Water Resources Research Act Program

Details for Project ID 2009MT191B

Quantification of Coal-Aquifer Baseflow in Montana Rivers Using Carbon Isotopes

Institute: Montana
Year Established: 2009 Start Date: 2009-03-01 End Date: 2011-02-28
Total Federal Funds: $29,000 Total Non-Federal Funds: $62,280

Principal Investigators: Elizabeth Meredith

Project Summary: Coalbed methane production requires the extraction of large quantities of water from coalbeds. Ground water in Powder River Basin coalbeds generally flows north from Wyoming to discharge areas in Montana. However, no research has quantified the amount of water contributed by coalbeds to surface water flows in Montana. Because water produced by coalbed methane wells is managed to maximize evaporation and transpiration (consumptive use), the withdrawal of water from coal aquifers will reduce surface flows. It is important to quantify the contributions of these coal aquifers in order to assess the potential impact of coalbed methane production to surface water resources in southeastern Montana.

The objective of this project is to quantify the amount of ground water contributed by the Knobloch coal aquifer to the Tongue River, Powder River, Rosebud Creek and Otter Creek, in the Powder River Basin of southeastern Montana. Quantification will be accomplished through isotope mixing models based on naturally-occurring, stable isotopes of carbon. Carbon isotopes (in dissolved inorganic carbon form) have been shown to effectively fingerprint water associated with coalbed methane production because isotope fractionation in water associated with methane generation is very different from surface water.

Samples will be collected from selected locations along the four drainages, including up- and down-gradient from the Knobloch coal outcrop and near the headwaters of the streams. The Knobloch coal will be sampled from several Montana Bureau of Mines and Geology monitoring wells. An isotope mixing model will be used to quantify the contribution of Knobloch coal aquifer discharges to baseflow in the four drainages. Results will include numerical calculations of how much water in four Montana streams originates from the Knobloch coal aquifer and estimates of how much water in these streams originates from coalbeds in general. The results from this study will benefit many types of water users, including irrigators, recreationalists, regulators and aquatic species.