Chemical compositions evolution of groundwater and its pollution characterization due to agricultural activities in Yinchuan Plain, northwest China.

Yinchuan Plain is a typically intensive cultivated region in the northwest of China. The irrigation return infiltration from Yellow River is the main source of groundwater recharge. Deep soil layers, sandy vadose zones, and dense irrigation canals make the groundwater susceptible to the return flow which contains pollutants originating mainly from agriculture applications, particularly from the extensive use of nitrogen fertilizer and manure. The pollution levels of phreatic water and confined water in NWS areas (non-water source areas) and WS areas (water source areas) of Yinchuan Plain in 2004 and 2014 were evaluated by the single-factor evaluation method, fuzzy comprehensive evaluation method, and average benchmark coefficient method, respectively. Piper trilinear diagram and scatter plots of major ions were used to classify water types and chemical facies, and further analyze the causes of groundwater pollution and the variation tendency of agricultural pollution. The results show that in 2014, about 50% of the groundwater samples were heavily polluted in Yinchuan Plain, the pollution level of phreatic water and confined water in NWS areas was up to level 5. And the groundwater within the standard in 2004 was heavily polluted in 2014 in WS areas, three-nitrogen pollution was the most serious pollutant, and the organic pollution level was grade IV. From the scatter plots of ions, it can be seen that the increase in concentrations of major ions was affected by evaporation-condensation and cation exchange reaction, but the complex ion contents indicated that groundwater was affected by human activities. The intensive agricultural activities, such as over fertilization, artificial irrigation, have led to concentrations increase of some chemical composition in groundwater.