CBM Produced Water Management in the Powder River Basin
CBM producers in the Wyoming portion of the Powder River Basin store the majority of produced water (about 64 percent) in surface impoundments to allow it to evaporate, to be sprayed into the air to enhance evaporation, or to infiltrate into the shallow subsurface or shallow alluvial aquifers (see figure below; Table 4.1). Twenty percent of the CBM produced water is discharged directly to surface water, either after treatment or without treatment if treatment is not required. Although the CBM produced water in the Powder River Basin generally has the lowest total dissolved solids (TDS) concentrations of all the produced water from the western CBM basins, only 13 percent is put to beneficial use, primarily as managed surface irrigation or subsurface drip irrigation. Use of produced water for subsurface drip irrigation requires an underground injection control (UIC) permit (see Chapter 3), inasmuch as the amount of water applied per unit of land is intentionally controlled to promote drainage below the crop root zone and into shallow alluvial aquifers. Only 3 percent of all Wyoming Powder River Basin CBM produced water is disposed of by deep-well reinjection, which also requires a UIC permit. In the Wyoming portion of the basin, 26 million barrels (3,350 acre-feet) of CBM produced water were reinjected in 2008; over the period from 2000 to 2008, 235 million barrels (30,300 acre-feet) were reinjected. In contrast, in 2008 alone in the Wyoming portion of the basin, nearly 77,000 acre-feet of CBM produced water were discharged into surface impoundments, while approximately 15,400 acre feet were directly applied to identifiable beneficial use for irrigation (including managed surface irrigation and subsurface drip irrigation).
In the Montana portion of the Powder River Basin, the two principal water management methods are permitted discharge and managed surface irrigation. The majority of produced water (61 to 65 percent) from CBM operations is discharged to surface water bodies, as treated discharge (see figure below); a 2010 Montana judicial ruling now prohibits the discharge of any untreated CBM produced water to any state waters. Managed surface irrigation comprises 26 to 30 percent of the discharge, of which 7 percent is apportioned to UIC subsurface drip irrigation. Impoundments are used for only 5 percent of the CBM produced water in Montana, and recently the Montana Supreme Court has declared the use of impoundments for disposal of CBM produced water to be unconstitutional. Industrial use of the water for dust control constitutes the final 4 to 5 percent of the produced water management.
The reason for the differences between the two states regarding the selection of management options for CBM produced water is that Montana currently has only two permitted CBM operations. One of these operations produces more than 95 percent of all produced CBM water in Montana and has a preexisting permit for the discharge of about 61 percent of all its produced water into the Tongue River.