Nitrogen contamination in groundwater in an agricultural region along the New Silk Road, northwest China: distribution and factors controlling its fate.

Nitrogen contamination is a global concern and has been a serious problem in agricultural areas. The present study was carried out in an intensively irrigated region of northwest China along the New Silk Road, Yinchuan Plain, where the residents depend on the groundwater as the primary source for drinking. To understand the nitrogen contamination in the aquifer system, the distribution of nitrate and ammonium and its controlling factors were studied based on hydrochemical, hydrogeological, and isotopic analyses. 11.37 and 40% of phreatic water samples are categorized as NO3-N and NH4-N pollution in accordance with the WHO standards. A total of 59.52% of confined water samples has high NH4-N values, exceeding the permissible limit for drinking purpose. The results indicate NO3-N predominates in the shallow water and NH4-N predominates in the deep water for the single phreatic aquifer. For the multilayer structure area, NO3-N predominates in the phreatic aquifer of the western and the southern parts of the plain; NH4-N predominates in the phreatic aquifer of the middle and the northern parts of the plain, and in the confined aquifers where groundwater pumping had been performed. The mixture of synthetic fertilizer and manure/sewage is primarily responsible for the phreatic water based on isotopic analysis. In the confined aquifers, higher NH4-N concentrations are mainly attributed to intensive pumping under higher pumping rates. The results of this study can be used as a scientific basis for the future research on nitrogen in the plain. They can also be used by scholars and decision makers who are interested in groundwater protection and sustainable development.