Fate of nitrate during groundwater recharge in a fractured karst aquifer in Southwest Germany

Anna Neva Visser, Moritz F. Lehmann, Hermann Rügner, Fernando Mazo D’Affonseca, Peter Grathwohl, Nia Blackwell, Andreas Kappler, Karsten Osenbrück

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

Over the past decades, fractured and karst groundwater systems have been studied intensively due to their high vulnerability to nitrate (NO3) contamination, yet nitrogen (N) turnover processes within the recharge area are still poorly understood. This study investigated the role of the karstified recharge area in NO3 transfer and turnover by combining isotopic analysis of NO3 and nitrite (NO2) with time series data of hydraulic heads and specific electrical conductivity from groundwater monitoring wells and a karstic spring in Germany. A large spatial variability of groundwater NO3 concentrations (0.1–0.8 mM) was observed, which cannot be explained solely by agricultural land use. Natural-abundance N and O isotope measurements of NO315N and δ18O) confirm that NO3 derives mainly from manure or fertilizer applications. Fractional N elimination by denitrification is indicated by relatively high δ15N- and δ18O-NO3 values, elevated NO2 concentrations (0.05–0.14 mM), and δ15N-NO2 values that were systematically lower than the corresponding values of δ15N-NO3. Hydraulic and chemical response patterns of groundwater wells suggest that rain events result in the displacement of water from transient storage compartments such as the epikarst or the fissure network of the phreatic zone. Although O2 levels of the investigated groundwaters were close to saturation, local denitrification might be promoted in microoxic or anoxic niches formed in the ferrous iron-bearing carbonate rock formations. The results revealed that (temporarily) saturated fissure networks in the phreatic zone and the epikarst may play an important role in N turnover during the recharge of fractured aquifers.

Original languageEnglish
Pages (from-to)1153-1171
Number of pages19
JournalHydrogeology Journal
Volume29
Issue number3
DOIs
StatePublished - 1 May 2021
Externally publishedYes

Keywords

  • Germany
  • Groundwater recharge
  • Karst
  • Nitrate
  • Stable isotopes

ASJC Scopus subject areas

  • Water Science and Technology
  • Earth and Planetary Sciences (miscellaneous)

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