Potential Arsenic Contamination of Sediments in Coal Bed Methane Produced Water Retention Ponds

The state of Wyoming, especially the Powder River Basin, is among the most productive areas in terms of coalbed methane (CBM) extraction in the country. CBM extraction requires high quantities of aquifer water to be pumped to the surface. This decreases water pressure in the coal-seam and allows natural gases to be extracted. The pumped water is then directed into a retention pond. The produced water is suspected to contain a trace amount of arsenic (As) that can be concentrated into pond sediments. Arsenic is a highly toxic element and the potential for contamination is problematic, as As can contaminate ground water from a sediment source. The objective of this study is to understand the potential for arsenic contamination of the sediments of the CBM produced water ponds located in the Powder River Basin, WY. Sediment cores (up to 1 m long) were collected at multiple sites in the Powder River Basin and sectioned in the lab. Arsenic in the samples was extracted and the concentration in the extract was determined using ICP-MS. In addition, pH and electrical conductivity (EC) were also measured. Most of the samples had pH of 7-8 and with increasing sediment depth, no significant trends were observed. In contrast, for a few sites, the pH of the samples dropped from ~8 on the surface to ~4 at the bottom of the cores. EC and arsenic concentration measurements are ongoing.