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Management and Effects of Coalbed Methane Produced Water in the Western United States
does not have active CBM production at present. In examining the known and potential effects of CBM produced water discharges on the environment, the committee focused its efforts on the Powder River Basin, with its relatively shallow coalbeds and less saline produced water and management of CBM produced water primarily through disposal in surface impoundments and surface water.
CBM PRODUCED WATER HYDROGEOLOGY: THE IMPORTANCE OF ESTABLISHING HYDRAULIC CONNECTIVITY
The degree of hydraulic connectivity between water-bearing coalbeds targeted for methane production and shallow alluvial or water table aquifers that support human activities and natural habitats is an important factor in determining the consequences of water withdrawal during CBM extraction. In this context the concept of the age of the water in a coalbed is also significant because the age, or length of time the water has resided in the coalbed, is one indication of the degree to which the coalbed aquifer is connected to surface water and shallow groundwater. “Old” or “fossil” water in coalbeds is water that has not been replenished in the coalbed by infiltrating precipitation within human lifetimes or even thousands to millions of years. This lack of natural recharge may be due to discontinuities within coalbeds or between a coalbed and associated geological units, and/or to the location of recharge areas far from downgradient portions of a coalbed. Such fossil water can be considered a “nonrenewable” resource.
Thorough in situ physical studies to determine the degree of hydraulic connectivity between CBM aquifers and shallow groundwater aquifers have been completed only in the San Juan Basin. The data, including geochemical analyses to determine the age of the water, establish a lack of hydraulic connectivity between CBM aquifers and shallow groundwater resources. The great depths from which CBM and water are extracted in the Uinta and Piceance basins, relative to shallow groundwater systems in these areas, make widespread hydraulic connectivity unlikely. Existing data in the Raton and Powder River basins suggest a lack of widespread hydraulic connectivity between CBM aquifers and other groundwater aquifers, but these studies have been limited in scope and have been generally site specific. Consequently, the connectivity between coalbed water and other water resources is not well defined in most western CBM basins and leads to uncertainty in the consequence of long-term produced water withdrawals on other aquifers.
Mathematical models have been used to characterize the effects of CBM water withdrawal on surface water flows and shallow groundwater levels but have not been calibrated using actual measurements of drawdown in the surface water bodies or shallow aquifers. Such measurements can provide reliable inputs against which model results can be tested. Current mathematical models cannot yet characterize complex water/rock interactions, differences in hydraulic properties, or boundary conditions present in CBM basins with con-