Hierarchical cluster analysis of arsenic and fluoride enrichments in groundwater from the Datong basin, Northern China

Abstract

To better understand the occurrence of high F and As in groundwater of the Datong basin, a total of 486 groundwater samples were collected for hierarchical cluster analysis (HCA) of eighteen hydrochemical parameters. Groundwater samples can be divided into thirty-six and nineteen groups for shallow and deep groundwater, respectively. Results show that high F samples in shallow groundwater contain F as high as 22mg/L and mainly occur in the discharge area in the basin center, and the highest F concentration of deep groundwater samples is 8.3mg/L which mainly occur in the western mountain front area. The groundwater with elevated HCO3 concentration favors F enrichment in the Datong basin. Nearly all of the high F samples are oversaturated with respect to calcite and undersaturated with respect to fluorite, indicating that fluorite solubility is a limit for F enrichment. Besides, evapotranspiration has a stronger effect on fluoride enrichment, especially for the shallow groundwater. For the both F and As enrichment samples in deep groundwater, the desorption of Fe-(hydr)oxides is suggested to be the major mechanism. High As samples of shallow and deep groundwater mainly occur between Senggan River and Huangshui River. The highest arsenic concentration reaches up to 469μg/L, and all samples of high arsenic groundwater have low concentrations of NO3 and SO4, indicating the prevailing reducing conditions in the aquifer system at Datong. The reductive dissolution of Fe-(hydr)oxides driven by sulfate reduction and biodegradation of organic matters is postulated to be the major process controlling arsenic enrichment in groundwater.