Sources Of Nitrogen In Groundwater And Influencing Factors In Intensive Agricultural Region Of Northern China

Groundwater is an important water resource for production and living in North China. Intensive agricultural production Water has a vital influence on groundwater quality in this region, especially the elevated nitrate-nitrogen concentration. In this study, Huantai County, a representative intensive agricultural region in Shandong Province in north China, was chosen as the study area. A variety of nitrate-nitrogen identification technologies and methods were applied for quantitative evaluation of the groundwater nitrate pollution levels in the study area and its space-time change pattern, to analysis the process of migration and transformation of NO3--N in the county and its influencing factors on groundwater NO3--N content, to track the nitrogen source and calculate different N sources contribution to the groundwater NO3--N. The main results were as follows:On the basis of sampling campaigns in 2002,2007 and 2010 in the studied area, another sampling campaign including 800 groundwater sampling locations was conducted in 2014. NO3--N concentrations in 800 groundwater sampling locations were normally distributed, and were in the range of 0.07～52.6 mg L-1, with the average concentration of 21.4 mg L-1.75.8% of the wells was above 10 mg L-1, which is the maximum permissible limit for drinking water set by WHO and 49.38% above 20 mg L-1. From 2002 to 2007, groundwater NO3--N concentration on average rose at an annual average of 1.1 mg L-1, rapidly rose from 11.6 mg L-1 in 2007 to 29.9 mg L-1 in 2010, and began to fall at an annual average of 2.1 mg L-1 between 2010 and 2014. The main reason for the fluctuation in NO3-N concentrations in different years was the change of the amount of applied chemical fertilizer., and with the reduction of applied nitrogen fertilization amount, groundwater nitrate nitrogen also showed a trend of gradual decline.Two models of IsoSource and SIAR were applied to analyze the contribution percentage of five potential nitrogen pollution sources (chemical fertilizers (FN), sewage (SN), animal manure (MN), soil organic nitrogen (SON) and precipitation (RN)) to groundwater NO3--N. The results showed that 45%～ 46.9% of groundwater NO3--N were from fertilizer,40.7%～51.7% from sewage,2.6%～10.7%from livestock manure and 0.8%～1.7% from rainfall in the study area. Therefore, the main contributors to groundwater NO3--N in the studied area were chemical fertilizer and sewage.Denitrification occurred in groundwater, with the characteristics of spatial heterogeneity, mainly distributed in the north of the study area, belonging to the runoff discharge area seriously polluted by nitrate, while denitrification was not obvious in the central and south of the study area. Based on comprehensively considering the denitrifying conditions of groundwater DO concentrations, the ratio of δ15 and δ18O, Rayleigh equation was used to calculate the fractionation coefficients of δ15N and δ18O when denitrification occurred, which were -15.9%o and -10.4%o, respectively, and the enrichment coefficient were -1.45%o and -0.75%o, respectively.Groundwater hydrochemistry types evolved from relatively low total dissolved solid types of HCO3--Ca2+, Mg2+ or HCO3--Na+, Ca2+, Mg2+ in the southern regions to median total dissolved solid types of HCO3-, Cl-,SO42--Na+, Ca2+, Mg2+ or HCO3-, Cl-,SO42-Na+, Mg2+ in the central regions. In the northern regions, groundwater hydrochemistry types evolved into median total dissolved solid types of Cl-, SO42--Na+, Ca2+,Mg2+ or Cl-, SO42--Na+, Mg2+. The key drivers for the different groundwater hydrochemistry types were intensive agricultural production measures, mainly including application of chemical fertilizer and flooded well and sewage irrigation, and the hydrological geology and other factors in the study area.Coupling with hydrogeochemistry of major constituents in groundwater, K-means clustering method and a tree-based classification model were used to classify potential NO3--N pollution activities in 800 groundwater sampling locations. Results showed that the 800 sampling locations (419) could be classified into three categories of agricultural production area:a combination of sewage irrigated agriculture and farming (SIA+F,419 sampling locations)、well irrigated agriculture (WIA,180 sampling locations), and sewage-irrigated agriculture (SIA,201 sampling locations), indicating that industrial wastewater and livestock farming in the region had important influence on groundwater NO3--N pollution.For groundwater NO3--N pollution in the studied area, excessive nitrogen fertilizer application was the key influencing factor, and industrial sewage and livestock farming also have had impact. In the near future, we need to further deepen the understanding of NO3--N pollution, further research the transformation processes of nitrogen in the vadose zone, and pay close attention to ecological circular agriculture in combination with planting and breeding ecological agriculture to balance the nutrients between grain and vegetables planting and husbandry to reduce agricultural non-point source pollution.