『Abstract
Sources and transformation processes of nitrate in groundwater
from shallow aquifers were investigated in an agricultural area
in the mid-western part of South Korea using a multi-tracer approach
including δ2H and δ18O values of water,
δ15N and δ18O values of nitrate, Cl/Br ratios
and chlorofluorocarbons (CFCs). The study area was comprised of
four land-use types with natural areas at higher altitudes, upland
areas with fruit orchards, paddy fields and residential areas
at lower elevations. The isotopic composition of water was suitable
for distinguishing groundwater that had infiltrated in the higher
elevation natural areas with lower δ2H and δ18O
values from groundwater underneath paddy fields that was characterized
by elevated δ2H and δ18O values due to evaporation.
δ18O-H2O values and Cl-
concentrations indicated that groundwater and contaminant sources
were derived from three land-use types: natural areas, residential
areas and paddy fields. Groundwater age determination based on
CFCs showed that nitrate contamination of groundwater is primarily
controlled by historic nitrogen loadings at least in areas with
higher nitrate contamination. Nitrate sources were identified
using the stable isotope composition of nitrate and Cl/Br ratios.
Higher δ15N-NO3- values
and Cl/Br ratios of 300 to 800 in residential areas indicated
that waste water and septic effluents were major nitrate sources
whereas lower δ15N-NO3-
values and Cl/Br ratios of 100 to 700 in upland areas suggested
that synthetic fertilizers constituted a major source of nitrate
contamination of aquifers. With only few exceptions in the natural
area, contributions of atmospheric nitrate were insignificant
due to the resetting of δ15N-NO3-
values via immobilization and re-mineralization of nitrate in
the soil zone. In groundwater underneath paddy fields, 30% of
samples had δ18O-NO3-
values at least 2‰ higher than expected for nitrate formed by
chemolithoautotrophic nitrification; these samples were also characterized
by low DO and NO3-N concentrations and elevated
Cl and Mn concentrations indicating anthropogenic contamination
and denitrification in the aquifer. These conditions were observed
primarily in aquifers on floodplains. Statistical comparison between
land-use groups revealed that Cl/Br ratios were more diagnostic
for the impact of different land-use types on groundwater quality
than stable isotope compositions of nitrate. This indicates that
the former is an additional efficient tracer for the effect of
land use on groundwater quality in agricultural areas. We conclude
that the combination of groundwater age dating together with the
use of chemical and isotopic parameters is a highly effective
but yet underutilized approach for elucidating the sources and
the fate of nitrate in aquifers in Asia.
Keywords: Nitrate sources; Nitrification; Denitrification; Stable
isotopes; Groundwater age』
1. Introduction
2. Study area and methods
2.1. Study area
2.2. Field investigation and chemical analyses
2.3. Stable isotopes of water and nitrate
2.4. Groundwater age dating
3. Results and discussion
3.1. Stable isotopes of water
3.2. Recharge sources and contaminants
3.3. Groundwater age
3.4. Stable isotope composition of nitrate
3.5. Processes affecting groundwater nitrate under natural conditions
3.6. Sources of groundwater nitrate in the upland areas
3.7. Fate of nitrate in paddy fields
3.8. Cl/Br ratios
3.9. Land-use controls on groundwater contamination
4. Conclusions
Acknowledgments
References