『Abstract
Geological and geographical parameters including land use, stratigraphic
structure, groundwater quality, and N- and O-isotopic compositions
of nitrate in groundwater were investigated to elucidate the mechanism
of groundwater pollution by NO3-
in the agricultural area of Katori, Chiba, Japan. An aquitard
distributed in the western part of the study area has produced
two unconfined aquifers. The average concentrations of NO3- and dissolved oxygen (DO) were high
in the aquifer above the aquitard (7.5 and 7.6 mg/L, respectively)
and in the aquifer of the eastern part of the study area that
was not influenced by the aquitard (11.9 and 7.8 mg/L, respectively);
however, the levels in the aquifer under the aquitard were relatively
low (2.2 and 3.7 mg/L, respectively). The δ15N and
δ18O values of NO3-
generally increased exponentially in the groundwater that flowed
into the aquifer under the aquitard as the concentration of NO3- decreased, although this decrease
in NO3- also occasionally occurred
without isotopic changes. These results indicated that the aquitard
prevented the penetration of NO3-,
DO, and gaseous O2. Under the aquitard, denitrification
and dilution with unpolluted water that entered from natural forested
areas reduced the NO3- concentrations
in the groundwater. The major sources of NO3-
in groundwater in the study area were estimated to be NH4-chemical fertilizer, livestock waste, and manure.
Keywords: Groundwater; Nitrate; Aquitard; δ15N; δ18O;
Denitrification』
Introduction
Materials and methods
Study area
Anthropogenic nitrogen load
Stratigraphic characteristics
Observation at wells
Sampling of groundwater
Analysis of groundwater and fertilizer samples
Results
Distribution and flow of groundwater
Groundwater quality
Nitrate nitrogen
Dissolved oxygen
Major ions and pH
Isotopic compositions of nitrate
Discussion
Sources of nitrogen
Difference in nitrate concentration between A1 and A2a
Occurrence of suboxic conditions under the aquitard
Denitrification and dilution under the aquitard
Isotopic enrichment factors of nitrogen and oxygen during denitrification
Conclusion
Acknowledgments
References