Investigating Conduit-Matrix Exchange Processes in Karst Aquifers

Karst aquifers can be understood as dual flow systems including a highly conductive conduit system with low storage, and a low permeability matrix system with high storage. The interaction between matrix and conduits represents one of the characteristic features of karst aquifers, displayed especially during time variant conditions after recharge events, i.e. a characteristic rapid increase in discharge and slow recession. While it is relatively easy to obtain hydraulic parameters of the matrix, e.g. borehole tests, it is difficult to obtain information on conduit parameters and the conduit-matrix exchange process. This work demonstrates how the variation in the relative abundance of oxygen isotopes and the concentrations of pesticides (e.g. Atracine) in spring discharge can be employed as tracers to quantify the exchange process. In karst groundwater systems, it is frequently observed that, especially during dry weather conditions, concentrations of pesticides are considerably increased above average levels. Furthermore, although Atracine has been banned for years, it is identified in karst spring waters also during dry weather. It is believed that the recent occurrence of these substances is a result of (1) the long-term application in agriculture, (2) their storage in the matrix of the karst aquifer, and (3) their subsequent slow release. This hypothesis is supported by the variation in the relative abundance of 18O in groundwater. During a long-term recession, a systematic decrease in 18O was observed, which is interpreted as the release of isotopically lighter water, recharged during the winter period several years ago. The isotopic and pesticide concentrations were used to calibrate a double continuum transport model and to obtain quantitative information on the exchange parameters.