A Study on the Hydrochemical Evolution Property and Pollution Source Attribution of Groundwater in Highly Urbanized Areas: A Case Study of Shenzhen City

Accurate identification of groundwater pollution sources is crucial for the socio-economic development of a region. In highly urbanized areas, where human activities have a pronounced impact on groundwater, however, the hydrochemical evolution patterns and sources of pollutants remain unclear. Taking Shenzhen, a highly urbanized city in China, as a case study, this research employed a combination of multivariate statistical techniques and the Positive Matrix Factorization (PMF) model to elucidate the hydrochemical evolution and quantitatively parse the pollution sources of groundwater in such regions. The results revealed that the pH of groundwater in the study area ranged from 4.24 to 7.31, indicating weak acidity to neutrality. The exceedance rates for pH, NH4+, COD, Mn, and Fe were as high as 67.1%, 44.3%, 44.3%, 34.3%, and 31.4%, respectively. The Water Quality Index assessment revealed that 32.9% of the groundwater samples were classified as poor, highlighting the significant impact of human activities. Land-use types significantly affected groundwater quality, with urban areas exhibiting higher concentrations of the COD, NO3−, Mn, and Fe compared to agricultural and forested areas. The predominant hydrochemical type of groundwater in the study area was HCO3·Cl—Ca·Na, with rock weathering (primarily silicate weathering) being the dominant process controlling groundwater chemistry. The PMF model identified three major pollution sources in the highly urbanized region: domestic and industrial wastewater, enhanced water-rock interactions leading to the release of hydrochemical components, and agricultural fertilizers, contributing 43.9%, 37.0%, and 19.1% to groundwater pollution, respectively. Geostatistical spatial interpolation techniques demonstrated that in urban areas, groundwater quality was primarily controlled by the discharge of domestic and industrial wastewater, while in agricultural areas, excessive fertilizer application was the main driver of groundwater degradation. These findings provide a scientific basis for groundwater pollution prevention and sustainable utilization in highly urbanized regions.