Aqueous phase arsenic in weathered shale enriched in native arsenic

Soils and geologic deposits derived from marine shale often contain elevated levels of arsenic. The source of arsenic is iron sulfide minerals. Oxidative weathering breaks down the iron sulfide minerals and potentially releases undesirable levels of arsenic into the surrounding pore water. Equilibration experiments on a weathered marine shale containing naturally elevated levels of native arsenic and on a mixture of weathered shale and calcareous till were conducted under aerobic conditions to determine how much arsenic was released into pore water and the mechanism responsible for limiting soluble arsenic concentrations. Aqueous phase arsenic released from the shale was less than 10 µg/L for aerobic conditions. Soluble arsenic concentrations were low considering the high total As concentrations in the shale (30 µg As/g shale). Over 4 times as much solution arsenic was released from the higher pH shale–till mixture compared to the acid shale alone. Adsorption and desorption was the mechanism that controlled the concentration of aqueous phase arsenic. Key words: soluble arsenate, groundwater, solution chemistry, adsorption, background concentration.