Chemistry of trace elements in coalbed methane product water
Introduction
Demand for natural gas (methane) is increasing because it is an abundant and clean burning fuel [1]. Several countries are now producing and/or plan to extract methane from coal deposits. In the United States, because of energy shortage, Colorado, Wyoming, Montana, Utah, and New Mexico are now exploring extraction of methane from their coal resources. As an example, in the Powder River Basin, Wyoming (Fig. 1) it is estimated that there are 31.7 trillion cubic feet of recoverable coalbed methane (CBM) and this basin is now one of the most active areas of CBM development in the western United States [2], [3], [4].
Most CBM development in Wyoming is occurring in the eastern portion of the Powder River Basin (Fig. 1), which is semi-arid with average annual precipitation ranging from 30 to . This basin is bounded by the Black Hills on the east, the Hartville Uplift to the south, the Big Horn Mountains on the west, and the Yellowstone River to the north, and is generally high plains with elevations from 1800 to above mean sea level. Eastern portions of the basin are typical high plains dominated by grass/sagebrush communities, but the central and western portion contains the Powder River Breaks and has a more diversified landscape setting and associated flora. For a detailed description of the geology and flora of the Powder River Basin, see [5], [6], [7].
Methane is produced in hydrologically confined coalbeds through biogeophysical processes. Recovery of methane from coalbeds is accomplished with extraction wells. As shown in Fig. 2, extraction of CBM requires some of the coalbed aquifer water to be removed to reduce the pressure in a zone around the bottom of the well. The extracted water is called CBM product water, and it is estimated that a single CBM well in the Powder River Basin may initially produce between 8 and of product water per minute. The amount of product water produced varies with the aquifer system pumped, the density or spacing of wells, and the duration of pumping. The production of CBM began approximately in 1989 in the PRB of Wyoming. At present more than 16,000 wells are under production in the Powder River Basin, and this number is expected to increase to 30,000 or more. Estimates are that approximately 30 trillion liter of product water will be produced from CBM extraction in Wyoming.
Currently, two to ten extraction wells are combined together into one discharge point and released into constructed unlined holding ponds. Many holding ponds are constructed concurrently with initial well installation and probably are 6–7 years old. These ponds are used for livestock and wildlife watering and some are stocked with game fish. With time, product water in the unlined ponds may percolate and affect the quality of downstream shallow aquifers. The downstream local landowners, ranchers, and citizens depend on ground water resources for livestock water, irrigation, and drinking water. CBM product water in holding ponds is expected to go through desorption and dissolution, ion complexation (speciation), and adsorption and precipitation processes based on local watershed soil properties. These chemical processes in turn control the quality of CBM product water in holding ponds as well the water percolating to the shallow aquifers.
Limited data on the chemistry of CBM product water and associated holding ponds in the Powder River Basin are available [8], [9], [10]. More information on how trace elements of CBM product water change in associated ponds as a function of watershed soils will be useful. Such information may help the state and federal water quality managers, landowners and citizens, and CBM industry personnel address potential environmental concerns associated with current uses of CBM pond waters.
The goal of this study was to examine the chemistry of trace elements in CBM product water in the Powder River Basin, Wyoming. We focused on the chemical changes in CBM product water at discharge points and after this water was placed in associated ponds within three different watersheds across the Powder River Basin. CBM product water samples were collected from both sources (discharge and associated ponds) within each watershed and were analyzed for pH and concentrations of dissolved trace elements (Al, As, B, Ba, Cr, Cu, F, Fe, Mn, Mo, Pb, Se, and Zn). Analytical data for the water from each watershed were compared to determine potential changes between the CBM discharge points and associated ponds. Furthermore, dissolved trace element concentrations of product water at discharge points and in holding ponds were compared with maximum contaminant limits (MCL) given by the Environmental Protection Agency for the drinking water and with Wyoming state aquatic life chronic values.
Section snippets
Study sites
Project study sites in the eastern portion of the Powder River Basin extend from the south to northwest and include the Cheyenne River (CHR), Belle Fourche River (BFR), and Little Powder River (LPR) watersheds. The CHR drains the areas south and east of the Powder River Basin. The BFR drains east of the Powder River Basin, and the LPR drains the northern portion of the Powder River Basin. These are perennial rivers and tributaries of the Missouri River. Major coal formations include the
Results and discussion
The analytical data of CBM product water and associated pond water are presented in Table 1. Charge balance of the chemical data is less than 5%, suggesting an excellent agreement between the analyzed concentration of positive ions and negative ions. The pH of CBM product water at discharge points from CHR to LPR ranged between 6.99 and 7.21. However, the pH of discharge water increased to between 7.79 and 8.26 in associated pond waters. The highest increase in pH was observed for the LPR
Conclusions
Trace element analysis of CBM product water samples at discharge points and in associated holding ponds indicated the following: No consistent trends were observed in trace element concentrations in CBM product water across the Powder River Basin. Among three watersheds, CBM product water from the CHR showed relatively less change in trace element concentrations in pond water. Dissolved Al, Fe, As, Se, and F concentrations of product water increased considerably in BFR and LPR watershed pond
Acknowledgements
We thank citizens, landowners, and methane industry for their cooperation and the Campbell County Commissioners office, Gillette, Wyoming for their assistance and partial funding. We also thank anonymous reviewers for their excellent reviews and comments. We appreciate and thank Dr. J.D. Rodgers, Department of Renewable Resources, College of Agriculture, University of Wyoming for his help for editing the manuscript.
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