Hydrogeology of a Biosolids-Application Site Near Deer Trail, Colorado, 1993-99

Abstract

This report presents hydrogeology data and interpretations resulting from two studies related to biosolids applications at the Metro Wastewater Reclamation District property near Deer Trail, Colorado, done by the U.S. Geological Survey in cooperation with the Metro Wastewater Reclamation District: (1) a 1993–99 study of hydrology and water quality for the Metro Wastewater Reclamation District central property and (2) a 1999 study of regional bedrock-aquifer structure and local ground-water recharge. Biosolids were applied as a fertilizer during late 1993 through 1999. The 1993 Metro Wastewater Reclamation District property boundary constitutes the study area, but hydrogeologic structure maps for a much larger area are included in the report. The study area is located on the eastern margin of the Denver Basin, a bowl-shaped sequence of sedimentary rocks. The uppermost bedrock formations in the vicinity of the study area consist of the Pierre Shale, the Fox Hills Sandstone, and the Laramie Formation, parts of which comprise the Laramie-Fox Hills hydrostratigraphic unit and thus, where saturated, the Laramie-Fox Hills aquifer. In the vicinity of the study area, the Laramie-Fox Hills hydrostratigraphic unit dips gently to the northwest, crops out, and is partially eroded. The Laramie-Fox Hills aquifer is either absent or not fully saturated within the Metro Wastewater Reclamation District properties, although this aquifer is the principal aquifer used for domestic supply in the vicinity of the study area. Yield was small from two deep monitoring wells in the Laramie-Fox Hills aquifer within the study area. Depth to water in these wells was about 110 and 150 feet below land surface, and monthly water levels fluctuated 0.5 foot or less. Alluvial aquifers also are present in the unconsolidated sand and loess deposits in the valleys of the study area. Interactions of the deeper parts of the Laramie-Fox Hills aquifer with shallow ground water in the study area include a general close hydraulic connection between alluvial and bedrock aquifers, recharge of the Cottonwood Creek and much of the Muddy Creek alluvial aquifers by the bedrock aquifer, and possible recharge of the bedrock aquifer by a Rattlesnake Creek tributary. Some areas of shallow ground water were recharged by infiltration from rain or ponds, but other areas likely were recharged by other ground water. Data for shallow ground water indicate that ground-water recharge takes less than a day at some sites to about 40 years at another site. Depth to shallow ground water in the study area ranged from about 2 feet to about 37 feet below land surface. Shallow ground-water levels likely were affected by evapotranspiration. Ground water is present in shallow parts of the bedrock aquifer or in alluvial aquifers in four drainage basins: Badger Creek, Cottonwood Creek, Muddy Creek, and Rattlesnake Creek. These drainage basins generally contained only ephemeral streams, which flow only after intense rain. Introduction The Metro Wastewater Reclamation District (MWRD) treats municipal sewage from the Denver area at their plant in Denver, Colo. (fig. 1). Biosolids are solid organic matter recovered from a sewage-treatment process that meet State and Federal regulatory criteria for a beneficial use such as soil amendment or fertilizer (Colorado Department of Public Health and Environment, 1998). In 1993, the MWRD acquired property (about 15 square miles; fig. 2) on the eastern plains of Colorado in Arapahoe and Elbert Counties east of Deer Trail, Colo. (fig. 1). Beginning in late 1993, MWRD biosolids were trucked from Denver about 75 miles east to MWRD property near Deer Trail. The MWRD applied biosolids as an agricultural fertilizer to nonirrigated farm land on their property. In 1995, the MWRD traded some of their property (land that included Muddy Creek in Elbert County); the resulting part of the original MWRD property became known as MWRD’s central property (fig. 3). In 1995, the MWRD also acquired additional property in the same area: the MWRD’s north property (about 14.5 square miles) and the MWRD’s south property (about 50 square miles) (fig. 3). The MWRD property near Deer Trail is farmed; besides biosolids, other fertilizers and pesticides could have been applied to the property in the past. These applications are called anthropogenic applications because materials of a chemical nature were added to the site by humans. Biosolids applications can affect soil and water quality. When biosolids are applied to agricultural soil, soil quality either can be improved by biosolids applications through increased nutrients and organic matter or degraded through the accumulation of excessive nutrients or trace elements (Berti and 2 Hydrology of a Biosolids-Application Site Near Deer Trail, Colorado, 1993–99 EL PASO CO LINCOLN CO ARAPAHOE CO ADAMS CO DENVER CO WELD CO LARIMER CO MORGAN CO LOGAN CO