Nishikiori,T., Takamatsu,T., Kohzu,A., Nakajima,Y. and Watanabe,M.(2012): Distribution of nitrate in groundwater affected by the presence of an aquitard at an agricultural area in Chiba, Japan. Environ. Earth Sci., 67, 1531-1545.

『日本の千葉の農業地域において半帯水層の存在により影響された地下水中の硝酸塩分布』


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


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