Characterization of biodegradation processes in a karst aquifer using environmental isotope data

Full text: The European community has recently adopted a new Water Framework Directive (WFD) that extends to protect all groundwater types and sets a legally binding objective of good status for those waters (6th Environment Action Programme 2001). Europe has only begun to address this issue, recognizing the importance of groundwater protection. While the increased and more efficient food production derived from chemical use the accumulative effect of adding more nutrients and persistent organic pollution can have adverse effects on the quality of water resources. The continuing pollution causes problems not only on the availability of high quality freshwater resources but generated also economic and social pressures due to increasing conflict between land use and the demands for protection of the water resources. The presentation shows results of hydrograph separation technique in three karstified aquifers (Franconian Alb, South Germany) using isotopic (18O) and geochemical tracers (Sr2+). The main runoff components were characterized for better understanding of spatial and temporal distribution of runoff generation processes. Each of the three catchments within the Franconian Alb responded differently to storm rainfall. The extent to which the runoff components contributed to stormflow varied among catchments as a function of the different drainage systems. Event water showing a contribution of 10 % to 68 % during flood peaks was identified by means of a detailed observation, stable isotopes and electrical conductivity variation. The high contribution of up to 68 % to stormflow could be explained if event water flows through the sinkholes directly and a significant part of the runoff flows horizontal (interflow) beneath areas of thin soil in a extended epikarst system to the sinkholes. The other two components originated from water stored in the shallow groundwater (soil zone, epikarst) and the deep groundwater and also contributed significantly to stormflow. The proportion of pre-event water which was stored in the soil zone and the epikarst before the event extends from about 16 % to 44 % in the three catchments. Combined hydraulic, isotope- and tracer-based approaches in one of the catchments characterize the flow paths and mean transit times of water indicating that low sulfate and nitrate concentrations can probably explained by biodegaradation processes in the saturated zone and soil zone. A multi-tracer experiment using artificial tracers with different pore diffusion coefficients was performed in the karst aquifer. The results of the field experiment, environmental 3H output concentrations, mathematical modelling and hydrograph separation demonstrate possible existence of diffusion processes between mobile fracture water and immobile matrix water. δ34S and δ18O-SO42- values reflect the existence of biodegradation as the key attenuation process of aquatic sulfate in the porous matrix and dead end pores of the saturated bedrock. A comparison of δ34S and δ18O-SO42- values from deep wells with the calculated isotope composition of the residual sulphate from the observed karst aquifer probably documents the involvement of sulphite during the biologically mediated reaction supported by enzymatic activities. Because δ18O-SO42- values of the residual sulphate approaches a constant value and the pertinent δ34S values increase it was assumed that the δ18O-SO42- of the residual sulphate was controlled by isotope exchange with water. Results of hydrograph separation can demonstrate that pre-event water showing a contribution of 16 % was stored in the soil zone of the karst aquifer. Increasing δ15N and δ18O values of nitrate reflecting changing in nitrate concentration by denitrification processes. The isotope values of nitrate collected during storm runoff and baseflow conditions are in areas where different sources of NO3- typically plot. The source of nitrate contamination in the karst could be determined and originates probably from synthetic fertilizer but influenced from atmospheric nitrate too. The two different sources of oxygen- atmospheric and synthetic fertilizer- could be pronounced in samples with highest δ18O values of nitrate and simultaneously a observed dilution effect of nitrate concentrations influenced from areas covered by forests. δ18O values in atmospheric nitrate reached values with a mean of 64.5 per mille and can influence isotope values caused by kinetic isotope effects accompanying denitrification of nitrate. (author)