Coalbed Methane Produced Water In The Western Us

and facilitate reinjection. Shallow well reinjection is not commonly used in these basins for disposal of CBM produced water but may require treatment under UIC permitting requirements.

Although deep-well reinjection is largely used as a disposal method, ancillary benefits may include enhanced hydrocarbon recovery, depending on the formation into which the water is injected, the quality of the produced water, and the water's age (see Box 2.1). Aquifer replenishment and storage may be an ancillary benefit from shallow-well reinjection, again depending on the formation into which the water is injected (see discussion in Chapter 5). The committee did not find any evidence of adverse effects from deep-well reinjection of CBM produced water and did not know of any cases where shallow-well reinjection was used in the Raton-New Mexico, San Juan, Piceance, or Uinta Basins.

Discharge to Ephemeral and Perennial Streams and Surface Impoundments

Recalling that the outfall which discharges CBM produced water into a stream or an impoundment usually represents a combination of CBM produced water combined from several CBM wells (a well "pod") (see also Chapter 3), produced water discharge volumes and concentration of chemical constituents at outfalls may differ from day to day. Treatment of the produced water prior to discharge to either ephemeral or perennial streams or impoundments may also be required to meet permitted discharge requirements.

The only basins where substantial discharge occurs to ephemeral and perennial streams are the Powder River Basin and the Raton Basin of Colorado. Surface discharge is most common at production wells with high volumes of produced water and low concentrations of dissolved solutes (see Chapter 2), although treatment to reduce salinity and other constituents or to manage sodium adsorption ratios (SAR) may be required under the provisions of a state-specific National Pollutant Discharge Elimination System (NPDES) permit. Additional treatment may be required under provisions of an NPDES permit to reduce fluoride, barium, and/or ammonium concentrations. In many instances in the Powder River Basin, little or no treatment is required to meet NPDES standards because of the low levels of most chemical constituents. Some of the ancillary benefits of discharge of produced water to streams, depending on the quality and timing of the flows, include streamflow augmentation, stream habitat restoration, and wildlife and waterfowl habitat enhancement. Although Table 4.2 identifies possible ancillary beneficial uses associated with discharge of CBM produced water to ephemeral or perennial streams, the committee did not find significant evidence or documentation substantiating intentional streamflow augmentation, habitat restoration, or quantified aquifer recharge using CBM produced water.

A substantial majority of the produced water of the Raton Basin in Colorado is currently directly discharged into ephemeral and perennial streams. This practice is due, in part, to the lack of clearly defined regulatory protocols and also because some of the water produced in the Colorado portion of the basin is of relatively low salinity, with low TDS concentrations (see Chapter 2).

A primary mode for disposal of CBM produced water in the Wyoming portion of the Powder River Basin (64 percent of all CBM produced water; Box 4.1) is discharge of untreated CBM produced water into constructed and existing ponds, constructed storage basins, and lined or unlined impoundments. The purpose of impoundments is primarily to facilitate evaporation or infiltration of produced water into the underlying soil. Ancillary benefits of disposal of CBM produced water in impoundments may be livestock or wildlife watering. In numerous instances in Wyoming, evaporation from these impoundments may be enhanced by atomizing or high-pressure spraying of CBM produced water into the atmosphere above impoundments; atomization cannot occur downstream of the impoundment and the atomization process is designed to drain atomized water back into the impoundment. Approximately 3,500 impoundments for storage of CBM produced water have been constructed in the Powder River Basin in Wyoming (Fischer, 2005) (see also Chapter 5 for discussion of documented effects to groundwater beneath impoundments). The use of impoundments in other basins is negligible or nonexistent except for temporary storage prior to deep-well reinjection.

During the first few years of CBM development in the Wyoming portion of the Powder River Basin, operators were permitted either to construct dams in ephemeral channels or modify existing on-channel dams and impoundments for temporary storage of CBM produced water. Recognizing the potential interference of these on-channel impoundments with priority water rights of downstream water rights holders, permitting of impoundments by the State Engineer's Office may require a bypass around an impoundment to address downstream water rights. In the Powder River Basin, approximately 2,500 impoundments are on-channel (Fischer, 2005).

An additional, relatively recent requirement being applied to some off-channel impoundments is lining with impermeable materials to minimize the amount of water leaking from impoundments to shallow alluvial groundwater. Presently, about 200 unlined off-channel impoundments in Wyoming may facilitate infiltration or recharge of underlying groundwater (Fischer, 2005). Often, no shallow groundwater is present beneath the impoundments to a depth of several hundred feet so shallow groundwater is thus not recharged or impacted. Specific provisions apply to the location of off-channel impoundments: they may not be sited within 500 feet of a designated water feature (nor less than 500 feet from the outermost floodplain or shallow channel alluvium), as identified on a U.S. Geological Survey 1:24,000 scale topographic map, including perennial and ephemeral streams, dry washes, marshes, and lakes. New guidelines for construction of impoundments, pre-construction groundwater monitoring, and compliance groundwater monitoring once discharge of produced water into the impoundment has commenced have recently been instituted in Wyoming (see Chapter 3).

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