Hydrogeology and water balance in karst terrain Jacques Mudry Chrono-Environnement, Besançon (France) 1 Introduction Karst is defined as "all superficial and underground landforms produced by dissolution of specific rocks, mainly limestone". At the same time, it is a morphological structure (requiring thermodynamic and hydrodynamic conditions), a hydrosystem (input-box-output), and an aquifer (water resource, man impact, protection). Different academic specialties study karst: Physical Geography, Speleology, Geology, Hydraulics, Hydrogeochemistry, Pedology, and also engineering specialties: Geophysics, Hydrogeology, Geotechnics. Comprising external and internal landforms, karst is characterized by its internal draining pattern gathering water from up to downstream. This pattern originates in dissolution action on sedimentary and tectonic pre-existing discontinuities. This dissolution can be roughly synthetized by: CaCO3 + CO2 + H2O  Ca2+ + HCO3- This equation requires CO2 providers, sometimes deep gas (thermal karst), but mainly soil in the most common case of meteoric karst. As carbonates outcrop under various climates, karst is therefore widely dispread in the world. The common feature of all these contexts is this flow structuration on the pattern of enlarged fractures. This structuration transforms a rather homogeneous media into a more or less heterogeneous one, depending on geological duration of the process. 2 karst heterogeneity We can observe three types of heterogeneities in karst: (i) a vertical heterogeneity with epikarst and vadose zone vs saturated zone; (ii) a spatial one with conduits (karstified fractures) vs microfissured blocks; (iii)a heterogeneity of inputs with karrenfields with diffuse infiltration vs swallow holes with concentrated infiltration. Behavior of karst is strongly influenced by this heterogeneity and anisotropy of karst systems. 3 hydrodynamic and hydrochemical consequences Due to the existence of a decompressed-open shallow zone called epikarst, infiltration process is more complicated than in a classic inter granular media. Water fills first this first reservoir which recharges, by means of tectonic vertical discontinuities, the saturated zone. Due to the existence of very permeable conduits and lowly permeable microfissured blocks, water exchanges occur between both media, conduits recharging blocks during flood episodes, and blocks supplying conduits during low-water periods. This heterogeneity of functioning induces a scale effect: the bigger the system, the more permeable it is, because big lumps of limestone include bigger fractures than small ones. Another consequence concerns residence time. In karst, contrary to more homogeneous aquifers, a contrast between long residence time waters (tenths or hundreds of years), stored in blocks, and shortest transits (hour to day) can be displayed. This way of behavior, added to the structure of the draining pattern enables existence of the big karst springs, with high flood discharges (sometimes >100 m3/s), but also with high depletion discharge (several m3/s). Existence of significant volumes (up to million m3/s), transiting the conduits induces 'piston-effect' which leads to yield 'pre-event' water at the beginning of floods. The consequence is a delayed mass transfer vs pressure transfer: water from the rainfall is not that flows at the spring at the discharge peak. Chemical composition of karst springs depends of the water transit through different media. The soil cover and the epikarst, submitted to evaporation, act as buffer for the hydrograph peak and marks water with characteristic tracers that are produced, stored or concentrated inside: e.g. chloride, provided by precipitations and concentrated by evaporation, and total organic carbon (TOC), produced by biological processes of pedogenesis. These tracers can be combined to the typical tracers of carbonate reservoir, e.g. magnesium and 13 C. As an example of transit time identification through vadose zone: the low-noise underground laboratory (LSBB, southeastern France) displays 1 month delay between rainfall and discharge peak in an artificial gallery, and 1 additional month for the infiltration itself to reach this point. Total: 2 months to pass vertically through 400 m of limestone. As an example of mixture identification in the saturated zone: the Fourbanne system (Jura mountains, Eastern France) displays well the behavior of the same water body, identified in a swallow hole by uranin tracing. This colored body is re-sampled within the conduit network, and finally in the general outlet of the system. As it is the same water, its chemical evolution will display mixtures with other components. The main results are that in the vadose conduit, the composition is similar to the input, and that the transit in the saturated zone involves a mixture with larger residence time water (making increase magnesium concentration), and poorer in agricultural tracers (making decrease nitrate concentration), probably infiltrated in forest areas, which has transited a larger time throughout the saturated zone. 4 Consequences for exploration and water protection In the context of coexistence of very and very low permeability, prospecting karst water resources is hard work. Historically, since the Antiquity, karst springs have been captured, but due to the easy drainage of conduits, often display low discharge in low water periods. Implementing boreholes requires remote sensing, in order to identify conductive fractures. But setting precisely a borehole is difficult, because geophysics does not meet high parameter contrasts. The main problem is that low permeability media are designed to propose equally probabilistic sites, but with a low exploitable discharge, whilst karst conduits, with potentially high exploitable yields are difficult to locate. Georadar may be a promising technique, mainly for shallow investigation. But the most permeable structures are also subjected to particle transit (turbidity) generally associated with microbiological problems (bacteria, viruses, protozoa), due to the insufficient transit time to kill these microorganisms. The relevant parameters for intrinsic vulnerability assessment are (i) the structure of rainfall episodes; (ii) the nature and thickness of the layers overlying the saturated zone (topsoil, subsoil, geological layers, unsaturated zone); (iii) the infiltration conditions (swallow hole, doline, karrens); and (iv) finally the importance of the karst conduit network. As an example a part of Besançon's resources, Arcier spring, tapped since 2nd century, has been protected by 'leopard skin' zones: swallow holes, draining inhabited intake areas, are considered as very vulnerable spots protected by four remote immediate protection zones, as well as the spring itself. 5 Hydrologic balance In karst regions, meteorological cover of the recharge zones is often sparse, due to the lowly peopled catchment areas. Simple and parsimonious methods are required, in order to be able to use data measured in these stations. The most common relevant parameters measured in weather stations are precipitation and temperatures. Flowchart, using more complete dataset of bigger stations (solar radiation, wind...), attempt to correlate these parameters with those of the local stations, and to reconstitute local series. Water balance in a Mediterranean mountain system (Fontaine de Vaucluse, Southeastern France), shows that actual ET is rather constant according to altitude belts, and therefore that increase of raw precipitation with altitude induces a parallel increase of effective rainfall. The highest altitude belts of the recharge area account therefore much more than the lowest ones in the water balance of the system. The consequence is the prominent role of karst mountains in the recharge of piedmont granular aquifers. As an example: the molassic Valence plain, Southeastern France. Most of the resource of the lower aquifer is provided by effective rainfall in the karst aquifer bordering it. This is particularly significant in semiarid or arid zones, as in the Saharan Atlas, in Southern Algeria, where the limestone, due to its high altitude, displays more rainfall and lower temperatures. Added with existence of shallow soils, effective rainfalls are very important for the recharge of piedmont aquifers. Estimating effective rainfalls requires the knowledge of the recharge zone. If the lowest altitude of this zone is known (that of the spring), estimating its maximum altitude is necessary, in order to map the recharge area. Stable isotopes enable to evaluate average altitude of the recharge zone, and the problem is that this average includes contribution of upper zones, rainier, and lower zones, less rainy. Accounting for the rain and the stable isotope gradients, we can estimate the highest zone of the recharge area, using a set of equations. 6 Conclusion Heterogeneous in its structure, in its residence times, in its vulnerability, in its ability to provide drinking water, the karst aquifer is a very interesting aquifer, as well as a scientific object as an economic and societal one. But this aquifer is probably the most difficult to study, and therefore to exploit for drinking water and finally to protect.