The Source Of Nitrate Pollution In Groundwater In An Outer Suburb Of Beijing

Taking an outer suburb of Beijing as the research object in the study, there were many potential sources of nitrate contamination in groundwater, and the groundwater had been polluted by nitrate. Based on the analysis of hydrogeological condition in the study area, the water chemistry characteristics of the groundwater in the study area were summarized by statistical analysis method, according to the data in different periods. On the basis of the isotopic signatures of nitrogen and oxygen, the sources of nitrate pollution were determined qualitatively, and the contribution rate of different pollution sources was calculated by using the mixed isotope model. The main research results are as follows.（1） The main hydrochemical type in groundwater was HCO₃-Ca·Mg, followed by the HCO₃·SO₄-Ca·Mg, and HCO₃-Ca·Mg·Na. According to the analysis of the spatial distribution and temporal variation characteristics of TDS, Cl-, and NO₃- in groundwater, it was concluded that the concentration distribution of the three indexes was consistent, which was higher in the central and southern of the study area.（2） The concentration of DO in groundwater was higher in the study area, and the value of δ¹⁵N did not increase obviously with the decrease of NO₃--N, which showed that the condition of the aquifer was not suitable for the occurrence of denitrification. The value of δ¹⁵N-NO₃- and δ¹⁸O-NO₃- in groundwater between each sample point was very close to each other（ except for DW11 and DW12 in wet season）, indicating no significant effect on the sources of pollution. Preliminary judgment can be drawn that the main sources of NO₃- in groundwater include ammonia fertilizer, soil nitrogen, animal manure and sewage.（3） In the top and front area of the alluvial-proluvial fan, the concentration of NO₃--N in groundwater was in the range from 3.91 to 16.22 mg/L. The relationship between δ¹⁵N and NO₃--N, the correlation of NO₃--N and Cl-, and factor analysis results showed that the groundwater in this area was mainly affected by sewage in the dry period. In the wet period, in addition to sewage, Cl-/Br- vs Cl- concentration in groundwater also showed the influence of septic tanks, mainly leaching by precipitation on animal manure made groundwater NO₃--N concentration rising.（4） In the plain part of the alluvial-proluvial fan, the sampling sites were located in the farmland and densely populated areas, the main sources of nitrate in groundwater were sewage and fertilizer（manure and chemical fertilizer）. Cl-/Br- vs Cl- concentration, the relationship between δ¹⁵N and NO₃--N showed that the sources of nitrate were sewage and fertilizer（manure and fertilizer） during wet period, the pollution sources followed the order of fertilizer > sewage or manure > soil nitrogen. The relationship between δ¹⁵N and NO₃--N, and factor analysis results showed that in the dry period the main sources of nitrate in groundwater was chemical fertilizers, followed by sewage.