Geochemical processes controlling trace elemental mobility in coalbed natural gas (CBNG) disposal ponds in the Powder River Basin, WY

Abstract

A concern with CBNG produced water disposal ponds is the potential migration of trace elements into the underlying shallow aquifers. Objectives of this research were to 1) examine the chemistry of CBNG produced water in outfalls and corresponding disposal ponds and 2) determine possible leaching of trace elements from the disposal pond sediments into shallow aquifers. The CBNG outfall water, disposal pond water, and sediment samples were collected from the Powder River Basin (PRB), WY during the summer months of 2006 to 2009. Water samples were analyzed for pH, major cations, anions, and trace elements. Geochemical analysis was performed using MINTEQA2 model to determine speciation, complexation, and mineral precipitation and dissolution processes to determine the mobility of trace elements in CBNG disposal ponds. Two types of SAR calculations were made: 1) practical SAR (SARp) was calculated based on Na+, Ca2+, and Mg2+ concentrations and 2) true SAR (SARt) was calculated based on MINTEQA2 calculated activity of Na+, Ca2+, and Mg2+. Sediment samples were analyzed for trace elements using toxicity characteristic leaching procedure (TCLP) to predict potential mobility of trace elements into the shallow aquifer. Research results suggest the pH of the CBNG produced water outfalls increases substantially in corresponding disposal ponds due to degassing of CO2 (carbon dioxide) from the disposal ponds. Observed high SAR values in CBNG disposal ponds compared to outfalls were attributed to increase in Na+ and decrease in Ca2+ concentrations. The decrease in Ca2+ concentration is attributed to the precipitation of calcite. Trace element concentrations such as As (arsenic), Cr (chromium), and Cu (copper) appear to increase in disposal ponds. Barium (Ba) and Mn (manganese) concentrations in the disposal ponds decreased substantially by precipitating as carbonate minerals into the sediments. Results of this study suggest that very minimum or no leaching of trace elements could occur from disposal pond sediments. However, further research is required to understand the role of SAR and sediment mineralogy in leaching of trace elements from CBNG disposal pond in the PRB, WY.