Timescales and development of groundwater pollution by nitrate in drinking water wells of the Jahna-Aue, Saxonia, Germany

1 Nitrate pollution from agricultural activities often persistently affects groundwater quality due to long residence times in the vadose and saturated zone. In this study we used a lumped parameter approach to estimate the residence time of groundwater and nitrate from the agriculturally used Jahna-Aue drinking water catchment in Saxonia, Germany. Inverse modeling of measured concentrations of tritium and tritiogenic 3He revealed consistent mean residence times between 25 and 50 years for the young, nitrate-rich groundwater component, and high contributions (>75%) of an old, tracer-free, and nitrate-poor groundwater. The obtained age distributions are in accordance with the complex hydrogeological situation of the investigated catchment, suggesting that the shallow and therefore most vulnerable part of the aquifer is not connected to the production wells. High residence times are supported by low concentrations of CFCs and by radiogenic 4He as an independent age indicator. CFC concentrations only yield lower age limits due to identified problems with CFC contamination. Using the tracer-calibrated age distributions, future nitrate concentrations in the production wells most probably will remain below the drinking water limit because of the high dilution with old, nitrate-poor groundwater. Deterioration of the groundwater quality with respect to nitrate may occur if the groundwater pumping regime is changed so that the fraction of the younger, nitrate-bearing water is increased.