Natural (Geogenic) Origins of Arsenic, Cobalt, and Lithium in Groundwater from Bedrock at CCP Disposal Facilities

Coal combustion products (CCP) constituents are natural (geogenic) components of earth materials and may be released to groundwater by natural processes at CCP facilities. Alternative source demonstrations should consider natural sources of constituents of interest (COIs), such as arsenic, cobalt, and lithium. Successful demonstration of natural occurrence of CCP constituents could save millions of dollars at CCP sites by avoiding unnecessary groundwater corrective action for naturally occurring constituents. Studies at multiple CCP sites underlain by shale, chalk, and/or alluvium demonstrate these COIs occur naturally in minerals present in the geologic formations. A fraction of the geogenic COIs is commonly present in geochemically available solid phases, in quantities much greater than those measured in groundwater. Naturally occurring COIs may not necessarily be present in upgradient monitoring wells since COI concentrations in rock can be spatially variable and groundwater flow gradients can be locally upward. Past mining activities may also facilitate release of COIs from coal and bedrock. Techniques used to perform these studies on geologic materials include X-ray fluorescence; X-ray diffraction; Tescan-integrated mineral analysis for identification of mineral grains; and electron microprobe analysis and laser ablation inductively coupled plasma mass spectroscopy for COI analysis of mineral grains. These are supplemented by selective sequential extractions, cation exchange capacity, leaching tests, and hydrogeochemical evaluations. Lines of evidence supporting geogenic sources of COIs include: increase of COIs in groundwater with depth coupled with upward groundwater flow gradients; monitoring wells screened in proximity to coal seams and/or former mining sites; presence of natural host minerals for COIs in rock, including pyrite, clay minerals, calcite, and iron oxides;and aqueous leaching tests on rock to demonstrate release of geogenic COIs to groundwater.