ARSENIC ASSOCIATION IN SEDIMENTS FROM SHALLOW AQUIFERS OF NORTHWESTERN HETAO BASIN, NORTHERN CHINA

High As groundwater (up to 1000 μg/L) is widely distributed in the northwestern Hetao Basin, an arid region with slow groundwater flow and with the most serious endemic arsenicosis area in Inner Mongolia. There are about 76,000 people exposed in 35 villages from 9 towns. Most water samples have elevated salinities, with Cl and/or HCO₃ as the dominant anion and Na as the dominant cation. High concentrations of As in shallow aquifers are associated with strongly reducing conditions, as evidenced by high concentrations of dissolved organic C (DOC), NH₄⁺, H₂S, As(III), dissolved CH₄, and relatively low concentrations of NO₃^- and SO₄^2-. In Na-HCO₃-dominated groundwaters, As, B and F were simultaneously enriched.

Bulk chemical analysis of sediment samples from two 50 m boreholes was completed for As-contaminated aquifers, the groundwaters of which have As concentrations more than 800 μg/l. Sediments consist of sands, silts, and clays that are oxidized in the upper 20m and reduced below. Total As concentrations in 47 sediment samples collected from two high-As boreholes have a range of 7.7-34.6 mg As/kg (Shahai village) and 6.8-58.5 mg As/kg (Tuanjie village). The highest As concentrations were found at 15-25 m depth. Another borehole was drilled for comparison in Erdaoqiao village where groundwater As concentrations are quite low (2 μg/l). The As concentration ranged 3-21.8 mg/kg. The variation of As contents in the sediments with depth from Shahai village were correlated with the concentrations of Fe, Sb, B, V, total C, and total S. Generally, the abundance of elements varied with grain size, with higher concentrations in finer fractions of the sediments.

Up to one-third of arsenic in the sediments could be extracted by ammonium oxalate, suggesting that Fe oxyhydroxides may be the major sink of As in the aquifer. Only 2% of the arsenic is hydroxylamine hydrochloride-extractable and taken to be associated with Mn oxides. Results show that approximately 35 % of total As was extracted by PO₄ (strongly absorbed As), also a higher percentage of oxalate-extractable As (As in amorphous iron oxides), concentrated HNO₃ and H₂O₂ extractable As (As with sulfides and organic matter), HCl-extractable As (acid volatile sulfides, carbonates and amorphous metal oxides). The release of As into solution is likely caused by reductive iron dissolution.