Hydrodynamic and microbial processes controlling nitrate in a fissured-porous karst aquifer of the Franconian Alb, Southern Germany

Concentrations and stable isotope compositions of nitrate from 11 karst springs in the Franconian Alb (Southern Germany) were determined during low flow and high flow conditions to assess sources and processes affecting groundwater nitrate. During low flow, nitrate concentrations in groundwater were around 0.10 mM in springs draining forested catchments, whereas in agricultural areas nitrate concentrations were typically higher reaching up to 0.93 mM. The isotopic composition of groundwater nitrate during low flow (δ ¹⁵N values of -3.1 to 6.7 ‰, δ¹⁸O values of +2.1 to 4.0 ‰) in concert with concentration data suggests that nitrate is formed by nitrification in forest and agricultural soils. In addition, synthetic fertilizer N that has undergone immobilization and subsequent remineralization likely constitutes an additional nitrate source in agriculturally used catchments. During recharge conditions, concentrations and δ¹⁵N values of groundwater nitrate changed little, but δ¹⁸O values were significantly elevated (up to 24.5‰) suggesting that around 25% of the nitrate was directly derived from atmospheric deposition. Groundwater dating revealed that low nitrate concentrations in groundwater (≥70 years) are consistent with a mixture of old low nitrate-containing and young water, the latter being affected by anthropogenic N inputs predominantly in the agriculturally used catchment areas during the last few decades. Thermodynamic and hydrogeological evidence also suggests that denitrification may have occurred in the porous rock matrix of the karst aquifer. This study demonstrates that a combination of hydrodynamic, chemical, and isotopic approaches provides unique insights into the sources and the biogeochemical history of nitrate in karst aquifers, and therefore constitutes a valuable tool for assessing the vulnerability of karst aquifers to nitrate pollution in dependence on land use and assessing their self-purification capacity.