This paper is the result of the water resources and environmental geological survey engineering.

Objective Nitrate contamination of groundwater is becoming more and more serious resulting from intensive human activities.

Methods Hydrochemistry and stable nitrogen and oxygen isotopes were used to trace the nitrate sources and transformation groundwaters in the Zhaotong basin, China. As well as, the origin of nitrate in the spring and well waters was quantitatively analyzed by SIAR model.

Results The results showed that: (1) The water quality of boreholes is good, while approximately 19% of the spring samples are not drinkable due to nitrate exceed the drinking water standard, and 13% of the well samples are unsuited to irrigation due to the high nitrate concentrations; (2) The ranges of δ¹⁵N_NO3 in spring and well waters were 2.4‰-18‰ (mean of 7.9‰) and －4.5‰-39.7‰ (mean of 17.3‰), respectively, and the values of δ¹⁸O_NO3 ranged from －8.8‰ to 39.3‰ (mean of 2.5‰), and from －16.4‰ to 26.7‰ (mean of 0‰), respectively. Based on the hydrochemical data and stable isotopic compositions, nitrification was the dominant process in the study area; (3) Nitrate in spring and well waters were mainly from manure & sewage, soil nitrogen, and ammonium nitrogen fertilizer. The SIAR model showed that the contributions of manure & sewage, soil nitrogen, and ammonium nitrogen fertilizer to spring were 48%, 28% and 24%, respectively, and to well water were 87%, 6% and 7%, respectively.

Conclusions Manure & sewage end-member accounted for 89% and 72% in the groundwaters from residential and agricultural areas, respectively, while it only accounted for 27% in the groundwater from forestry land, indicating that the more strongly affected by human activities, the more serious nitrate contamination of groundwater.