Assessment of the geochemical conditions for the release of arsenic, iron and copper into groundwater in the coastal aquifer at Yumigahama, western Japan

摘要

Arsenic contamination in groundwater is a hazard worldwide, and Japan is no exception. Arsenic has been detected in perched groundwater in the Holocene Yumigahama sandbar, where such water should be As-free due to prevailing oxidizing conditions. Groundwater drawn from shallow hand-pump wells is used for agriculture in the area. This study aims to evaluate the geochemical conditions of the Yumigahama aquifer by examining mobilization of As, Fe and Cu in soil under extreme pH conditions, and also the influence of fermentation. Soil samples were eluted with 1M HC1 and 0.5M NaOH, and fermented with 0.28M glucose. Groundwater and treated samples were analysed for As, Fe and Cu by atomic absorption spectrophotometry. Bulk soils were analysed by X-ray fluorescence. Groundwater has near-neutral pH (6.1-7.9) and As, Fe and Cu ranges are 0.01-42.2μg/L, 0.01-65.4mg/L and 0.7-73.6μg/L respectively. Based on an average of 5.4mg/L As in the soil, about 5% of As was eluted after treatment with HC1 or NaOH, whereas 0.3% was removed by glucose fermentation. Lower pH favoured Fe elution (0.1%-0.6% of 36,354mg/L Fe in the soil), whereas at higher pH, 1% to 5% of the 10.7mg/L of Cu in soil was easily mobilized. From the experimental results, As is probably hosted by FeOOH when acidic conditions prevail and by organic matter in alkaline conditions, whereas Fe is released from FeOOH (lower pH). Nitrate and phosphate from fertilizers probably contribute to modification of the aquifer redox conditions. Change of redox conditions due to agricultural practice, presence of even low concentrations of organic compounds and microbial activity can influence the release of As, Fe and Cu from soil into water.