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!l :l :ll 'II. v v v J.. ,,.\ ~ River downstream from Denver. In 1992, the USGS, in cooperation with the MWRD, began a study of the quantity and quality of ground-water discharge to this reach of the South Platte River. Purpose and Scope This report describes the quantity and quality of ground-water discharge in a reach along the South Platte River from Denver to Fort Lupton. Physical and biochemical processes that affected the quantity and quality of ground-water discharge to the South Platte River also are discussed. This report presents (I) measurements of surface-water discharge rates at 14 sites and measurements of ground-water discharge rates at 30 cross sections in the river, (2) continuous measurements of ground-water levels relative to river stage at two locations in the study area, (3) water-quality data for water samples collected from 30 locations in the river, and (4) water-quality data for water samples collected from 12 monitoring wells screened in the alluvial aquifer immediately adja- cent to the river and from 159 (150 single-depth and 9 multi-depth) shallow piezometers screened in bed scdiments underlying the river. Data were collected from August 1992 through January 1993 and in May and July 1993. Background Hydrology and Description of Study Area The study area is a 26-mi reach of the South Platte River and the adjacent alluvial aquifer between 64th Avenue in Denver and Highway 52 in Fort Lupton (pI. I). Land use in the vicinity of the river is predom- inantly urban between 64th Avenue and 88th Avenue and is predominantly irrigated agriculture and mining for sand and gravel in the South Platte alluvium between 88th Avenue and 124th Avenue, whereas irrigated agriculture is most important between l24th Avenue and Highway 52. An exception to the agricultural setting downstream from 124th Avenue is the city of Brighton, located near I 60th Avenue. Based on long-tenn surface-water discharge measurements at the South Platte River at 64th Avenue at Commerce City streamflow-gaging station (station number 06714215) and the South Platte River at Henderson streamflow-gaging station (station number 06720500) (fig. I), the low-flow period for this reach of the South Platte River extends from about September through March. The two most important features affecting surface-water discharge in the study area during periods oflow flow are effluent discharges from the MWRD treatment plant and surface-water diversions to irrigation ditches. Rates of effluent discharge from the MWRD plant vary daily from about 80 to 250 ftl Is and generally peak about noon. The MWRD effluent is discharged to the river from two points---the north complex outfall and the south com- plex outfall. The two discharge points are located side by side. The north complex has a design capacity of about 110 ftl Is, and the south complex has a design capacity of about 177 ftl Is. In addition to effluent dis- charge from the MWRD plant, inflows to this reach of the South Platte River include effluent discharges from the South Adams County (design capacity of6.7 ftl/s) and the city of Brighton (design capacity of 4.1 ftl/s) wastewater-treatment plants and include tributal)' inflows from Sand Creek, Clear Creek, Niver Creek, and Big DI)' Creek (pI. I). Irrigation ditches in the study area generally operate from May through the middle of October. However, the Burlington Ditch, located about I mi upstream from the MWRD plant, diverts water from the river from October through March for storage in reservoirs. The Burlington Ditch has a water right in excess of350 ftl/s (Camp Dresser and McKee, Inc., 1992); therefore, it can potentially divert all of the flow in the South Platte River during periods of low flow. At these times, discharge of effluent from the MWRD plant may constitute more than 90 percent of the river downstream from the MWRD plant Four ditch sys- tems downstream from the MWRD, the Fulton (water right> 200 ftl/s), Brantner (water right> 110 W/s), Brighton (water right> 40 ftl/s), and Lupton Bottoms (water right> 150 ftl/s) (pI. I), also may remove sub- stantial volumes of river water during the irrigation season. The South Platte River alluvial aquifer in the study area is a relatively narrow band of un consoli- dated gravel, sand, silt, and clay that parallels the river. Mineralogically, the aquifer material is dominated by quartz and feldspar. The aquifer ranges in width from about 2 to 5 mi (Hurr and Schneider, 1972). Depth to ground water varies from 0 to about 30 ft, and the sat- urated thickness varies from 10 to 40 ft (Hurr and Schneider, 1972). Ground water generally flows in a downvalley direction, subparallel to the river. Gaggiani and others (1987) reported that the rate of incremental ground-water discharge to the South Platte River between Henderson and Fort Lupton varied from 0 (ftl/s)/mi in March 1968 to 3.2 (ftl/s)/mi in November 1968. INTRODUCTION 3