RESULTS AND DISCUSSION
Parameters measured at the wellhead such as temperature and pH and the major and minor element composition of the 47 samples are presented in Table 2. The temperature ranges from 13.8 to 28.7^o C with a mean of 19.6^o C and the pH of the water has a mean of 7.3 and a range of 6.8 to 7. 7. Total dissolved solids (TDS) ranges from 370 to 1,940 mg/L with a mean of 840 mg/L. For comparison, the national drinking water standards recommendation for potable water is 500 mg/L and seawater is about 35,000 mg/L. These samples suggest that TDS in waters in the Wyodak-Anderson coal zone increases from south to north and from east to west (Fig. 6). This trend may be a result of increased water-rock interaction along a flowpath, an increase or change in composition of the ash content of the coal, or other factors not yet recognized. The increase in TDS is generally a result of an increase in the sodium and bicarbonate content of the water. The preliminary data may support other basin-wide trends in constituents.

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.

ACKNOWLEDGMENTS
This study would not be possible without the cooperation of many of the companies and operators in the Powder River Basin who kindly gave permission to sample their coalbed methane wells and provided support in locating well sites and sometimes replumbing wellhead configurations. Thanks to Ocean Energy, Barrett Resources, Pennaco Energy, CMS Energy, Hi-Pro Production, Western Gas Resources, and Big Basin Petroleum. Jim Crock of the U. S. Geological Survey Mineral Resources Team kindly provided analyses of mercury in water samples by fluorescence spectroscopy.