9%
	* No degradation was observed in the control
	The biodegradation capacity of the microflora towards ETBE and MTBE (100 mg.L -1

Table 4 . Comparison of carbon and hydrogen enrichment factors ( ) and values ( 2 H/ 13 C) for aerobic biodegradation of ETBE by enrichment culture from site PZ1 at different conditions and other strains reported in the literature. All the values have associated their ± 95% confidence intervals (CI)

 4

	Culture	C [‰ ] R	2	B [%] N H [‰ ]	Λ Λ Λ Λ	Reference
	Pure cultures
	Aquincola tertiaricarbonis L108 Rhodococcus ruber IFP2001 Pseudonocardia tetrahydrofuranoxydans K1	-0.8 ± 0.1 0.98 95 10 -11 ± 3 -0.8 ± 0.1 0.96 96 16 -11 ± 4 -1.7 ± 0.2 0.98 -73 ± 7	13 ± 1 10 ± 1 49 ± 4	Rosell et al. 2007 Rosell et al. 2007 McKelvie et al. 2009
	Enrichment from well PZ1
	Oxic at 30ºC	-0.4 ± 0.3 0.94 90 5 ns (-0.3)	na	This study
	Hypoxic at 30ºC	-0.7 ± 0.2 0.95 89 5 ns (+12)	na	This study
	Oxic at 12ºC	-0.5 ± 0.2 0.98 86 7 ns (+6)	na	This study
	Hypoxic at 12ºC	-0.6 ± 0.1 0.95 85 5 ns (+5)	na	This study
	N: number of data points; na: not applicable; ns: not significant, B: maximum percentage of ETBE biodegradation
	analysed

Acknowledgments

M. Rosell was supported by a Beatriu de Pinós postdoctoral grant (2008 BP-A 00054) from the Autonomous Government of Catalonia, Spain (Agència de Gestió d'Ajuts Universitaris i de Recerca, AGAUR).


Christophe Didier
email: isrm.christophe.didier@ineris.fr

Jean-Marc Watelet

IMPACT OF CLIMATE CHANGE ON THE STABILITY OF SHALLOW MINES

Keywords: climate change, mining, cavity, collapse, water table, chalk, flood

The present paper deals with climate change impact on stability of underground cavities due to its indirect effect on ground water levels. In France, several back-analyses have confirmed that variations of underground water tables play a key role in the genesis of surface disorders above shallow mine workings. The paper presents the forecast impacts of climate change on the behaviour of underground water tables as well as the consequences of those variations on the stability of disused underground cavities. A particular collapse that affected the Paris Basin during a major flood of the Seine River, one century ago (January 1910), with dramatic consequences, illustrates the importance of water variation impacts on the rock mass behaviour. The high sensitivity of the extracted material (very pure chalk) to water as well as the dynamics of the mine flooding process are described. The most realistic mechanisms and scenario enabling to explain massive failure of the underground workings are then discussed.

INTRODUCTION

The existence of a global warming process of the global climate is now well admitted by the scientific community. The rise of the average temperatures of air and oceans, as well as the observation of the de-icing of ice caps and the rise of mean sea level constitute some clear indicators. The various studied prospective scenarios, depending on the efficiency of prevention policies engaged by governments show that, up to 2100, the average rise in temperature on the surface of the Earth could vary between 1,5 and 6 °C [START_REF]Synthesis on Climate Change[END_REF]. Such a temperature increase would be likely to generate serious disturbances on climatic parameters (e.g. modification of rainfalls, disturbances of water tables). Numerous international studies are being performed in order to make progress in the understanding and prediction of the consequences of the presumed climate change. Among these studies, very few relate to the matter of underground cavities.

The paper deals with the impact of climate change on stability of underground cavities. This topic may become a serious problem in France due to the existence of very numerous (hundred of thousands) shallow underground cavities, natural or manmade, well known or totally unlocated, undermining French territory. A large majority of those shallow workings appears to be very sensitive to the variations of their environmental parameters. Several recent examples have confirmed that, among those, the level and variations of underground water tables have a key role in the genesis of surface disturbances. Major trends concerning the forecast impacts of climate change on underground water tables as well as the consequences of those variations on the mechanical behaviour of shallow underground workings (impacts on strata and discontinuities) are thus briefly presented.

Among others, the specific case of a ground collapse (with dramatic human consequences: 7 deaths and 7 severe injured persons) that affected the Paris Basin during a major flood of the Seine River, one century ago is then described, as a perfect illustration of the importance of water variation impacts on rock mass behaviour. The chalk mine of Lorroy (city of Château-Landon), collapsed in January 1910, in direct connection with the flood of the Seine River. The course of events described by the witnesses clearly highlights the role of water table rising on the collapse triggering. As a matter of fact, sudden and massive water outflows are described inside the galleries (hitherto dry) during the morning of the catastrophe.

In addition to a brief description of the chalk mine context, the high sensitivity of the extracted material (a very pure chalk) to water as well as the dynamics of the mine flooding process are described further. The most realistic scenario enabling to explain massive failure of the underground workings is then discussed, according to some hypothesis due to the lack of data.

FORECAST IMPACT OF CLIMATE CHANGE ON UNDERGROUND CAVITIES

Impact of climate change on water table behaviour

A recent report of the International Panel on Climate Change (IPCC) points out that the level of many aquifers in the world tends to decrease during these last years [START_REF]IPPC Technical Paper VI: Climate Change and Water[END_REF]. This usually appears however as a consequence of overexploitation of the resource much more than of climate change. Because of the lack of available data and of the high inertia of the water tables' behaviour, significant modification of the refill of the aquifers related to climate is not yet documented. However, the IPCC insists on the lack of research carried out on the subject and thus on the necessity to develop urgently scientific knowledge on this topic.

The complexity of the subject has strongly contributed to the very limited number of reference papers available. Gradually however, taking advantage of the improvement of predictive models, a convergence of experts' opinion seems to emerge. According to it, an increase of the magnitude of water level variations should be observed in a close future as a consequence of the predictable modification of rainfall regime. As a matter of fact, in European countries, climate experts forecast an increase of rainfall amount during winter time (more important refill of water tables inducing water level rising) and, on the contrary, a severe reduction during summer periods (lack of refill inducing water level lowering). This general tendency needs to be considered with great care. The regional variability of both climatic conditions and water table behaviour may strongly influence the hydraulic mechanism. For example, a water table subject to a Mediterranean climate can react very differently of another one located north of the Loire River, under an oceanic climate. In addition, Nastev and al. specify that an increase in the frequency of extreme rainfalls would not result automatically in an increase in water tables refill. Such events could rather enhance run-off contribution and thus contribute to catastrophic floods [START_REF] Ippc Nastev | Hydrogeology and numerical simulation of the regional groundwater flow of the St. Laurence Lowlands of South-Westren Quebec[END_REF].

In a given climate change context, the probable amplification of summer droughts should increase the needs for underground water exploitation, in particular in the areas of intensive irrigation. This may amplify the magnitude of water level variations resulting from natural climatic conditions.

Impact of water on underground cavities

Water plays a key role in the behaviour and stability of underground mine workings. In addition to the dissolution of some minerals, humidification or drainage of rock mass (and especially successive alternations between these two states) can have important mechanical impacts. Interaction between strata and water appears to be a very complex problem, especially because water impact results from a combination of various effects, some of them resolutely damaging stability and others being, on the contrary, improving it in some specific contexts.

Taking into consideration experience feedback, most rock mechanics experts agree to consider, in a purely observational sense rather than in theory, that modification of the hydraulic context (especially flooding or dewatering of underground cavities) is likely to increase probability of mine workings failure in the short term. The disturbance (whatever its origin) of an "unstable equilibrium" established within disused underground workings often contributes to failure development. As a consequence, mining areas subject to severe "water level variations" constitute a very unfavourable context regarding ground stability.

Many examples described in the international literature confirm this by reporting development of ground instabilities in the weeks (or months) succeeding flooding or dewatering of cavities. One will take care not to conclude that, in the absence of water level variations, these areas would remain stable indefinitely. It is much more appropriate to consider the influence of water variations as a "worsening or triggering factor", likely to initiate a failure process development in a potentially unstable rock mass [START_REF] Hawkins | Sensitivity of sandstone strength and deformability to changes in moisture content[END_REF]. Generally, such a ruin is, as a matter of fact, irretrievable in the long term due to mechanical considerations. Water level fluctuations are thus more likely to anticipate failure process than to initiate it by its own. Those variations will mainly affect, in the short term at least, the shallowest water tables, most rapidly influenced by rainfall regime. It is thus the shallowest cavities that should be seriously affected by climate change. In France, these are mainly the mines having extracted raw building material (e.g. chalk, gypsum, limestone), the underground refuges (notably war saps) as well as the very numerous (and often very old) mine workings that were developed near the outcrops. These kinds of exploitations were generally carried out right above the natural water level. One will exclude on the other hand many mining areas, which were excavated deep enough not to be affected by some minor water level fluctuation in a close future.

Effect of water on mine workings strength

As mentioned above, impact of water on the rock mass strength results from a combination of several effects. In addition to the dissolution of specific minerals (e.g. salt, gypsum, chalk…), humidification or drainage of rock mass (and even more an alternative succession of both) makes it possible to explain, under certain conditions, some strength reduction of mine workings.

One may consider at first approximation that rocks presenting high values of porosity and permeability are more predisposed than others to be affected by water variations. Thus, except for the most impermeable rocks (e.g. some granites and quarzites) not very sensitive to the hydric variations or, on the contrary, strata presenting an extremely sensitive behaviour to water content (chalk, clays, some marls), most rocks lose between 30% and 50% of their strength when saturated compared to dry (figure 1). It is however important to remember that strata hosting the cavities present already high water content values (often close to 80% according to our experience). This pre-existing high water content value results from water infiltration from surface, capillarity forces which contribute to drain deeper water as well as relatively high ambient hygrometric atmosphere, specially when it is not renewed any more by ventilation after mine closure. The phenomenology of such a strength reduction is only partially understood, in particular in its quantitative dimension. Among the various mechanisms interacting, few of them are rapidly quoted below:

Effects of water on discontinuities

Strata are generally fissured or cracked mediums. Fluids circulate within these discontinuities (faults, fractures, bedding planes). Those flows are determined by complex hydro-mechanical coupled processes, depending in particular on the opening and the surface roughness of discontinuities as well as on the flow and velocity of fluids. The possible effects in terms of opening or closing of discontinuities can have direct impact on the rock mass stability. The relatively frequent context of a fracture or a fault filled with clayish material, for example, may be taken into consideration. The "lubrication effect" due to flooding may be sufficient to initiate failure of a rock structure already close to instability.

Suction effect

The "suction effect" describes the effect of the capillarity forces being opposed to the extrusion of the water contained in a rock sample in contact with a medium presenting a lower value of water content than the sample itself (the surrounding mining atmosphere for example). For rocks like partially saturated chalk, this suction effect (generated by desaturation process) can provide to the rock an "apparent cohesion" which contributes, due to the generated containment, to increase the shear strength of the rock (or compressive strength if there is no containment). In case of a progressive flooding of the galleries, this suction effect decreases until being totally cancelled (saturation state) with, in certain circumstances, significant effects on the mine workings behaviour (strength reduction). The consequences are even more important when the rock structure is subject to successive cycles of saturation-desaturation (as a result of seasonal variations of water tables levels). In this case, cumulating suction stress variations can generate "rock fatigue", likely to ease, or even initiate, the propagation of macro-and micro-cracks networks affecting the rock mass.

Effective stress effect

The concept of effective stress consists in retracting the fluid pressure within the pores to the total stress value. It can be expressed, into simplified terms, as the stress developed within the "rock skeleton" alone. In a saturated medium (specially in flooded mines), the effective stress is lower than the total stress. At first sight, this "discharge effect" may contribute to decrease the vertical compressive stresses transmitted by the overburden weight and to the rock skeleton. However, in rock mechanics, the concept of failure is more connected to the concept of "stress deviator" (difference between major and minor stresses) that to the only compressive stress. Indeed, the rock strength is strongly dependant of a potential "containment stress" which contributes to reduce the lateral distortion of a rock sample. If the "discharge effect" due to mine flooding contributes to decrease the vertical stress that develops within the rock skeleton, it also reduces the horizontal stress guaranteeing a containment stress of the pillar cores. The combined reduction of these two stresses does not always ensure more stability to the rock mass. On the contrary, in certain contexts it can lead the rock structure closer to its failure threshold.

Physicochemical effects

Some types of rocks may undergo physicochemical reactions when they are in contact with water or drained. Some of them may expand or shrink with very important magnitude (montmorillonite) or appear sensitive to splitting (illite, kaolinite). In general, clayey minerals present in the marly horizons appear to be the most sensitive to this kind of phenomena, which is mainly due to the structure of clayed minerals in layers of very small dimensions and to the development of electrostatic forces between the surface of those layers and the ions in solution present in underground water. It will not be drawn categorical conclusions concerning these phenomena, certain argillaceous or marly formations being very sensitive to water whereas others not at all.

AVAILABLE EXPERIENCE FEEDBACK

Today, the scientific community does not have to its disposal well documented databases enabling to characterise the impact of climate change on water tables behaviour and, even less, on its indirect impact on stability of underground cavities. The available experience feedback of impact of water variations on rock mass stability has thus to be found in circumstances having led, for any other reasons than global climate change, to important variations of water tables levels that have generated disturbances on ground surface. Three French examples are provided below, the last one being described in a more detailed way.

Flooding of the Lorraine Iron Field

INERIS has been strongly involved in the analysis and the monitoring of the flooding process of disused Lorraine iron mines located North-East of France [START_REF] Didier | Post-Mining Management in France: Situation and Perspectives[END_REF]. Several collapses occurred within the iron field during (or shortly after) the flooding of the "Northern" and "Centre" under-basins (many families living in the towns of Auboué and Moutiers for example had to be evacuated) that followed the stopping of the dewatering system resulting from mine closure. To better understand the phenomena concerned and to be able to predict more accurately the consequences of the future flooding of the third sub-basin, various studies had been carried out by the teams of the Scientific Network on Post Mining [8]. These studies related both to laboratory tests and in situ experimentations (artificial flooding of two pillars in the Tressange mine). On this basis, numerical modelling was developed in order to reproduce accurately the observed phenomena and mechanisms. Some of the principal results highlighted are presented here: the compressive strength reduction of the iron ore according to its water content is close to 50% for a water content increasing from 80% to 100% (sample becoming totally flooded); several creep tests under variable load and controlled hygrometry highlighted significant drops of elastic modulus and an increase of the strain values measured with water content increasing from 90% to 100%. This can be interpreted as an increase of irreversible strains and a stronger tendency to the long term failure of the sample. The use of acoustic sensors during these creep tests clearly highlighted the increase of acoustic emissions (development of microscopic cracks reflecting damage development) as soon as the sample begins to be flooded (figure 2); the ambitious in situ full scale experiments (flooding of two mine pillars during a few months) enabled to highlight, through tomography tests notably, that the "fringe" of weathered rock at the periphery of the pillars was larger at the end of the experiment than before flooding; the weathering effect induced by flooding has not been spatially homogeneous. It appeared to be much higher in sectors where strata were already damaged (important density of cracks) or included minerals very sensitive to water (clay, marls).

Impacts of water tables elevation in Picardy during spring 2001

Several towns of the Picardy Plateau (northern part of France) were severely affected during spring 2001 by numerous collapses of buildings undermined by shallow cavities mainly located in chalk (figure 3). These ground disturbances were due to the loss of strength of the rock structures (roof, pillars) that occurred at the time of important rainy events having induced an exceptional elevation of water tables. If, in the long term, the failure of these very ancient cavities, unknown for most of them and badly maintained for the others, was probably inevitable because of the bad condition of the mine workings, degradations and failures were, for sure, accelerated by water intrusion within the cavities.

Year 2000 was characterized by an exceptionally significant rainfall recording (about 950 mm of water, for an annual average of about 600 mm usually). Those rainfalls strongly contributed to supply the water tables and, in particular, the one located in the chalk bed. The average piezometric level, usually located between -15 and -40m below surface, brutally increased during spring 2001 (-5m below surface in average), inducing severe floods in the topographic depressions. This water table increase generated the outbreak of many surface collapses in numerous regions located North of France (Artois, Picardy, Normandy). One will quote for example:

• sudden flooding (more than 10 m of water table increase within a few days) of cellars making them inaccessible (figure 3 Beginning of flooding

Collapse of the Lorroy chalk mine

Context

Sometimes, interactions between climatic phenomena and underground instability are occulted by repercussions of those exceptional weather phenomena and their consequences on a region or a even whole country. As an example, at the time of the 1910 catastrophic floods which affected severely Paris and its region, the chalk mine Lorroy (city of Château-Landon) (77) collapsed as a direct consequence of the flood of the Loing River (affluent of the Seine River). In spite of the dramatic consequences of the event (7 people killed and 7 were severely injured), this catastrophe is very poorly known because it occurred during the flood of the Seine River which severely affected Paris and its surroundings and monopolised at length the interest of the media.

The Lorroy site is located about 50 km south of Paris, along the Loing River that notches a limestone plateau causing a steep hillside of about 40m high. Extraction activity began in the area around 1850 with galleries dug in the slope. The very pure and white chalk was initially extracted by peak, then by explosive. The mines were exploited by "arched form galleries" of 3 to 5 m of opening (figure 4). The pillars sections were globally increased with deepening of the mine workings but their fitting and their geometry were designed at random. Overburden thickness varies between a few meters near the outcrop to about thirty meters in the deepest sectors. In 1910, 6 underground chalk mines were registered over the city territory. Several were already collapsed, one of them, in 1897, already during a previous flood of the Loing River. Part of the still existing mine was not any more object of active extraction, most of those galleries being then reconverted for mushroom cultivation. Thus, on the 1910 collapse site, the active extraction galleries (called "Beaulieu") were located south (figure 6), next to northern galleries used for mushroom (called "Guillon").

The disaster and the investigation

The catastrophe occurred on Friday January 21 th , about lunch time. It was characterised by the sudden collapse of the slope combined with an associate major slide of the ground located at the slope base (figure 5). The landslide destroyed 3 houses built along the Loing channel and caused death by burial of 7 people. It is difficult to evaluate precisely the volume of material moved (probably over 100.000 m 3 ). The collapse of the rock mass extends on about one hundred metres long and 40 metres wide. The debris accumulation filled the channel and broke the closest lock as well as a dam. Several months were necessary to clear the site. Rivers. On the Thursday, the day before the accident, the flood had reached its local maximum but no water had yet entered the mine. The testimony of the mushroom grower exploiting the northern galleries of the collapsed mine is, concerning this subject, of first interest. He specified formally that no trace of water penetration was yet visible underground on Thursday 20 th evening. But, the morning after, day of the accident, about 9:00 (that is to say three hours before the catastrophe), at the time of its visit of underground workings, he declared that "water was invading the galleries on 20 to 40 centimetres high". It seems that water entered the mine through old shafts dug for water supply (usually located about 10 metres lower) used for washing the extracted chalk before its conditioning (figure 6). The main conclusions drawn from the investigation clearly highlighted the origin of the catastrophe to the collapse of the disused mine workings located north of the chalk mine. The initial failure of this "small pillars sector" seems to have been initiated by the flooding of the galleries basis. Then, due to dynamic shock, this triggering collapse would have propagated to the other parts of the exploitation even though, according to the mine operator, the southern galleries were not flooded the morning of the collapse. The global subsidence of the rock mass would have then generated a concomitant slide of the hill towards the channel.

Several compressive strength tests were carried out on cubic samples taken in the nearby mines. They confirmed the very sensitive behaviour of this chalk to water. The values of compressive strength, in totally saturated conditions, appeared to be extremely low with an average of about 0.5 MPa for an average value of 1.5 MPa for usual water content values (80% corresponding to not submerged galleries). The propensity of the chalk "to lose any consistency when it has been submerged for a few hours" is clearly underlined in the report.

Some calculations of the apparent stress based on the tributary area method based on the available indicated vertical stresses developing in the pillars varying from 0.9 to 1.3 MPa depending on the size of the pillars as well as the thickness of the overburden. These results consolidated the assumption of a mine collapse due to a sudden drop of rock mass strength generated by the water intrusion within the mine.

Remaining questions

Despite the seriousness of the investigation performed, several questions remain today unanswered. INERIS who fortuitously discovered this unique case study will undertake in the following months a research program to better understand some of those "shadow zones". First of all, looking carefully at the mining map, if a collapse of the old highly extracted and moreover flooded sectors appears quite easy to understand, it is more difficult to explain the extension of the failure to much larger pillars located southern. The concept of "barrier pillar" needs to be studied in this context, specially considering the width/height ratio of the pillars. Was the dynamic wave generated by the collapse of the northern part sufficient to enable propagation of the instability or was there a more complex mechanism, including for example shearing stress due to the hillside proximity?

It will also be of great interest to analyse precisely the sequence of the catastrophe and particularly the overlapping between cavity failure and landslide triggering. As rock mechanics experts, it does not appear evident to us that the potential failure scenario considered would have be as catastrophic as it really was. Necessary coupling processes between rock mechanics and soil mechanics as well as between geotechnics and hydrogeology will be of first interest. This could be useful to make progress in our ability to determine the way to deal properly with the necessity to predict safely, but reasonably, the concept of "over-accident".

The existence of remaining accessible mine in the surroundings will enable INERIS to carry out some new characterisation tests to precise the constitutive law governing this material. Some monitoring equipment will also be implemented underground to characterise the evolution of the geotechnical and environmental parameters within the mine. According to the results, numerical modelling will be carried out to better understand the sequence of events that lead to the 1910 catastrophe but also to identify if the still existing mines remain sensitive to major hazardous events or no.

CONCLUSIONS AND PERSPECTIVES

The various scenarios of climate change for the 21st century envisage, for France, a serious modification of the rainfall regime likely to generate increased fluctuations of water tables. The forecast increase of water tables level variations seems to be the major parameter likely to influence the behaviour of the underground cavities.

Important scientific gaps remain, concerning, for example, the mechanical impact of seasonal water level fluctuations on potentially unstable underground cavities but also the impact on rock mass of potential physicochemical modifications of water seeping from the surface.

In order to better appreciate this impact, INERIS chose to carry out a dedicated research program that will contribute notably to monitor several cavities, located in various geological and climatic contexts, for which water plays a key role. Some of them are located in chalk, others in gypsum and a last one in limestone. The monitoring devices previously developed by INERIS will feed the available database. Today such data concerning impact of climate change on water table behaviour and consequently on underground cavity stability are greatly lacking.

This experimental program, coupled with case studies analysis (like the one of Château-Landon), will have a greater interest if same initiatives are taken in the coming years in other countries to enlarge the available database to numerous contexts. INERIS would be pleased to take part of an international network of organisms interested to develop knowledge and research initiatives concerning interaction between climate change and rock mechanics, not only dealing with underground cavities but also landslides and rock falls.

Figure 1 :

 1 Figure 1: Compressive strength variation depending on water content [5] & [6]

Figure 2 :

 2 Figure 2: Evolution of the acoustic emissions during creep test with controlled hygrometry [8].

  .1); • occurence of numerous sinkholes on surface, some of them under houses built more than 100 years ago (figures 3.2 et 3.3). Some sinkholes were due to backfill run-out (figure 3.4) in old chalk mine shafts abandoned several centuries ago (extraction of chalk by local farmers for fertilizing acid soils). • the number of rescue interventions performed by the authorities were multiplied by ten to fifteen during the crisis period of spring 2001 compared with a "normal" activity period.

Figure 3 :

 3 Figure 3: (1) Flooded cellar; (2 & 3) Collapses of buildings due to undermining cavity failure;(4) Backfill run out of an old chalk mine.

Figure 4 :

 4 Figure 4: Some views of a remaining chalk mines in Lorroy, close to the one collapsed in 1910.

Figure 5 :

 5 Figure 5: View of the Lorroy site after the disaster of the chalk mine collapse. An investigation was quickly undertaken by the Mining Authority to identify the reasons of the catastrophe. It made it possible to establish that the collapse occurred right at the peak of the flood of both Seine and Loing

Figure 6 :

 6 Figure 6: Schematic view of the collapse and associate landslide of the Lorroy hillside on January 21 th , 1910.