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" " ~, l ~. 3 ~ ~ ,,", , -' u::.....I::...; i.'~ Quantity and Quality of Ground-Water Discharge to the South Platte River, Denver to Fort Lupton, Colorado, August 1992 Through July 1993 By Peter B. McMahon, Kenneth J. Lull, Kevin F. Dennehy, and Jim A. Collins Abstract Water-quality studies conducted by the Metro Wastewater Reclamation District have indicated that during low flow in segments ofthe South Platte River between Denver and Fort Lupton, concentrations of dissolved oxygen are less than minimum concentrations set by the State of Colorado. Low dissolved-oxygen concentrations are observed in two reaches of the river-they are about 3.3 to 6.4 miles and 17 to 25 miles downstream from the Metro Wastewater Reclamation District effluent outfalls. Conccntrations of dissolved oxygen recover between these two reaches. Studies conducted by the U.S. Geological SUlvey have indicated that ground-water discharge to the river may contribute to these low dissolved-oxygen concentrations. As a result, an assessment was made of the quantity and quality of ground-water discharge to the South Platte River from Denver to Fort Lupton. Measurements of surface-water and ground-water discharge and collections of surface water and ground water for water-quality analyses were made from August 1992 through January 1993 and in May and July 1993. The quantity of ground- water discharge to the South Platte River was determined indirectly by mass balance of surface- water inflows and outflows and directly by instan- taneous measurements of ground-water discharge across the sediment/waterinterface in the river channel. The quality of surface water and ground water was determined by sampling and analysis of water from the river and monitoring wells screened in the alluvial aquifer adjacent to the river and by sampling and analysis of water from piezometers screened in sediments underlying the liver channel. The ground-water flow system was sub- divided into a large-area and a small-area flow system. The precise boundaries of the two flow systems are not known. However, the large-area flow system is considered to incorporate all allu- vial sediments in hydrologic connection with the South Platte River. The small-area flow system is considered to incorporate the alluvial aquifer in the vicinity of the river. Flow-path lengths in the large-area flow system were considered to be on the order of hundreds of feet to more than a mile, whereas in the small-area flow system, they were considered to be on the order of feet to hundreds of feet. Mass-balance estimates of incremental ground-water discharge from the large-area flow system ranged from -27 to 17 cubic feet per second per mile in three reaches of the river; the median rate was 4.6 cubic feet per second per mile. The median percentage of surface-water discharge derived from ground-water discharge in the river reaches studied was 13 percent. Instantaneous measurements of ground-water discharge from the small-area flow system ranged from -1,360 to 1,000 cubic feet per second per mile, with a median value of -5.8 cubic feet per second per mile. Hourly measurements of discharge from the small-area flow system indicated that the high rates of discharge were transient and may have been caused by daily fluctuations in river stage due to changing effluent-discharge rates from the Metro Wastewater Reclamation District treatment plant. Higher river stages caused surface water to infiltrate bed sediments underlying the river chan- nel, and lower river stages allowed ground water to discharge into the river. Although stage changes apparently cycled large quantities of water in and out of the small-area flow system, the process probably provided no net gain or loss of Abstract 1