Tracing nitrate pollution sources and transformations in the over-exploited groundwater region of north China using stable isotopes.

Nitrate contamination in groundwater has become an environmental problem of widespread concern. In this study, we used environmental isotopes (δ15N-NO3- and δ18O-NO3-) and an isotope mixing model (SIAR) to identify the main sources of nitrate pollution, and factors controlling nitrate pollution, and to quantify the relative contributions of potential NO3- sources in an over-exploited groundwater region, north China. The results showed that human activities had dramatically increased the mean concentration of groundwater NO3- reaching 124.4 mg/L. In Hutuo River pluvial fan region, groundwater nitrate came from many kinds of pollution sources and the predominant sources were sewage and/or manure. Nitrification might be one of the most important nitrogen transformation processes and groundwater intensely exploited was a major inducing factor for the NO3- pollution. The highest contribution of groundwater NO3- was sewage and/or manure which the percent in the Hutuo River valley plain unit, upper pluvial fans of Hutuo River and central pluvial fans of Hutuo River was 54.9%, 55.0% and 61.8%, respectively, followed by soil N, NH4+ in fertilizer and rain, NO3- fertilizer, and NO3- in precipitation. We suggested that the local government must strengthen the sewage treatment for the collection of domestic sewage, and must prohibit over-exploitation of groundwater in order to prevent NO3- contamination of in groundwater.