Monte Carlo Simulations as a Decision Support to Interpret δ¹⁵ N Values of Nitrate in Groundwater

Intense farming is often associated with the excessive use of manure or fertilizers and the subsequent deterioration of the groundwater quality in many aquifers worldwide. Stable isotopes of dissolved nitrate (δ¹⁵ N and δ¹⁸ O) are widely used to determine sources of nitrate contamination and denitrification processes in groundwater but are often difficult to interpret. Thus, Monte Carlo simulations were carried out for a site in lower Bavaria, Germany, in order to explain δ¹⁵ N observations in a porous groundwater system with two aquifers, the main aquifer (MA) and several smaller perched aquifers (PA). For evaluating potential contributions, frequency distributions of δ¹⁵ N were simulated deriving from (I) the mixing of different nitrate sources, related to land use, as input to groundwater, combined with (II) transport of nitrate in groundwater and (III) microbial denitrification. Simulation results indicate a source-driven isotopic shift to heavier δ¹⁵ N values of nitrate in groundwater, which may be explained by land use changes toward a more intensified agriculture releasing high amounts of manure. Microbial denitrification may play a role in the PA, with simulated δ¹⁵ N distributions close to the observations. Denitrification processes are however unlikely for the MA, as reasonable simulation curve fits for such a scenario were obtained predominantly for unrealistic portions of nitrate sources and related land use. The applied approach can be used to qualitatively and quantitatively evaluate the influence of different potential contributions, which might mask each other due to overlapping δ¹⁵ N ranges, and it can support the estimation of nitrate input related to land use.