The origin and fate of arsenic in coalbed natural gas produced water ponds

Coalbed natural gas (CBNG) produced water contains small amounts of trace metals which can accumulate over time in produced water detention ponds. Within the Powder River Basin (PRB) of Wyoming, high concentrations of trace metals in pond water and their effect on shallow groundwater are potential concerns. A pond with a maximum arsenic concentration of 146 mug/L was studied in detail to determine the potential for groundwater pollution and to explain the cause for the high concentration of arsenic. Outfall and pond water quality, subsurface hydrology, infiltration, isotope signatures, and trace metal balances were examined to assess the hydrology and geochemistry of the pond. The results indicated minimum or no infiltration of pond water and no measurable contamination of the shallow groundwater. The high arsenic concentrations in the pond were determined to be the result of semi-continuous inputs of CBNG produced water with low arsenic concentrations (0.20 to 0.48 mug/L) over a six year period, exasperated by low pond volumes during drought conditions. Because of reduced infiltration and high evaporation rates, arsenic became concentrated over time. Reduced infiltration was most likely caused by the high sodium concentration and high Sodium Adsorption Ratio (SAR) of the CBNG produced water, which disrupt soil structure. The findings for the pond and the techniques used may serve as a template for future impact assessments of other CBNG produced water ponds and are relevant for the approximately 4000 ponds currently permitted in the PRB and for future ponds. Further studies are recommended in the use of playa landforms to store marginal quality produced water.