TRACKING CBM INFILTRATION POND IMPACTS UPON GROUNDWATER USING STRONTIUM ISOTOPES

Disposal of water from coal-bed methane (CBM) production is an important environmental, agricultural and legislative issue. Current practice in the Powder River Basin of Wyoming and Montana includes collecting the water in infiltration ponds. Discharge from these ponds is by means of infiltration and evaporation; there is no overland flow. Recent data indicate that these ponds have the potential to negatively impact the underlying groundwater. This observation is important because Wyoming regulations require that the class of use of groundwater be protected in all situations. Our study looks at an infiltration pond in the Coal Creek area near Ucross, WY. The infiltrating CBM water has caused an 8-fold increase in total dissolved solids (TDS). We have used hydraulic measurements, major ion chemistry, and strontium (Sr) isotope data to constrain infiltration processes, and isolated the source of the increased TDS through paste extract analysis.

CBM production and subsequent water discharge into the pond began in December, 2003. As of June 2004 over 10 million gallons of water have been added to the pond. Before filling began, monitoring wells at varying depths were installed within and surrounding the pond, and along a nearby creek. Prior to the release of CBM production water, the ground-water table was 17 feet below the floor of the pond. After 6 months of discharge into the pond, the groundwater level has risen 13 to 23 feet.

The Sr concentration in conjunction with the ratio of radiogenic 87-Sr to 86-Sr has been shown to be an effective aquifer fingerprint. This is true in the Coal Creek area where the CBM wells produce water with a ratio of 0.711 while the local ground and surface waters have a more radiogenic signature of 0.714. In the vicinity of the pond, the strontium isotope ratio varies from the 0.711 CBM input water to 0.712 within 0.25 feet of the pond floor, to 0.713 in a shallow well within the pond. Wells at 23 and 36 feet show no shift in ratio as of six months after filling began. Near the bottom of the pond the CBM Sr is already being diluted by the Sr in the soil by 1/3, while in the shallow well the dilution is by 2/3. Interpretations of these measurements could potentially explain the flow of infiltrating water, the causes of changes in water quality, and can help determine the best course of action regarding water disposal in CBM mining.