Source Apportionment of Inorganic Nitrogen in Shallow Groundwater in the Chengdu Plain Area Based on Multiple Quantitative and Qualitative Methods

With urban construction, economic development, and population growth, groundwater quality has become an important factor restricting the green development of Chengdu's cities,the accumulation of nitrates and nitrites in the body can harm human health in particular. However, the source of inorganic nitrogen is complex, so it is of great significance to accurately understand the formation process of inorganic nitrogen in groundwater, for utilizing and managing groundwater resources scientifically and rationally. Based on water chemical analysis, the self-organizing neural network (SOM), combined with principal component analysis(PCA), the absolute principal component score-multiple linear regression receptor model (APCS-MLR) and positive definite matrix decomposition (PMF) are used to quantitatively identify the influence of anthropogenic and natural factors on groundwater quality in the basin. The results show that: (1) The average nitrate nitrogen content in the study area are 2.59mg/L, the exceedance rate is 2.44%, the average ammonia nitrogen content is 0.1mg/L, the exceedance rate is 6.1%, and the average nitrite content is 0.02mg/L. (2) According to the characteristics of SOM, the number of clusters 1, 2 and 3 affected by geological background is much larger, indicating that the groundwater quality in the study area is greatly affected by the geological background. (3) According to the PCA/APCS-MLR result, the contribution rates of evaporative salt dissolution, reduction, domestic pollution, silicate dissolution and unknown sources to groundwater quality are respectively 31.63%, 13.02%, 9.17%, 32.15% and 14.03%, and natural sources are the main factors(the comprehensive contribution rate is 76.81%). From the perspective of individual influencing factors, the contribution rate of domestic pollution to NO ₃ ^- -N in the study area is the highest (38.85% contribution rate), and the reduction effect is the main influencing factor of NH ₄ ⁺ -N (49.72%). (4) According to the PMF result, the contribution rates of water-rock interaction, domestic pollution, reduction, waste liquid leaching and soil dissolution filtration to the groundwater quality in the study area are 32.28%, 11.51%, 16.01.20.68% and 19.52%, respectively, and natural sources are the main factors affecting the quality of groundwater in the study, with a contribution rate of 67.81%, and the impact of human activities are not small, with a contribution rate of 32.19%. From the perspective of individual influencing factors, domestic pollution (Factor2) contributed the highest, human activities are the main factors affecting the content of NO ₃ ^- -N in groundwater in the study area, and water-rock interaction (Factor1) is mainly from natural sources in the groundwater in the study area, and human activities have little impact. Therefore, the combination of qualitative and quantitative methods can complement and verify each other, so as to improve the accuracy and comprehensiveness of the analysis results and more effectively reflect the actual situation of the study area, and providing a reliable reference for subsequent water environment management in the study area.