Jacques Mudry
email: jacques.mudry@univ-fcomte.fr

Hydrochemistry, a tool for understanding karst groundwater flows

Keywords:

Karst is a highly heterogeneous medium (i.e. properties vary from one point to another one), and can be studied by variations of water chemical composition, which reflect in space and time differences in origin (runoff on impervious media, diffuse seepage, inflows from evaporites…) or residence time throughout the various aquifer compartments (soil, epikarst, vadose and saturated zones. This natural background is supplemented by anthropogenic inputs (domestic, urban or industrial wastes, leachates from agricultural soils and livestock byproducts...). Any natural (cave, chiasm) or artificial (tunnel, mine) penetrable gallery is a means of observation and sampling, at least of infiltration water, and sometimes of water from the saturated zone. Several cave sites in Europe provide examples for understanding the behavior of karst flows, both through the vadose and the saturated zones.

As the easiest access point to permanent flows throughout the whole karst system was the spring, which is the outlet of the saturated zone, the whole karst behavior (volume of reserves, regulation of the dry season's discharge…) has been assigned for decades to the saturated zone. Speleological observations joint to examination of man-constructed tunnels, enabled to observe, to sample and even to monitor groundwater flows, either in the vadose or in the saturated zones. Artificial galleries (Fig. 1A) display the advantage in a random sampling of water flows without any bias due to the drainage pattern of cave network structure. Conversely, a cave (Fig. 1B) provides a 'natural' sampling pattern in a hierarchical conduit network. This means that the cave, being an active conduit (or at least a palaeoconduit), is more subjected to enable sampling flows from different permeability contexts, whilst tunnels are devoid of organized inner structure, and may miss for instance major fractures generally well connected to cave conduits or pathways. Nevertheless, both types of underground voids are complementary and enable studying flows in the karst aquifer.

-Flow types

Depending on the gallery level and according to the piezometric groundwater level, different flow components can be observed in caves:

 'vauclusian' deep flooded shafts typically drain the saturated zone, below the base level. They can be considered as representative of flow through the deep compartment of the karst aquifer, as much in terms of hydrodynamics than in terms of physical, chemical and isotopic contents. But of course, this terminal conduit is the unique collector that drains all flows merging towards the spring. This conduit does not drain only 'old' or 'pre-event' water during low water periods, but also a mixture between 'pre-event' and 'event' waters during flood periods. This does not mean that all water flowing in low waters can originate only from the low permeability blocks of the saturated zone, a part of this 'reserve' water being permanently in transit through the vadose zone;  in the case of 'jurassian' springs, cave networks develop within the fluctuation zone that separates vadose from saturated zones. This type of cave is very interesting, enabling to observe both the saturated and unsaturated flows. Generally, the main underground brook at the floor of the cave or in the deepest part of the cave, displays the same mixture possibilities with infiltration water as the former case. But this cave type enables also sampling dripping water, from lateral fractures (or at roof), and from speleothems;  caves entirely dug within the unsaturated karst can sample only this second flow type, except in the case of hydraulic loading episodes, when flood water is evacuated via dry caverns situated several, and sometimes even several tenths of meters above the spring level;  very shallow caves allow to encounter flows draining the epikarstic aquifer.

According to their situation vs. groundwater level, caves enable to sample saturated flows, vadose flows or both, the nature of the unit drained being under control of the hydrometeorological events: water inflows through the unsaturated zone sample always infiltration fluxes (more or less rapid), whilst water conduits through or at the surface of the saturated zone can convey either reserve water, from the saturated zone or not, or a mixture of preevent and event water in flood periods.

-Interest of hydrochemical methods

Along its cycle, water passes through different media (Fig. 2, from [START_REF] Hunkeler | -Hydrochemical methods[END_REF]. From atmosphere where rainfalls leach marine and continental aerosols, through soil where it leaches CO 2 and organic acids, and then through epikarst where carbonic acid is neutralized by carbonate dissolution, and below, through vadose and finally saturated zones where minerals dissolve with a slower kinetics, water acquires a chemical composition enabling to use it as a an environmental tracing. Environmental, also called natural tracing is based on interpretation of contrasts (in space), and/or variations (in time) of physical (temperature, turbidity, electrical conductivity, pH, Redox potential…), chemical (major and minor ions, natural organic matter…), isotopic (oxygen, hydrogen, carbon, nitrogen, sulfur, strontium…) or biological (bacteria, viruses, protozoa…) composition of water. Water content carries a composite message which reflects its story through the different media that interpretation and deconvolution assignable to a compartment of the hydrosystem.

4 -Case study of the Sierra del Rey-Los Tajos karst aquifer (Southern Spain, Fig. 3) Each sector of the Sierra del Rey-Los Tajos aquifer has a different pattern of hydrological functioning. Los Tajos sector presents a high degree of functional karstification in its unsaturated zone, as shown by the variability of hydrochemical responses of the overflow springs when precipitation events occur (Fig. 4, [START_REF] Mudarra | Monitoring groundwater in a discharge area of complex karst aquifer to assess the role of saturated and unsaturated zones[END_REF][START_REF] Mudarra M - | Importancia relativa de la zona no saturada y zona saturada en el funcionamiento hidrogeologico de los acuıferos carbonaticos[END_REF]. The karst draining here favors rapid infiltration and the flow of rainwater toward the springs, which provokes a rapid decrease in electrical conductivity. On the contrary, the main Auta spring, which is responsible for draining the saturated zone, responds to rainfall more slowly and with a time lag. This is due to the existence of a low degree of functional karstification in the saturated zone, with a slow drainage and a high natural regulation capacity. Both the saturated and unsaturated zones participate in the hydrological functioning of the Los Tajos system, but almost totally independently of each other. On the other hand, the Sierra del Rey springs respond to precipitation with rapid, severe hydrochemical variations, within 1 day. In this case, the springs drain a sector of the aquifer with a high degree of functional karstification, both in the saturated and in the unsaturated zones, and both are of similar participation in the hydrogeological functioning of the system.

-Case studies of accesses to the unsaturated zone: Nerja cave (Southern Spain) and LSBB gallery (Southern France)

Nerja cave is a shallow cave situated in the South-Spanish Mediterranean coast. Completely ubicated in the epikarst, this cave is a privileged site for sampling infiltration water below speleothems. Despite the thin limestone cover (only 35 m), a significant lag (Fig. 5) is observed after the rainfall: 8 months in normal period, falling to 2 months in exceptionally rainy period. The LSBB is a previous military 4-km gallery intersecting vadose vertical water flows below the Vaucluse plateau. Several water inflows are intersected and enable sampling [START_REF] Emblanch | -Utilisation du magnésium et du carbone organique total dans l'estimation du volume d'eau écoulé avant l'arrivée des eaux d'infiltration "directe" en période de crue[END_REF]. Figure 6 displays the variability of magnesium, indicating residence time in this vadose zone, and Total Organic Carbon, characteristic of short transits through the unsaturated zone. Indeed, magnesium having slow dissolution kinetics separates water fallen several months ago (springs A and B) from water fallen several hours or days before (springs C, D and GAS). And Organic Carbon separates water infiltrated a very short time ago (springs C and GAS).

-Case study of the En Versenne-Fourbanne karst network (Eastern France)

The Fourbanne karst system [START_REF] Charmoille | Traçage hydrochimique des interactions hydrauliques et mécaniques entre les volumes perméables et peu perméables au sein des aquifères fracturés carbonatés[END_REF][START_REF] Mangan | Hydrogeology : Objectives, Methods, Applications[END_REF]) includes (Fig. 7A) the Verne and Luxiol swallow holes, diffuse infiltration, and the passageways of the Fontenotte cave, which plunge into the saturated zone and lead to the karstic Fourbanne spring. The water's path was traced using fluorescein at the Verne loss, and this traced water has been sampled in Verne, Fontenotte, and Fourbanne. Nitrate concentrations of the traced mass appear in Figure 7B. Verne and Fontenotte display the same nitrate concentrations (as well as magnesium concentrations), showing the absence of mixing of other water with the water from the swallow holes (pinpoint infiltration), between the swallow holes and the underground river. At the Fourbanne spring, on the other hand, this localized infiltrated water is mixed with water of different composition (diffuse infiltration and/or saturated zone), with a higher magnesium concentration -larger residence time in the same limestone -and a lower nitrate concentration, less influenced by leaching from agricultural land (Fig. 7B).

-Conclusion

Underground cavities, cave networks as well as man-constructed tunnels are exceptional structures enabling sampling of water inflows. Understanding the karst behavior requires the knowledge of interaction between rapid flow in conduits and slow flow in microfissured blocks. Hydrochemistry appears as a very powerful tool to separate these origins. Hydrogeologists and cavers must work together in this task in the future.

Figure 1 :

 1 Figure 1: natural vs man-constructed gallery A: Example of natural cave network: the Gour cave, Jura Mountains, Eastern France(after CDS 25, 1991)

Figure 4 :

 4 Figure 4: Box plots showing mean, maximum, minimum, percentiles 05, 25, 75 and 95 for electrical conductivity (EC), Temperature, Alkalinity, Mg 2+ , pCO 2 , TOC and NO 3-for each spring[START_REF] Mudarra | Monitoring groundwater in a discharge area of complex karst aquifer to assess the role of saturated and unsaturated zones[END_REF]

Figure 7 :

 7 Figure 7: 3 monitoring spots in the Fourbanne karst hydrosystem. (after Charmoille, 2005) A: organization 1: swallow hole; 2 vadose conduits down to the saturated zone (cave); 3: outlet


Cécile Noel

Jean-Christophe Gourry

Ioannis Ignatiadis

Fabienne Battaglia-Brunet

Christophe Guimbaud

Monitoring of the biodegradation of toluene-contaminated sand in

teaching and research institutions in France or abroad, or from public or private research centers.

Monitoring of the biodegradation of toluene-contaminated sand in columns by SIP measurements, CO 2 content and its 13 C/ 12 C isotopic signature.

Cécile Noel (1,2) , Jean-Christophe Gourry (1) , Ioannis Ignatiadis (1) , Fabienne Battaglia (1) , Christophe Guimbaud (2) .

(1) BRGM, Orléans, France (2) LPC2E, CNRS, Orléans, France

Hydrocarbon contaminated soils represent an environmental issue as it impacts on ecosystems and aquifers. Bioremediation uses the ability of bacteria naturally present in the ground to degrade hydrocarbons. It represents an effective solution to fight the pollution but in situ monitoring before and during soil treatment is difficult and challenging. Indeed, where significant subsurface heterogeneity exists, conventional intrusive groundwater sampling can be insufficient to obtain a robust monitoring as the information they provide is restricted to vertical profiles at discrete locations, with no information between sampling points.

In order to obtain wider information, complementary methods can be used like geo-electrical techniques. Induced polarization (IP) seems to be the more promising to study the effects of biodegradation processes. Indeed, laboratory and field experiments have shown an enhancement of real and imaginary parts of electrical conductivity while bacterial treatment is progressing (Abdel Aal et al., 2006 ; Atekwana et Atekwana, 2010).

Moreover, microbial activity induced CO 2 production and isotopic deviation of carbon (Aggarwal and Hinchee, 1991). The ratio δ 13 C(CO 2 ) will come closer to δ 13 C(hydrocarbon).

From these findings, the French project BIOPHY, supported by the French National Research Agency (ANR), proposes to use electrical methods and gas analyses to develop a non-destructive method for monitoring in situ biodegradation of hydrocarbons in order to optimize soil treatment. Laboratory experiments in columns are carried out to demonstrate its feasibility.

Our objectives were to monitor aerobic microbial activity in toluene-contaminated sand columns using complex electrical resistivity measurements (SIP, Spectral Induced polarization and GEIS, Galvanostatic Electrochemical Impedance Spectroscopy) and measuring concentration and δ 13 C isotopic ratio of produced CO 2 .

MATERIAL AND METHODS

Toluene aerobic biodegradation by Rhodococcus wratislaviensis (R. w.) is studied. Columns (height 50cm ; diameter 15cm) are constructed in Kynar® and filled with sand from Fontainebleau. They are fed up with a fluid containing toluene (carbon source), bacterial growth medium and H 2 O 2 in low concentration to supply O 2 and thus stimulate aerobic metabolic bioprocesses.

Complex electrical conductivity measurements are performed regularly to monitor the bioactivity and progress of toluene degradation. Two methods are used: SIP and GEIS. An electrical current with known amplitude and frequency is applied at two ring electrodes, positioned at both ends of the columns. This induces an electrical current in the porous media and the associated voltage is measured between two Cu/CuSO 4 potential electrodes. The change of amplitude and phase of the received waveform allows measuring complex conductivity σ*, expressed as a sum of the real (σ') and imaginary (σ'') parts: with et . CO 2 concentration and δ 13 C(CO 2 ) is measured by infrared laser spectroscopy. Chemical composition of fluid is also monitored: toluene quantification, δ 13 C(toluene) and δ 13 C(DIC) (Dissolved Inorganic Carbon) and alkalinity.

RESULTS AND DISCUSSION

We show the results for two columns on Fig. 1: a biotic and an abiotic control.

For the biotic column, we notice an enhancement of σ'' during decrease of toluene concentration. Moreover, there is an increase of CO 2 concentration in the air just after increase of HCO 3 -concentration in the pore fluid ; and δ 13 C(CO 2 ) approaches δ 13 C(toluene). On the contrary, for the abiotic column, there is no significant variation of the different parameters. The δ 13 C(CO 2 ) was not able to be measured because of the lack of CO 2 in the air of the column. Regarding these results, we can say that microbial activity is characterized by CO 2 production and δ 13 C(CO 2 ) isotopic deviation, and also by an evolution of complex electrical resistivity in correlation with chemical analyses. Aerobic biodegradation of toluene can be monitored by SIP and gas measurements at laboratory scale and it will be tested soon on the field scale.

LITERATURE

Fig. 1 :

 1 Fig. 1: Results for biotic (left) and abiotic (right) columns. a) and c) Variation over time of real and imaginary parts of complex conductivity at 0.5Hz and toluene concentration ; b) and d) CO 2 and HCO 3 concentrations ; δ 13 C(CO 2 ) and δ 13 C(toluene).

  CITED G. Z. Abdel Aal, D. L. Slater, E. A . Atekwana, 2006. "Induced-polarization measurements on unconsolidated sediments from a site of active hydrocarbon biodegradation." Geophysics 71(2): H13-H24. P. K. Aggarwal and R. E. Hinchee, 1991. "Monitorlng in Sltu Biodegradation of Hydrocarbons by Using stable carbon isotope." Environmental Science & Technology 25: 1178-1180. E. A. Atekwana and E. A. Atekwana, 2010. "Geophysical Signatures of Microbial Activity at Hydrocarbon Contaminated Sites: A Review." Surveys in Geophysics 31: 247-283.

-9 April 2014 (Oléron Island, France)