Identification of localised recharge and conduit flow by combined analysis of hydraulic and physico-chemical spring responses (Urenbrunnen, SW-Germany)

Karst aquifers are highly vulnerable to contamination due to the rapid transport of pollutants in conduit systems. Effective strategies for the management and protection of karst aquifers, therefore, require an adequate hydrogeological characterisation of the conduit systems. In particular, the identification and characterisation of conduits transmitting rapid, localised recharge to springs is of great interest for vulnerability assessments. In this work, it is demonstrated that localised recharge and conduit flow in a karst aquifer (Urenbrunnen catchment, southwest Germany) can be characterised by jointly analysing the hydraulic and physico-chemical responses of a spring to recharge events. Conduit volumes are estimated by evaluating time lags between increases in spring discharge and associated changes in the electrical conductivity and temperature of the discharged water. These estimates are confirmed by the results of a combined tracer and recharge test. Variations in electrical conductivity are also shown to assist in the quantification of the fast recharge component associated with short-term recharge pulses. However, spectral analysis of temperature fluctuations reveals that highly mineralised surface waters locally infiltrate into the aquifer during the winter and spring without causing significant electrical conductivity variations in the spring water. Hence, the most consistent conceptual model is obtained by a combined analysis of both physico-chemical parameters.