Hydrogeochemical characterization and quality assessment of groundwater based on integrated-weight water quality index in a concentrated urban area

Clean and safe groundwater is the basic guarantee for social and human sustainable development. With the increasing groundwater pollution, it is essential to characterize hydrogeochemistry and assess groundwater quality accurately for water supply purpose. In this study, investigation of groundwater was conducted in the urban area of Xi’an, which has a more than 3100 years glorious city history. 97 groundwater samples were collected from domestic tube wells for physical and chemical analysis. Results showed that groundwater in the study area was predominantly the HCO₃–Ca and HCO₃–Ca·Mg type, which were controlled by multiple processes of water-rock interaction, evaporation, cation exchange etc. Some samples fall in Zone 4 (mixed type) and Zone 2 (SO₄–Na type) in Piper diagram, indicating the complex influence of both rock-water interactions and anthropogenic activities. To assess groundwater quality reasonably, an innovative integrated-weight water quality index (IWQI) was proposed by combining objective and subjective weights through additive model. The calculated weights showed that integrated weights balanced the relationship between subjective expertise about impacts of chemical components on human health risk and objective entropy information of ion concentration. The high integrated weight for F⁻ (0.237), NO₂–N (0.104) and HCO₃⁻ (0.103) indicated their significant influences on groundwater quality. According to the IWQI, overall situation of groundwater in the study area was described as good, while only 9.4% of groundwater samples was of medium to poor quality and unsuitable for drinking. Investigation and historical documents data showed that this poor groundwater quality in the city centre can be attributed to the low terrain, special characteristics of loess deposit, modern pollution in recent decades and the migration of ancient pollutants over one millennia. The sensitive analysis of IWQI indicated the innovative IWQI could describe the overall water quality reliably, stably and correctly, and have the potential suitability for extensive application.