SPATIAL VARIATION IN AS AND F CONCENTRATIONS OF SHALLOW GROUNDWATER FROM THE SHAHAI TOWN, INNER MONGOLIA

As a typical town in the Hetao basin, Inner Mongolia, both high As concentration and F^- concentration were observed in shallow groundwater. The groundwater As and F^- were believed to be of geogenic origin. Primary investigation showed that high As groundwaters generally occurs in shallow alluvial-lacustrine aquifers. High F^- concentration was mainly found in Na-(Mg)-HCO₃-(Cl) types of groundwater. Evaporation would also favor F^- enrichment in groundwater from arid-semiarid areas. However, spatial distributions of As and F^- have not been sufficiently investigated.

Twenty nine wells were selected for groundwater sampling in the Shahai town. Four multilevel wells were installed for monitoring groundwater chemistry at different depths (4-23 m). Results show that F^- concentrations range between 0.38 and 2.37 mg/L. Groundwater As exhibits a big variation, ranging between 0.37~709 μg/L. There is no good correlation between As and F^- concentrations. Groundwaters with low Eh values have high As concentration. However, F^- concentration is positively correlated with pH and negatively correlated with Ca concentration. The correlation coefficient is around 0.6 between F^- concentration and pH/Ca.

Arsenic concentration is lower in groundwater sampled from 10 m below land surface (BLS) than from 20 m BLS, while groundwater generally exhibits a decrease in F^- concentration with the increase in sampling depth. Low As concentration (<10 μg/L) was found in groundwaters at depths less than 10 m. Clay interlayers occurring as lentoid bodies are present in the aquifers at depths between 10 and 20 m. These interlayers usually contain high As and organic carbon, reaching 17.6 mg/kg and 3.95%, respectively. Organic matter, acting as electron donor and/or electron shuttles, leads to reduction of Fe oxides, and subsequently the releases of As from aquifer sediments. Groundwaters at depths less than 10 m normally contain high F^- concentration (> 1.0 mg/L). In addition to evaporation, elevated fluoride levels are the result of substitution of exchangeable F^- by OH^- and HCO₃^- and fluorite solubility with the presence of low Ca concentration.

Acknowledgement

The study has been financially supported by NSFC (No.40872160) and the Program for New Century Excellent Talents in University (No. NCET-07-0770).