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Analysis of major and trace constituents in streams flowing through six semiarid watersheds indicates that the stream chemistry is characterized by saturation with respect to common carbonate minerals (calcium, magnesium, iron, manganese, and lead). The solubility of the carbonate minerals may be a major control on the absolute and relative concentrations of calcium, magnesium, bicarbonate, iron, manganese, and lead; however, other mechanisms probably control the concentrations of cadmium and zinc. Statistical analyses indicated that the mean concentrations of the major ions in the two climatic areas studied are significantly (P=0.05) different from one another, with larger mean concentrations in the more arid area. Trace-metal concentrations were similar from one area to another and indistinguishable (P=0.05) from site to site for lead, cadmium, and zinc. Linear regressions of major ion concentration to specific conductance are similar in both areas for sodium, bicarbonate, sulfate, and chloride. Results of the study may be useful in providing a first approximation of stream chemistry in other watersheds with the same geologic setting, in selecting representative watersheds to allow investigation of geochemical controls, and in determining future changes in stream chemistry in the watersheds studied. INTRODUCTION Increasing national needs for energy have resulted*in a significant increase in the production of coal in Colorado. From 1965 to 1975 coal production in Colorado has increased by 68 percent (Colorado Division of Mines, 1976). Much of the present and future coal mining in northwestern Colorado is and will be surface strip mining within small, semiarid watersheds. Few data are available to define the hydrology and water quality of such watersheds. Without such data and an understanding of the processes controlling the hydrology and water quality of these watersheds, it is impossible to predict the impact of mining. To obtain the data needed to define the hydrologic and water-quality processes within the watersheds, the U.S. Geological Survey, in cooperation with the U.S. Bureau of Land Management, has begun the study of small, semiarid watersheds in Colorado. E X P L A N A T IO N 09 24 37 00 SA M PL IN G S IT E A N D N U M B E R

Water-quality characteristics of six small, semiarid watersheds in the Green River coal region of Colorado