Geologic factors in the evaluation of water pollution potential at mountain dwelling sites

December, 1973.Bibliography: pages [133]-135.In order to establish the relationship between the geologic setting and the occurrence of water pollution in mountain home developments containing individual sewage disposal systems, three areas in Colorado's Front Range were studied. Two of the areas were known to have biological contamination as confirmed by microbiological tests. Also, each area had adequate rock exposures to allow for detailed geologic study, the absence of thick soil profiles above bedrock and current development for mountain home location. In addition, each of the selected study areas differed in geologic setting, age of development and home density in the development. The extent of water pollution was established by a program of well and surface water testing for total coliform. Fecal coliform and inorganic contaminants were tested in selected wells. Detailed geologic maps were made of each area to locate features such as dikes or shear zones which might act as either barriers or conduits to ground water movement. Slope maps were prepared for the area by computer plots of digitized data of elevations taken from U.S. Geological Survey topographic maps. Determination of the water table profile and extent of alluvial fill in valley bottoms was accomplished using driller's well logs for each well in the area. In addition, soils were tested to determine their effective grain size, a joint and foliation study was conducted to determine the direction of pollutant travel should effluent enter these openings, driller's well logs were used to establish depth of soils and depth of weathering in bedrock and data from county health records were used to establish soil percolation rates. Data were compiled in the form of overlays on base maps of the areas involved. A topographic map with the geologic overlay was used in conjunction with various combinations of the field derivative overlays to indicate the pollution potential for specific areas. The overlays used in this procedure were compiled from. the following parameters: 1) slope, 2) depth of soil, 3) depth of intensely weathered bedrock, 4) local water table profile, and 5) soil percolation rates. These overlays indicated that the Glen Haven area is unsuitable for soil absorption sewage systems because of steep slopes, soil depth and depth of the water table. Most of the Tall Timbers area was indicated as unsuitable for soil absorption systems because of slope, soil depth and local geology. The Crescent Park area was categorized as safe (in part) for soil absorption systems, however, local areas within the subdivision were categorized as hazardous. From the results it was suggested that a procedure such as the one used in this study could be used for each subdivision proposed in the mountainous regions of Colorado. Thus more effective use of mountainous areas might be passable while maintaining a low probability of ground water contamination. Areas within each proposed subdivision would be classified as safe, hazardous, or unsuitable for soil absorption sewage systems. Unsuitable areas could be used as parks or greenbelts, hazardous areas would have low population densities and safe areas would be allowed to have higher population densities as long as other factors were favorable. In addition, procedures such as the one used in this investigation could be used to indicate mountain areas which should require a municipal sewage disposal system before development to ensure that the ground water system was not polluted.Supported by the Colorado Water Quality Control Commission and the United States Department of Interior, Office of Water Resources Research, no.87-194-2. (s) 14-01-001-1882