Identification of nitrate pollution sources of groundwater and analysis of potential pollution paths in loess regions: a case study in Tongchuan region, China

Abstract

Groundwater is the essential and predominant source of drinking and irrigation water supplies in most arid and semiarid regions, particularly in Loess Plateau of China. The threat to groundwater quality by nitrate pollution has always been of considerable concern in loess regions. In this paper, correspondence analysis, stoichiometric analysis, ratios of major ions and some qualitative analysis are employed to discuss the spatial variability of nitrate (NO3 −), identification of NO3 − pollution sources and its potential pollution paths based on 112 water samples in Tongchuan region on the middle edge of Loess Plateau of China. Results indicate that the NO3 − concentrations ranged from 0.25 to 173 mg/L, 20.54% of water samples exceeded the Standards for Drinking Water Quality from China (≤44 mg/L), and 15.18% of samples exceeded the Guidelines for Drinking Water Quality from World Health Organization (≤50 mg/L); the water samples with high NO3 − concentrations primarily concentrated in river valleys, and the types were mainly pore waters and individual surface waters. Sewage or manure from livestock effluent was the primary sources of NO3 − in some surface waters, pore water samples with high NO3 − were closely related to agricultural input or a mixing process. Groundwater recharges from surface water and vertical infiltration in river valleys with shallow groundwater depth were considered as the potential pollution paths for NO3 − ions in the study area based on nitrate nitrogen distribution in soil profile. Driven by concentration difference and local seepage system, these NO3 − ions diffused around and groundwater buried in great depth was consequently polluted. Therefore, river valleys should be listed as the prior protection areas in loess regions; in addition, the time lag between land use changes and groundwater quality response should also be paid attention to in nitrate pollution prevention, particularly in the thick unsaturated zones.