Tracing sources and fate of nitrate in multilayered karstic hydrogeological catchments using natural stable isotopic composition (δ15N-NO3− and δ18O-NO3−). Application to the Toulon karst system (Dordogne, France)

Abstract

• Nitrate contents and isotopes are measured in a large multilayered karst aquifer. • Primary and secondary sources of nitrate are determined, at the local scale, using nitrate isotopes. • Origins of nitrate, over a karstic hydrogeological catchment, are assessed by nitrate isotopes. • The fate of nitrate in a karst aquifer is specified/ elucidated/ quantified/ combining isotopes and hydrogeological approaches. In groundwater management, the determination of the origins of nitrate (NO 3 – ) endmember sources is the first step in improving the management over a hydrogeological catchment. The generally multiple source contamination by nitrate over the karst systems makes it difficult to identify the exact nitrate origins in surface water and groundwater. This work aims to assess nitrate vulnerability of a karst system fed by several karst aquifers. The Toulon karst system offers the opportunity to study the relationships between two multilayered karst aquifers. It shows an increase in mean NO 3 – concentrations over decades. However, the origins of the NO 3 – remain unclear. Natural stable isotopic composition of NO 3 – (δ 15 N -NO3 – and δ 18 O -NO3 – ) measurements combined with concentrations were used to identify the origins of nitrogen causing this increase. This information was also integrated into the local hydrogeological situation to improve knowledge of the contamination of both confined and unconfined karst aquifers. δ 15 N -NO3 − and δ 18 O -NO3 − in potential nitrate sources over the hydrogeological catchment were analysed in order to identify the range of values that can be measured in both rivers and groundwater. Over the Toulon Springs catchment, rivers are contaminated with nitrate coming from N synthetic fertilizer mineralized in agriculturalsoils. Most of the springs are also contaminated by this nitrate source, but some record an isotope composition of nitrate coming from sewage effluents. Toulon Springs are only contaminated with nitrate from N synthetic fertilizer mineralized in agricultural soils. Finally, from a hydrogeological point of view, the analysis of δ 15 N -NO3 − and δ 18 O -NO3 − values improves knowledge on the functioning of the study area and shows that the upper Jurassic karst confined aquifer has nitrate of the same origins (sources) as the upper Cretaceous unconfined aquifer. The results here highlight that the lower aquifer can be fed by the Upper Cretaceous unconfined aquifer, contrary to previous belief that the lower aquifer was confined. From a more fundamental standpoint, this work provides a set of parameters adapted for karstic hydrogeological catchment. It lays the foundation for the use of isotope biogeochemistry as a powerful tool for understanding the recharge of drinking water catchments and beyond the efficient management of water quality at the scale of karstic hydrogeological catchment.