Hydrochemistry of Fluoride in Groundwaters from the Permo- Triassic Aquifer System of Central Shaanxi Province, Northwest China

摘要

The district of Chengcheng lies within Weinan city in central Shaanxi Province. It suffers from serious water shortages, a dilemma that is amplified by elevated concentrations of fluoride in the groundwater, a primary source of domestic water supply. To determine the extent and origin of the fluoride problem, a study was undertaken involving 144 groundwater samples collected from two aquifers in the region: the shallow Quaternary overburden aquifer (QLB) (10 samples) and the more widely utilized Permo-Triassic-fractured rock aquifer (PTF) (134 samples). Spatial analysis of the hydrochemical data shows that concentrations of fluoride in the PTF increase from north to south, generally following the direction of groundwater flow. 50 % of the samples show fluoride in excess of the 1.0 mg/L drinking water quality standard. Statistical analysis of data from the Permo-Triassic-fractured rock aquifer shows the fluoride to correlate positively with Ca~(2+), Na~+, and HCO_3~- and suggests mineral dissolution involving fluorite (CaF_2) to be the primary source of the elevated fluoride. Analysis using PHREEQC reveals the groundwaters to be oversaturated with respect to calcite but undersaturated with respect to fluorite, and support such a hypothesis. Significantly, a positive relationship between NO_3~- and F~- for the PTF groundwaters suggests that trace amounts of fluoride in applied fertilizers may provide an additional source of fluoride. 28 water samples were also investigated using stable isotopes (δD and δ~(18)O) and reveal that aquifer recharge is primarily due to irrigation water derived from the Shibaochuan Reservoir at high elevation to the north west of the study area, the irrigation water undergoing significant evaporation prior to entering the aquifer. The concentrating effect of evaporation also contributes to the elevated fluoride problem.