『Abstract
Denitrification is an important natural attenuation process that
has been observed in many fissured and porous aquifers. However,
an important factor limiting denitrification in aquatic systems
is the microbial availability of electron donors. Pyrite as the
most abundant sulfide mineral in nature represents one of the
potential electron sources for denitrifiers to reduce nitrate,
but the reaction mechanisms coupling denitrification processes
to pyrite oxidation are still questionable. We utilized hydrochemical
data and stable isotopes of nitrate and sulfate in groundwater,
isotope ratios of sulfur compounds in aquifer sediments and tritium
based groundwater dating for assessing denitrification processes
in a pyrite-bearing porous groundwater system. The oxic part of
the aquifer with mean water transit times of approximately 60
years was characterized by nitrate concentrations of around 15
mg/l and δ15N values were similar to those typical
for nitrification. In contrast, in the anoxic part with mean water
transit times of up to 100 years, low nitrate concentrations accompanied
by elevated δ15N values were observed. Furthermore,
isotope data of groundwater sulfate and sulfur compounds in the
aquifer sediment suggest that pyrite oxidation is the dominant
source of sulfate in the aquifer. The trend of increasing δ15N
values and decreasing nitrate concentrations in concert with depleted
δ34S values of groundwater sulfate similar to δ34S
values of pyrite, FeS2, suggests that denitrification
is coupled to pyrite oxidation, particularly when water mean transit
time is elevated.
Keywords: Groundwater; Denitrification; Pyrite oxidation; Stable
isotopes; Depth profiles』
1. Introduction
2. Methods
2.1. Study site
2.2. Sampling
2.3. Analytical methods
2.4. Modelling of mean transit times
3. Results
3.1. Hydrochemistry
3.2. Isotopic compositions of groundwater nitrate and sulfate
3.3. Sulfur isotopic composition of reduced sedimentary sulfur
3.4. Transit times of groundwater
4. Discussion
4.1. Atmospheric nitrate deposition
4.2. Redox conditions
4.3. Denitrification
4.4. Mixing processes of old and young groundwater
4.5. Pyrite oxidation
4.5.1. Theoretical background
4.5.2. Isotopic composition of groundwater sulfate and sedimentary
sulfur
5. Conclusions
Acknowledgements
References