Karst aquifers represent an important groundwater resource world-wide. They are highly vulnerable to contamination due to fast transport through the system and limited attenuation of contaminants. The two main hydrogeological approaches developed for studying flow and transport are: inference of the
system structure from karst spring hydrographs and chemographs; numerical modelling of flow and transport using a theoretical distribution of flow and transport field parameters. These two approaches lack of validation by detailed field measurements and observations. The main objective of this thesis is to “fill the gap” existing between field and model data. Observations of flow and transport parameters at several locations within the system were used to develop a conceptual model. This model was then compared to the existing models.
The main field test site is the Milandre karst aquifer, located in the Swiss tabular Jura. Natural tracers (major ions, oxygen-18, specific conductance) and discharge were measured on the underground river, its main tributaries, percolation waters, and the main spring. These data were collected on a long-term basis in order to assess the spatial variability of the parameters, and on a short time scale (i.e. flood events) in order to investigate the dynamic processes. Complementary sites (Brandt and Grand Bochat) were used for more observations at the base of the epikarst.
The proposed conceptual model considers four sub-systems: the soil zone, the epikarst, the unsaturated zone, and the phreatic zone. Each has its own specificity with respect to flow and transport. The soil zone controls the actual infiltration into the system. It contributes efficiently to groundwater storage. It mixes quickly stored water with fresh infiltrated water. Its thickness determines land-use: thick soils are generally cultivated whereas thin soils are under forested areas. The solutes concentration of soil waters depends on land-use for pollution-related parameters (nitrate, chloride, sulfate, potassium, sodium). Moreover the soil zone is the main source of CO2 which controls the limestone dissolution-related parameters. The epikarst zone contributes largely to groundwater storage. It distributes groundwater into vadose flow through conduits, and base flow through low permeability volumes (LPV) in the unsaturated zone. It is the sub-system where dissolution-related parameters are mostly acquired.
The unsaturated zone is seen as a transmissive zone connecting the epikarst to the horizontal conduit network of the phreatic zone. In case of flood events, some dissolution still occurs in this sub-system.
The phreatic zone is the partly flooded conduit network draining groundwater to the spring. It collects waters issued from the unsaturated zone, mixes the tributaries, and drain the water towards the discharge area. The role of phreatic storage appears to be limited for both hydraulics and transport.
Tributary mixing is a prominent process that shapes spring chemographs during flood events. In steady-state conditions, base flow is mainly sustained by the epikarst reservoir. Tracer concentrations are stable as the chemical equilibrium is already reached in the epikarst. Waters issued from the different tributaries mix in the conduit network, and the spring chemistry is the result of this mixing.
During flood events, transient flow induces non-linear mixing of the tributaries. The respective contributions of the tributaries change throughout the flood, and the spring chemographs vary accordingly. In case of important recharge, waters issued from other sources than the epikarst participate to the flood. First, soil water reaches the phreatic zone. Its characteristics are a dampened isotopic signal, and ionic concentrations differing from those of the epikarst. Second, fresh water directly issued from rainfall, may reach the phreatic zone. Its characteristics are a varying isotopic signal, and diluted ionic concentrations. The mixing components participating to the flood are controlled by the actual infiltration volume (or height). The limestone dissolution process is effective for the fresh and soil components of flow. However mixing processes play a more important role than dissolution for shaping the spring chemographs.
From a practical point of view, the project confirmed the prominent role of the soil zone and the epikarst on the solute transport in karst systems. This was already integrated in karst vulnerability mapping methods recently developed (EPIK, PI, VULK).