Abstract: Early researches show that the shallow aquifer of the Nanyang district, at which the South to North Water Diversion Project starts, has been contaminated by nitrate. In this study, 28 groups of shallow groundwater samples are collected. By measurements of nitrogen and oxygen isotopes (δ¹⁵N and δ¹⁸O), combined with the linear mixing model based on the mass conservation law of stable isotopes, the quantitative results of the potential sources of nitrate contamination are obtained. The results show that the average concentration of NO₃^--N in shallow groundwater is 23.25 mg/L and 39.29% of all the samples are above the level Ⅲ of the Groundwater Quality Standard. The data from all the samples indicate the sewage and manure are the main reason of the nitrate pollution in the study area, which account for 73% of the overall level. The use of fertilizer is the secondary cause, which takes up 23%, and the natural factors are negligible compared to the anthropogenic activities.