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":1! c - o. '"'' ,,)~ v u "'- c,..,u Concentrations of DO In the South Platte River downstream from the MWRD are variable and dependent on numerous factors, including water tem- perature, anunonium concentrations in the river, time of day, and the relative rates of photosynthesis and respiration by the instream biological community. Concentrations of DO less than the minimwn 24-hour average DO concentrations established by the Colorado Department of Health and Environment (1993) can occur during periods oflow flow from about July through October (Camp Dresser and McKee, Inc., 1992). During periods of low flow, instantaneous day- time concentrations of DO in the South Platte River exhibit a cyclic pattern with distance downstream from the MWRD plant. The most important feature of the longitudinal DO profile is the presence of two reaches in which instantaneous DO concentrations are low relative to concentrations of DO upstream from the MWRD facility. The first reach extends from above the 88th Avenue bridge to the Fulton Ditch headgate, 3.3 to 6.4 mi downstream from the MWRD effluent out- falls. The second reach extends from I 60th Avenue to Highway 52 in Fort Lupton, 17 to 25 mi downstream from the MWRD effluent outfalls (pI. I). METHODS OF DATA COLLECTION This study used a variety of data-collection techniques to provide the information necessary to describe the quantity and quality of ground-water discharge to the South Platte River. Some ofthe techniques used to directly measure ground-water discharge to the river were developed specifically for this project to accommodate the physical and hydro- logic conditions present in the study area, Discharge Measurements Fourteen sites were established to measure surface-water discharge (table I). Six sites were located on the South Platte River, four sites were located on tributaries to the South Platte River, two sites were at irrigation ditches, and two sites were at wastewater-treatment facilities. Discharge measurements at the wastewater-treatment plants were obtained from the plant operators. The discharge-measurement sites on Sand Creek and Clear Creek were located at the gaging stations at those sites (Sand Creek at mouth near Commerce City, station number 394839104570300, and Clear Creek at Derby, station nwnber 06720000). The measurement sites on Niver Creek and Big Dry Creek were located at the mouths of the creeks. Measurements of surface- water discharge were made from August 1992 through January 1993 and in May and July 1993. Surface-water discharge measurements were made using methods described by Buchanan and Somers (1969). In general, discharge was measured by dividing the stream channel into a minimum of 25 equal-width increments perpendicular to the direc- tion of flow and measuring surface-water velocity in each increment. Ifthe water depth was less than 2.5 ft, velocity was measured at a depth of 0.6 times the total [u, not applicable) Tsble 1. Surface-water sites at which discharge was measured Station number At 64th Avenue at Commerce City. At 88th Avenue. At l04th Avenue. Upstream from the Brighton Ditch headgate. At 160thAvenue. At Highway 52. Sand Creek at mouth near Commerce City. Clear Creek at Derby. Niver Creek at mouth. Big Dry Creek at mouth. Brantner Ditch at headgate. Lupton Bottoms Ditch at headgate. Metro Wastewater Reclamation District. City of Brighton. Site type South Platte River Tributaries Irrigation ditches Wastewater-treatment plants Location 06714215 394839104570300 0672??oo METHODS OF DATA COLLECTION 5