Coalbed Methane Produced Water In The Western Us

of CBM produced waters are discharged to the land or surface water or are applied as irrigation water to soils.

Groundwater

The potential effects on groundwater quality and quantity are related to groundwater withdrawals and infiltration from surface disposal impoundments that store CBM produced water. The extent of groundwater drawdown depends on the density of wells, the rate of pumping water from the coalbed by CBM operators, and the length of time that pumping has been ongoing. The time for the CBM-bearing aquifer to return to its original water pressure or level is a function of the extent of drawdown; site-specific aquifer characteristics such as porosity, permeability, and depth to the coalbed aquifer; climatic and hydrogeological conditions; and proximity and connectivity to recharge sources. Due to the distance between the deep coalbeds and the shallow groundwater aquifers and to aquifer compartmentalization, CBM extraction in the San Juan, Raton, Uinta, and Piceance basins is unlikely to cause lowering of the water table in shallow alluvial aquifers. However, research in the Powder River Basin, which has relatively shallower coal seams, has shown that hydrostatic heads in the coalbeds have been lowered between 20 and 625 feet in CBM production areas. Estimated recovery of groundwater levels in areas of the Powder River Basin where CBM production has ceased in recent years varies from 65 percent in the center of the area near the locus of the CBM wells to 87 percent near the edge of the basin over 10 years. This drawdown has been measured only in the coalbeds from which CBM has been extracted and which are not necessarily the same as groundwater aquifers used extensively as water supplies. An important characteristic that has not yet been thoroughly substantiated is the degree of local hydraulic connection between coalbed aquifers from which CBM and water are withdrawn and other aquifers in the Powder River Basin. Although an EPA study found no conclusive evidence of drinking water contamination by hydraulic fracturing fluid injection associated with CBM wells in a 2004 study (see Box 2.1), lack of comprehensive datasets and studies, and continued development of domestic oil and gas fields, including CBM, since the release of that study have continued to focus attention on hydraulic fracturing. The EPA is conducting a broader analysis of the potential effects on groundwater quality and public health from hydraulic fracturing throughout the entire oil and gas industry.

A primary mode for disposal of CBM produced water, especially in the Powder River Basin ofWyoming and somewhat in the Colorado portion of the Raton Basin, is in surface impoundments. Infiltration and percolation of impounded water can dissolve and mobilize preexisting salts or naturally occurring constituents such as sulfate, selenium, arsenic, manganese, barium, chloride, nitrate and soil solution TDS below impoundments. Studies in

Wyoming indicated no apparent change in groundwater quality as a result of interaction with underlying shallow alluvial groundwater for a substantial majority of impoundments studied; an increase in TDS, selenium, and sulfate in groundwater beneath some impoundment facilities; and improved water quality beneath a small fraction of impoundments. A monitoring well network and a monitoring program are integral parts of CBM produced water management plans that include disposal in surface impoundments.

Surface Water

The potential effects of CBM production and produced water discharge to surface water include water quality effects to perennial and ephemeral drainages and stream depletion from dewatering of coalbed aquifers. Studies that have been conducted on the effects of CBM produced water discharge on perennial stream water quality have produced equivocal results. Background (historical) data prior to CBM development are limited, making assessing the influence of climatic influences on in-stream flows difficult. Specific conductance and SAR of water resources may not be the most meaningful diagnostic or representative measures of CBM produced water influence on receiving water bodies, particularly in the Powder River Basin. Isotope analyses may provide more representative characterization of the influence of CBM produced water on groundwater and surface water.

Carbon isotopic "fingerprinting" studies have distinguished the presence of CBM produced water in the Powder River near areas of CBM production. These carbon isotope fingerprints become less evident as downstream flows are influenced by tributaries that are not themselves influenced by CBM produced water discharges. Use of isotope ratios or other isotope signatures of CBM produced water presence and effects may be useful to monitor and assess the presence and effects of CBM produced water on surface water and groundwater resources.

The committee was unable to find any published data or reports documenting measurable stream depletions due to CBM water production in the basins studied. The reliability of results from stream depletion modeling studies for the Piceance, Raton, and Northern San Juan basins in Colorado has not yet been evaluated against actual stream measurements in areas of CBM production. Similarly, the general assumption of "tributary" groundwater as a primary model input does not comport with the data available from the San Juan Basin.

Soil Quality and Agricultural Production

Several site-specific research studies and natural resource inventories have documented that application of CBM produced water to some soils in of the Powder River Basin has altered plant ecology and resulted in adverse soil with ecological, chemical, and hydrologi-

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