Abstract
An inflatable Packer-Straddle Pump Assembly was used to collect depth-specific water samples from an experimental bore hole at intervals of 6 m between 28 and 58 m. Based on chemical and microbiological analyses, several redox zones in the aquifer were identified. For example, at depths of 28, 40, and 46 m, dissolved arsenic in the range of 18 to 23 µg/L and Fe(II) in the range of 1.3 to 1.6 mg/L together with the presence of iron-reducing bacterial activity suggested that reduction of ferric oxy-hydroxides was able to mobilize arsenic from sediment to groundwater. On the other hand, conditions were predominantly nitrate- reducing at 34 and 52 m where dissolved arsenic was below detection level and Fe(II) was around 0.2 mg/L. Although arsenic is present in the sediment throughout the depth of the bore hole, it was mobilized only under an iron-reducing condition. Arsenic mobilization was not observed under nitrate-reducing conditions at 34 and 52 m with a high nitrate concentration of about 4 mg/L. Nitrate from agricultural runoff accumulates in the surface water bodies and spreads in the aquifer through predominantly horizontal regional groundwater flow. Low hydraulic conductivity due to the presence of clay limits the flux of nitrate at 28, 40, and 46 m where conditions are iron reducing.
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