Monte Carlo simulations as a tool to interpret stable isotope analyses of nitrate in groundwater of a complex porous aquifer system

Lisa Wild1, Arno Rein1, Florian Einsiedl1
1 Technische Universität München, Chair of Hydrogeology, Arcisstr. 21, 80333 Munich, Germany

P 13.5 in Nitrate and the importance of denitrification for water supply

Stable isotopes of nitrate have been proven to be a useful tool to determine denitrification processes in groundwater systems. However, isotope exchange reactions between water oxygen and nitrogen intermediates that are formed during the denitrification pathway, hydrodynamic processes along flow paths (transport, dispersion) as well as mixing of different nitrate sources can alter the isotope signature. Based on the isotopic signature of nitrate it is often unclear if denitrification or simple mixing processes of different nitrate sources have led to the observed isotopic shift of nitrate in groundwater.  

Here we measured δ15N and δ18O of nitrate in a heterogeneous porous groundwater system in lower Bavaria, Germany, and performed probabilistic analyses by applying Monte Carlo simulations on the isotope data of nitrate. We want to clarify the open question to which extent theoretical assumptions of (i) mixing of different nitrate sources in the catchment, (ii) hydrodynamic processes, and (iii) microbial denitrification with different stable isotope enrichment factors have contributed to the isotopic signal of nitrate in the investigated area.

Therefore, in different scenarios, Monte Carlo simulations and analytical groundwater modelling were carried out with a data set from a complex porous aquifer to approach these questions. To clarify this, the isotope enrichment factors of nitrate were varied and it was determined to which probability and after which possible mean transit time denitrification processes could be clearly identified in groundwater.