|Open-File Report 00-372: Results, Discussion, and Acknowledgements|
Rice, C.A., Ellis, M.S., and Bullock, J.H., Jr.
Open File-Report 00-372
RESULTS AND DISCUSSION
Powder River Basin CBM water has sodium as the dominant cation and bicarbonate as the major anion with the remaining cations and anions contributing less than 16 percent of the TDS (Table 2, Fig. 6). The major element composition of water in this study is in close agreement with water sampled from Tongue River Member coals in June, 1999 by the Water Resources Division of the USGS (Bartos, T., USGS, personal communication; Swanson and others, 1999). The data differ significantly from values reported in Larson and Daddow, 1984 for waters from the Fort Union Formation in Campbell County. In particular, many of the water analyses in Larson and Daddow have sulfate concentrations in the hundreds to thousands of mg/L, whereas sulfate concentrations in waters from Tongue River Member coals collected in this study range from <0.01 to 12 mg/L with a mean of 2.4 mg/L. As mentioned earlier, data from Larson and Daddow may not represent water from specific coal beds or zones in the Fort Union Formation.
Low values of sulfate in the CBM waters analyzed in this report are consistent with water in contact with a coal reservoir that has undergone or is undergoing methanogenesis. Sulfate concentrations in the CBM water have a direct influence on the amount of barium found in the water because barite (barium sulfate) generally controls the solubility of barium in most natural waters (Hem, 1992). Barium concentrations in the water analyzed in this study are relatively high compared to most groundwater because of the low sulfate concentrations. During coalification and methanogenesis, water in contact with the coals is anoxic and reducing. Elements such as iron and manganese, which are soluble as reduced species (Fe2+ and Mn2+), have concentrations that are relatively high compared to surface water values as a result of the reducing environment. On contact with oxygen in the atmosphere at the surface, the dissolved concentrations of these elements may be expected to decrease significantly.
Trace element concentrations in water from the 47 CBM wells sampled in this study are given in Table 3. Concentrations for most of the elements are at or below detection limits. All of the concentrations for elements in Table 3 are below the maximum contaminant level (MCL) given by the Environmental Protection Agency (EPA) in the Drinking Water Standards (EPA, 1996). No noticeable trends in trace element concentrations are apparent.
U. S. Geological Survey Open File Report 00-372
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