『Abstract
　We investigated the expansion of NO3^--contaminated groundwater in the Sichuan Basin, China. Nitrogen concentrations and isotopic ratios of NH4⁺ and NO3^- were analyzes in groundwater and rain collected from four areas in this basin in order to evaluate the sources of nitrogen pollution. NH4⁺ in rain, for which δ¹⁵N values are strongly negative to slightly positive -13.4 to +2.3‰, appears to originate from fertilizers and excretory waste. NO3^- in rain (δ¹⁵N: -10.2 to -4.4‰) was attributed to NOx from automobile exhaust gas. In the studied area, well water sampled from farmyards was found to have the highest δ¹⁵NNO3 (average: +9.7±4.7‰), indicating contamination by domestic sewage as animal excrement. The lowest δ¹⁵NNO3 (average: -0.2±3.7‰), found in spring water, indicates that the studied groundwater samples are widely affected by air contaminants (mainly as nitrogen oxides) resulted from fuel combustions. The δ¹⁵NNO3 (average: +3.7±2.1‰) values of well water from farmland are between these levels, suggesting that NO3^- contamination results primarily from cultivation using nitrogen fertilizers, although the contribution from animal excrement cannot be excluded. These results demonstrate that the studied groundwater is widely polluted by locally derived nitrogen sources.

Keywords: ammonium and nitrate ions; groundwater pollution; nitrogen isotopes; pollutant sources; Sichuan Basin』

『要旨
　我々は、中国の四川盆地における硝酸イオンに汚染された地下水の拡大について調査した。窒素汚染の起源を評価するために、アンモニアイオンと硝酸イオンの窒素濃度および同位体比を、この盆地の４地域から採取した地下水と雨水について分析した。雨水のアンモニアイオンについて、δ¹⁵Nは非常に大きな負の値からわずかに正の値の-13.4〜+2.3‰であり、これは肥料および排水に由来すると思われる。雨水の硝酸イオン（δ¹⁵N：-10.2〜-4.4‰）は自動車の排ガスからのNOXに起因する。調査地において、農家の庭から採取した井戸水が最高のδ¹⁵NNO3 （平均：+9.7±4.7‰）をもつことがわかり、これは動物の排せつ物のような家庭下水による汚染を示している。δ¹⁵NNO3の最低値（平均：-0.2±3.7‰）は、湧き水で見られたが、調査した地下水試料が燃料の燃焼から生じた大気汚染物質（おもに窒素酸化物としての）に広く影響されていることを示している。農地からの地下水のδ¹⁵NNO3（平均：+3.7±2.1‰）はこれらのレベルの間にあり、動物の排せつ物からの寄与が除外できないけれども、硝酸イオン汚染は窒素肥料を用いた耕作から主に生じていることを示している。これらの結果は、調査した地下水が局所に由来した窒素源によって広く汚染されていることを表している。』

1 Introduction
2 Study area and background
3 Sampling and analytical methods
　3.1 Samples and pretreatments in the field
　3.2 Analytical methods
4 Results
　4.1 Nitrogen isotopic ratios of groundwater
　4.2 Concentrations and isotopic ratios of nitrogen in rain
　4.3 Nitrogen isotopic ratios of fertilizer
5 Discussion
　5.1 Sources of nitrogen in groundwater
　5.2 Sources of nitrogen in rain
6 Conclusions
Acknowledgments
References

Figure 2 δ15N values of groundwater samples in the Sichuan Basin. For the δ15N values of NO3- and NH4+ of groundwater from various sites, the cross represents spring water; the open triangle and closed circle represent well water from farmyard and that from farmland, respectively. The δ15N values of animal waste and local fertilizer and rain are included for comparison. The average of each set of data is represented by a left-leaning triangle with short line. The data on animal waste are referred from Kreitler (1979), Kreitler and Browning (1983), Heaton (1986), Fogg et al. (1998), Schulz et al. (2001) and Widory et al. (2004). 〔Li,X., Masuda,H., Koba,K. and Zeng,H.(2007): Nitrogen isotope study on nitrate-contaminated groundwater in the Sichuan Basin, China. Water Air soil Pollut., 178, 145-156.から〕

• Fogg,G.E., Rolston,D.E., Decker,D.L., Louie,D.T. and Grismer,M.E.(1998): Spatial variation in nitrogen isotopic values beneath nitrate contamination sources. Ground Water, 36, 418-426.
• Heaton,T.H.E.(1986): Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: A review. Chemical Geology, 59, 87-102.
• Kreitler,C.W.(1979): Nitrogen-isotope ration studies of soils and groundwater nitrate from alluvial fan aquifers in Texas. Journal of Hydrology, 42, 147-170.
• Kreitler,C.W. and Browning,L.A.(1983): Nitrogen-isotope analysis of groundwater nitrate in carbonate aquifers: Natural sources versus human pollution. Journal of Hydrology, 61, 285-301.
• Schulz,H., Gehre,M., Hofmann,D. and Jung,K.(2001): Nitrogen isotope ratios in pine bark as an indicator of N emissions from anthropogenic sources. Environmental Monitoring and Assessment, 69, 283-297.
• Widory,D., Kolppmann,W., Chery,L., Bonnin,J., Rochdi,H. and Guinamant,J.(2004): Nitrate in groundwater: An isotopic multi-tracer approach. Journal of Contaminant Hydrology, 72, 165-188.