Hydrology of Area 61, Northern Great Plains and Rocky Mountain Coal Provinces, Colorado and New Mexico

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8.0 GROUND -WATER QUALITY 8.1 Major Ions and Dissolved Solids Ground -Water Quality Deteriorates from West to East Across Area 61 The water quality deteriorates as water flows from sandstone, conglomerate, and basalt in plateaus and mesas to shale in stream valleys and plains. From west to east across the area, concentrations of dissolved solids increase and the ground water changes from bicarbonate to sulfate and chloride in composition (figs. 8.1 -1 and 8.1 -2). The regional chemical evolution of ground water is summarized in figure 8.1 -3. Area 61 is higher on the western side than on the eastern side (fig. 8.1 -2), and as ground water flows down this topographic gradient, it gains dissolved solids by reaction with bedrock. Geologic forma- tions on the western side of the area are predomi- nantly sandstone and conglomerate, which consist of minerals, such as quartz, that resist reaction with ground water. Precipitation and surface water that infiltrate these rocks and become ground water are calcium bicarbonate solutions containing few dis- solved solids. Because sandstone and conglomerate minerals do not react readily with ground water, the ground water is little changed as it flows through the rock. Geologic formations on the eastern side of the area are predominantly shale, which is composed of clay minerals and salts that react readily with ground water. Ground water flowing through shale thus becomes very mineralized. In the central part of the area, geologic formations consist of nearly equal quantities of sandstone and shale, and ground water there is intermediate in composition and quality between ground waters on the western and eastern sides of the area. Regional trends in ground -water quality are dis- rupted by topographic and geologic features. As ground water flows from stream divides to stream valleys, it gains dissolved solids (fig. 8.1 -2). Human activities, such as mining or irrigated agriculture, also add dissolved solids to ground water in valleys. Ground water in valleys with intermittent or ephe- meral streams is thus more mineralized than in stream- divide areas. In valleys with perennial streams, the streamflow may be sufficient to flush out dissolved minerals and cause water in alluvium and shallow bedrock to be less mineralized than in adjacent bedrock. Ground water in volcanic- capped mesas contains few dissolved solids because the vol- canic rocks are very permeable and rapidly transmit water. The origin of highly mineralized sodium chloride ground water near the Spanish Peaks, an intrusive stock, and related dikes is obscure, but the composition of the ground water may result from reactions with gases emanating from the igneous rocks. 72 Geologic formations are distinguishable by their ground -water quality. The Devils Hole - Farasita, Cu- chara Poison Canyon, and volcanic rock aquifers contain mostly calcium bicarbonate water with less than 500 milligrams per liter of dissolved solids. The Huerfano Formation contains mostly calcium sulfate water with 500 to 1,500 milligrams per liter of dis- solved solids. The Raton - Vermejo- Trinidad aquifer contains mostly sodium bicarbonate water with 500 to 1,500 milligrams per liter of dissolved solids. Cretaceous shale, limestone, and sandstone contain mostly sodium sulfate, calcium sulfate, and sodium chloride water with 1,000 to more than 5,000 milli- grams per liter of dissolved solids. Within each formation, water generally is least mineralized in topographically high areas and more mineralized in topographically low areas. Sources of ground -water quality data include Griggs (1948, p. 159 -176), Powell (1952, p. 30), McLaughlin and others (1961, p. 17 -18), Dinwiddie (1964, p. 15), Hart and Smith (1979, p. 50 -51), Water, Waste, and Land, Ltd. (1980, appendix E), Repplier and others (1981, pl. 5), Howard (1982, appendix K), Kaiser Steel Corp. (written commun., 1981), and unpublished U.S. Geological Survey data.