2012-09-06_REVISION - M2008070 (29)

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Wells in these two bedrock aquifer systems, the upper and lower Piceance Basin aquifers, typically range in depth from 500 to 2,000 feet and commonly produce between 2 to 500 gallons per minute of water (USGS, 1984). These Tertiary bedrock aquifers are stratigraphically separated from the base of the Cameo Coal Zone in the Cretaceous Mesaverde Group by from less than 1,500 feet of strata along the Douglas Creek Arch to more than 11,000 feet along the basin trough just west of the Grand Hogback (Johnson and Nuccio, 1986) (Figure A3 -2). Aquifer maps do not exist for the Piceance Basin, but water quality in the Piceance Basin is poor owing to nahcolite (sodium bicarbonate) deposits and salt beds within the basin (Graham, 2001). Only very shallow waters such as those from the surficial Green River Formation are used for drinking water (Graham, CDWR, 2001). In general, the potable water wells in the Piceance Basin extend no further than 200 feet in depth, based on well records maintained by the Colorado Division of Water Resources (CDWR). At least two wells in the area are approximately 1,000 feet in depth, but they are used for stock watering. A composite water quality sample taken from 4,637 to 5,430 feet deep within the Cameo Coal Group in the Williams Fork Formation exhibited a total dissolved solid (TDS) level of 15,500 mg/L, which is above the 10,000 TDS water quality criterion for a underground source of water (USDW) (Graham, CDWR, 2001). The produced water from coalbed methane extraction in the Piceance Basin is of such low quality that it must be disposed of in evaporation ponds or re- injected into the formation from which it came or at even greater depths (Tessin, 2001). It is unlikely that any USDWs and coals targeted for methane production would coincide in this basin. These targeted coals are generally located at great depth, of at least 4,000 feet. The thousands of feet of stratigraphic separation between the coal gas bearing Cameo Zone and the lower aquifer system in the Green River Formation should prevent any of the effects from the hydrofracturing of gas - bearing strata from reaching either the upper or the lower bedrock aquifers. Permeability of the coal and the surrounding sandstone and shale is generally quite low except near outcrop, creating little potential for these rocks to contain a USDW. Researchers (Reinecke et al., 1991) report that the permeability of gas - bearing coal and sandstone of the Cameo Zone is so low that the gas is over - pressured and has forced groundwater out of the zone, a condition that tends to disfavor the entrapment of methane. Tyler et al. (1998) state that high coalbed methane gas productivity requires geologic and hydrologic conditions, and that these conditions are not optimal throughout much of the Piceance Basin because of the absence of dynamic groundwater flow and the low permeability of the host rocks. The above conditions prevail in the central part of the basin, previously favored as a coalbed methane development fairway, and heavily targeted for exploration (Nowak, 1991). However, analyses by Tyler et al. (1998) suggest that a transitional zone, between the deeply buried coal and the outcrops at the boundaries of the basin, where groundwater Evaluation of Impacts to Underground Sources of Drinking Water by Hydraulic Fracturing of Coalbed Methane Reservoirs June 2004 A3 -4