Coalbed Methane Produced Water In The Western Us

drawdowns (AHA and GEC, 2002). The different sandstone zones within the Wasatch did not show drawdown of Wasatch water levels except for a few limited areas, suggesting limited connectivity of the units. Thus, although pumping of water in Wyoming has been much more aggressive and local to the CBM wells compared to Montana, the conclusions of this analysis in Wyoming are consistent with those reported by Wheaton and Meredith (2009) and in various MBMG reports (see references above).

Importance of Fossil water

Determining the extent to which CBM produced water is actually fossil water (see Chapter 2) is also important to analyzing the effects on groundwater drawdown. Multiple lines of evidence suggest that CBM produced water in the San Juan Basin and potentially also in the Raton Basin is fossil water with an age of thousands to tens of millions of years. Prior to extraction, the water rested underground in aquifers in these basins over geological timescales, without interacting with or being affected by surface events such as rainfall. Recharge of the San Juan and Raton coalbed aquifers is low because of hydrogeological compartmentalization and the fact that evaporation usually exceeds precipitation in the dry western climate. Data from the Powder River Basin suggest that some of the CBM aquifer water there is also likely at least thousands of years old in aquifers with limited connectivity (see Chapter 2).

Long-term implications of mining fossil water have not been studied or included as part of the discussion of management approaches for CBM produced water. Similarly, basin-wide and comprehensive analyses of the degree of hydraulic connectivity between CBM aquifers and other groundwater aquifers are needed to understand the degree to which CBM waters may be considered "fossil." Such studies have not been thoroughly completed for any basin except the San Juan.

Hydraulic Fracturing

In CBM operations where hydraulic fracturing is regularly used, expressions of concern by the public prompted a study by the U.S. Environmental Protection Agency (EPA) to assess the potential for contamination of underground sources of drinking water (USDWs) as a result of the practice (see also Box 2.1). The study (EPA, 2004) found that, while fracturing fluids contain various chemicals, the identities of which are not generally a reporting requirement for operators, no conclusive evidence of drinking water contamination by hydraulic fracturing fluid injection was found to be associated with CBM wells. Lack of comprehensive datasets and studies, and continued development of domestic oil and gas fields since the release of that report, have continued to focus attention on hydraulic fracturing. The EPA has announced it is conducting a broader analysis of the potential effects on water quality and public health from hydraulic fracturing throughout the entire oil and gas industry (EPA, 2010).

CBM Impoundments and Produced Water Quality

Surface impoundments hold produced water until it evaporates or infiltrates into the subsurface, or they store the water for future beneficial uses (see Chapter 4). In 2008, 64 percent of the CBM produced water in the Wyoming portion of the Powder River Basin was managed in surface impoundments (see Box 4.1). Surface impoundments are not used extensively in the other western CBM basins or in the Montana portion of the Powder River Basin (see Table 4.1 and Chapter 3), although some impoundments (lined and unlined) are used in the Raton Basin in Colorado. Impoundments strictly for storage or disposal (evaporation or infiltration) are no longer permissible in Montana. In the Raton Basin the Colorado Oil and Gas Conservation Commission (COGCC) has indicated some issues related to leaks or seepage from the impoundments either to the surface water or groundwater, but the committee was not able to identify specific data on the extent of any effects of seepage from the impoundments (Ash and Gintautas, 2009). Thus, the remaining discussion focuses specifically on impoundments in the Wyoming portion of the Powder River Basin.

As of2005, about 2,500 of the approximately 3,000 CBM impoundments in the Powder River Basin were "on-channel" impoundments sited within a water feature (including perennial and ephemeral streams and rivers, dry washes, marshes, and lakes) or within the floodplain or alluvium of a water feature. Roughly 200 impoundments were "off-channel" and unlined, with the intent to recharge underlying groundwater. The remaining offchannel impoundments are lined to reduce, minimize, or prevent leakage and infiltration into underlying soils. According to Wyoming state policy, off-channel impoundments may not be sited within 500 feet of a designated water feature (and must be located at least 500 feet from the outermost floodplain or alluvium; Fischer, 2005a).

In Wyoming, impoundments were initially permitted for the purpose of storage of produced water, although the intent was to facilitate disposal by evaporation, enhanced by atomization, infiltration, or for storage for land spreading or irrigation. Under Wyoming DEQpermitting provisions, a limited number of impoundments were permitted for the purpose of infiltration. Wyoming DEQpresently permits some off-channel impoundments for the purpose of infiltration, but not necessarily with the intent of recharging underlying groundwater. Changes to the guidelines for construction and monitoring of unlined impoundments in Wyoming are outlined in Chapter 3.

Potential groundwater effects from off-channel CBM produced water impoundments relate to the leaching of salts, metals, or metalloids that occur naturally in soils in or under the impoundments and that may be dissolved and mobilized by CBM produced water infiltrating beneath the impoundments (McBeth et al., 2003; Jackson and Reddy, 2007;

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