CHLORIDE CONCENTRATION PROFILES AND IMPLICATIONS FOR WATER MOVEMENT THROUGH THE UNSATURATED ZONE IN THE POWDER RIVER BASIN, WYOMING

The Powder River Basin, in northern Wyoming and southern Montana, contains a wealth of energy resources - the largest coal reserves in North America and abundant traditional oil and natural gas reserves. Energy development in the Basin has increased rapidly in the last decade as new techniques have made it economically viable to tap an unconventional energy source, coal-bed natural gas. The impacts of energy development on natural hydrologic processes within the Basin, which may not be fully known for many years, can be effectively assessed with a broad-scope study of natural, ambient processes. To this end, a study was conducted to examine natural recharge rates in the Basin. A total of 178 unsaturated-zone soil cores were collected at 44 locations in the Basin. Profiles of water content and chloride concentration were determined from the cores. The chloride mass balance method, which equates chloride accumulation rates in the subsurface with rates of atmospheric chloride deposition, was applied to estimate rates of water movement. Sampling locations were selected to provide broad geographic coverage of the study area; soil cores were collected at the toe, midpoint, and high point of broad ridges that typify the landscape in this semi-arid environment. Vertical concentration profiles of chloride generally show a bulge in concentration at depths of about 2 m, a pattern typical of semi-arid settings of the western United States. Total mass of accumulated chloride is variable, ranging between 400 and 7,000 kg/ha. Estimates of drainage rates through the unsaturated zone are also variable, ranging from much less than 1 mm/yr to more than 10 mm/yr. Sources of variability in estimates, including soil texture, landscape setting, and location, are addressed.