Abstract

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.

Highlights

  • Many countries around the world rely on groundwater for safe drinking water supply

  • A beta distribution was found as a best fit for manure and normal distributions for precipitation and mineral fertilizer

  • Probability Density Functions (PDF) were fitted to δ15N values observed in groundwater of the main aquifer and the perched aquifer, respectively, where logistic distributions could describe observations best

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Summary

Introduction

Many countries around the world rely on groundwater for safe drinking water supply. With the increasing pressures of climate change, population growth and rapidly developing economies on the quality and quantity of water resources, groundwater protection is a global challenge. Article impact statement: With the increasing pressures on groundwater quality, MC simulations could support decision makers in the assessment of nitrate contamination. In Europe, several regulatory frameworks aim to improve surface water and groundwater quality (European Commission 2000, 2015). In Germany, 28% of regularly sampled groundwater wells (EU monitoring network) showed nitrate concentrations exceeding the European drinking water limit of 50 mg/L in the years 2012 to 2015 (Keppner et al 2016)

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