Coalbed Methane Produced Water In The Western Us

the impoundment is constructed, the volume of the impoundment and of volumes and bal-ances7 of CBM produced water entering the impoundment over time, the means by which CBM produced water travels to the impoundment (whether through a pipe or over land), the length of time the water is in the impoundment, and the local climate—can influence the way in which produced water stored in the impoundment may affect the groundwater beneath the impoundment. A concern is the potential for impoundments, through infiltration and percolation of CBM water, to dissolve and/or mobilize naturally occurring constituents in the underlying soil, including sulfate, selenium, arsenic, manganese, barium, and TDS.

Several studies using monitoring wells beneath impoundments and groundwater near them indicated a wide range in the relationship between impounded water and underlying groundwater, including (1) an increase in TDS, selenium, sulfate, chloride, and nitrate in groundwater beneath some impoundment facilities; (2) no apparent impact or interaction with underlying shallow alluvial groundwater for a substantial majority of impoundments studied; and (3) improved water quality beneath a small fraction of impoundments. Ongoing groundwater investigations in Wyoming by the DEQ have included nearly 2,000 CBM produced water impoundments. Of these, 170 reports from groundwater monitoring wells have been submitted as a part of operator permit compliance and exceedances of TDS, sulfate, and/or selenium groundwater standards beneath 17 impoundments have been documented. These studies and their results have led to new compliance monitoring guidelines for CBM impoundments in the state and recommendations for further studies. These guidelines were put into place in April 2010 (see Chapter 3 for further details).

SURFACE WATER

Discharges of CBM produced water to surface water and/or impoundments can affect the receiving water quality, whether perennial streams or rivers, ephemeral drainages, or surface impoundments. The effects of discharges to perennial and ephemeral streams and rivers and impoundments in terms of water quality and water volume—whether enhancements or depletions—are discussed below. Because dewatering of aquifers as part of CBM production can also potentially affect streamflows, studies of stream depletion are addressed in this section.

7"Volumes" refer to the total amount of water discharged and "balances" refers to the accounting of the disposition of those volumes (in reference to how much has evaporated, infiltrated, seeped, or spilled).

Effects from Discharge of CBM Produced Water to Streams and Rivers, Ephemeral Drainages, and Impoundments

Substantial documented discharge of produced water to streams and rivers occurs in the Powder River Basin. Produced water management records of the COGCC also substantiate significant direct discharges of CBM produced water to ephemeral and perennial drainages of the Colorado portion of the Raton Basin.8 However, because of COGCC specifications regarding water management and discharge reporting, information is presently limited regarding quantitative effects of such discharges on surface water quality or quantity in the Raton Basin. Issues of concern in Colorado related to surface discharges include potential for erosion, soil damage, immersion of nonhydric vegetation, water and land discoloration, and development of algal mats. The Colorado Geological Survey is currently studying the interaction and effects of CBM production and produced water management on surface water and groundwater resources in the Purgatoire River Basin of Colorado (Ash and Gintautas, 2009).

Perennial Streams and Rivers

The concentration of CBM operations in the Powder River Basin and differences in regulation between Wyoming and Montana have generated a number of studies that have examined the potential effects of CBM produced water discharges on the Powder River and Tongue River drainages in Wyoming and Montana. The studies have largely focused on inorganic constituents or parameters, including specific conductance, sodium-adsorption ration (SAR), nitrogen (as measured in ammonium, nitrate, and nitrite), pH, iron, potassium, sodium, chloride, fluoride, calcium, magnesium, sulfate, and bicarbonate. One set of studies has examined changes in the isotopic signature of surface waters as a means of examining the influence of CBM produced water on the Powder River. Specific measurement and analysis of organic constituents has been the subject of only one study to date. Although limited studies have examined the concentrations of organic constituents in produced water (e.g., Orem et al., 2007; see also Chapter 2), the effects of these organic compounds on surface water, groundwater, aquatic life, and riparian vegetation in the Powder River Basin have not been investigated.

Two studies by EPA Region 8 examined whether CBM production and produced water management caused significant changes in water quality in the Powder and Tongue rivers in Wyoming and Montana. Dawson (2007a) reported no statistically significant increases in specific conductance (measured by TDS) or SAR values associated with CBM develop-

8P. Gintautas, COGCC, e-mail conversation, December 1, 2009.

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