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o HJ7 Evaluation of Possible Human-Induced Effects on Ground-Water Quality, St. Charles Mesa, Colorado, 1997 INTRODUCTION St. Charles Mesa (Mesa) is an upland terrace south- east of Pueblo that has an area of about 10 square miles (fig. I). The Mesa has been irrigated for agricultural purposes since the late 1800's (Dumeyer, 1975). The unconfined sand and gravel aquifer. originally deposited by . ~ ----=- --~l i~Y J lJENVCR \ '~ ~ \;~^S," ' I Pueblo. ' NIVEf:L-..--.-...J J \..N Study \ I area ~---- LOCATION MAP 104045' 104"30' 38 20. ____ Foun/ain Creek 50" Pueblo I'?('.~erl)oir \ 38" 15' o 1 2 3 4 5 MILES I I I I I I I I 111 o 1 2 3 4 5 KILOMETERS Figure 1. Location at study area. . U.S. Department of the Interior U.S. Geological Survey the Arkansas River, overlies an eroded shale-bedrock surface (Scott, 1969). Ground-water flow is generally from the southwest to the northeast, with the highest water-table elevations in the vicinity of Bessemer Ditch near County Farm Road lDumeyer, 1975; U.S. Geological Survey (USGS) Ground-Water Site Inventory data base). During the last 25 years, the Mesa has become increasingly urbanized as cultivated fields have been convel1ed to residential areas. However, much of the Mesa is still dedicated to agriculture and animal husbandry, both of which are potential sources of contaminants to the ground water. Septic systems are another potential source of contaminants to the ground water; all wastewater treat- ment on the Mesa is provided by septic systems. Because there has been a high water table on much of the Mesa, with ground-water levels within 10 feet of the land surface, the potential exists for septic-system flooding. As the vertical distance between the water table and a septic- system's leach field decreases, the treatment efficiency of the septic system can be decreased due to thinning of the unsaturated zone below the leach field (fig. 2). The extent of the high water table on the Mesa is described by Brendle (1999). The U.S. Geological Survey, in cooperation with Pueblo County, collected ground-water samples on the Mesa during July and August 1997 to evaluate whether ground-water quality has been affected by the byproduets of human activities. ineiuding septic-system effluent. Samples were obtained from 24 domestic and irrigation wells and I spring. The samples were analyzed for chem- ical constituents and bacteria that can be indicative of the byproducts of human activities: nitrate, ammonia, dissolved organic carbon (DOC), methylene blue active substances (MBAS) (detergents and other natural and synthetic substances), total coliform and Escherichia coli (E. coli) bacteria, and caffeine. USGS/WRIR 99-4085