Characterizing possible nitrate sources in a karst area with help of stable isotope measurements and Monte Carlo simulations

Arno Rein1, Thomas Hanke1, Anja Wunderlich1, Florian Einsiedl1
1 Lehrstuhl für Hydrogeologie, Technische Universität München

O 8.1 in Isotopen- und Tracermethoden in der Hydrogeologie

15.04.2016, 14:15-14:30, Audimax A, Geb. 30.95

 

 

For characterizing nitrate sources and related biogeochemical processes in aquatic habitats, the analysis of stable isotopes has been established as a well suited methodology during the past years. In the present work we have determined δ15N and δ18O values of dissolved nitrate over a period of approximately 10 years, as well as nitrate concentrations in a study area located in the Franconian Alb, Germany. Furthermore, measured tritium contents linked with a lumped-parameter model have been used for groundwater age-dating. Among others, our study aimed at evaluating whether there is a temporal trend in the isotopic composition of dissolved nitrate in a karstic groundwater system after changes in agricultural management strategies. Moreover, we probabilistically investigated the contribution of different processes that could also have led to the observed isotopic shift to heavier isotopes in dissolved nitrate.

 

In hypothetical studies applying Monte Carlo simulations and analytical groundwater modeling we addressed as a baseline question to which extent and probability (i) mixing of different sources within the catchment, (ii) transport processes in groundwater or (iii) denitrification processes (microbial degradation and isotopic enrichment) could have contributed to the measured isotopic signal. Observed increasing δ15N values accompanied by elevated nitrate concentrations indicate a source-driven shift away from ammonium-based fertilizers to manure derived from animal waste. This finding could also be supported by first simulation results where mixing and hydrodynamic processes seemed to contribute to a higher extent (higher probability) to observed δ15N values, as compared to the contribution of denitrification. The latter is assumed to take place in the rock matrix where groundwater exhibits high mean transit times (>100 years), however this water contributes only to a minor portion to groundwater flow. Conduit water, which is characterized by mean transit times below 60 years (and predominantly low potentials for microbial denitrification) is the main component of groundwater flow. This aspect is currently investigated in more detail.



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