We investigated the expansion of NO3--contaminated groundwater in the Sichuan Basin, China. Nitrogen concentrations and isotopic ratios of NH4+ and NO3- were analyzed 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 delta N-15 values are strongly negative to slightly positive -13.4 to + 2.3%, appears to originate from fertilizers and excretory waste. NO3- in rain (delta N-15: -10.2 to -4.4%) was attributed to NO (x) from automobile exhaust gas. In the studied area, well water sampled from farmyards was found to have the highest delta N-15(NO3) (average: +9.7 +/- 4.7%), indicating contamination by domestic sewage as animal excrement. The lowest delta N-15(NO3) (-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 delta N-15(NO3) (+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.