Causes of groundwater hydrochemical characteristics and assessment of health risk to humans in the karst area of southeastern Chongqing

ZHANG Guiquan; ZHU Aiping; REN Kun; ZENG Jie; PENG Cong; PAN Xiaodong; LIANG Jiapeng; SONG Chen; ZHANG Wenping; TANG Weiwei; JIANG Dansi

doi:10.11932/karst20250506

Abstract：This study conducted comprehensive water sampling and testing at 71 groundwater sites in a typical karst area of southeastern Chongqing during the summer.The research utilized methods including Gibbs diagrams,ion ratios analysis,multivariate statistical technique,the entropy-weighted water quality index(EWQI),and assessment models of human health risk to investigate the hydrochemical composition,water quality status,and health risks of groundwater in the region. The results indicate that the hydrochemical type of groundwater in the karst area of southeastern Chongqing is predominantly HCO3-Ca(61.97％),followed by mixed types of HCO3-CaMg(30.98％)and a small proportion of HCO3SO4-Ca(7.04％).The dominant cations is Ca2+,while the primary anions is HCO3-.The average cation concentrations follow the order:Ca2+＞Mg2+＞Na+＞K+(60.67 mg·L-1,10.09 mg·L-1,2.45 mg·L-1,and 1.23 mg·L-1,respectively),while the average anion concentrations are ranked as HCO3-＞SO42-＞NO3-＞Cl-(195.04 mg·L-1,26.95 mg·L-1,9.04 mg·L-1,and 3.69 mg·L-1,respectively).The pH values range from 6.98 to 8.17,with a mean of 7.58,indicating weakly alkaline conditions.The total dissolved solids(TDS)range from 101.48 to 578.44 mg·L-1,with an average value of 300.13 mg·L-1.The coefficients of variation(CV)of the major ions Cl-,SO42-,and Na+in the groundwater exceed 100％,suggesting complex sources. The hydrochemistry of groundwater in the karst area of southeastern Chongqing is predominantly controlled by rock weathering.The ions of K+and Na+primarily originate from dissolution of and halite,with a limited role played by cation exchange.In some localized areas,elevated Na+levels are influenced by domestic sewage inputs.Ca2+,Mg2+,and HCO3-are mainly derived from the weathering and dissolution of carbonate rocks such as calcite and dolomite.Cl-is partly sourced from halite dissolution but predominantly reflects anthropogenic contamination from domestic wastewater.NO3-is primarily attributed to the application of agricultural fertilizer.SO42-originates from the dissolution of sulfur-bearing minerals,industrial and mining activities,and gypsum dissolution,with the gypsum dissolution contributing minimally. The water quality evaluation of the karst area in the southeastern Chongqing was conducted using the EWQI method.The EWQI values range from 7.29 to 182.68,with an average value of 21.83,indicating overall high water quality.Specifically,the proportions of high-quality,good,and poor are 92.96％,5.63％,and 1.41％,respectively.Correlation analysis revealed a strong relationship between TDS,TFe,Mn,Al,and the EWQI.Multivariate linear regression further confirmed that groundwater is mainly affected by TFe,TDS,Al,Mn,and NO3-.Some sampling points exhibited poor water quality(Class Ⅳ),greatly affected by elevated levels of TFe,Mn,and Al.According to the Groundwater Quality Standard(GB/T 14848-2017),the single-factor evaluation classified water quality as Class Ⅰ to Ⅴin proportions of 7.04％,47.89％,32.39％,9.86％,and 2.82％respectively.Overall,groundwater quality is mainly affected by industrial and mining activities associated with local mining operations. The assessment of health risk of groundwater in southeastern Chongqing show that most of the risks are within safe levels,with an exceedance rate of 4.23％.However,in some areas,TFe and NO3-surpass the comprehensive health risk threshold for children(HI=1),indicating elevated hazard risks,with the highest risk value of NO3-reaching HInitate=2.17277.Therefore,these areas require increased attention.