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EFFECT OF GROUNDWATER DISCHARGE ON DISSOLVED-OXYGEN CONCENTRATIONS IN THE SOUTH PLATTE RIVER AT LOW FLOW

National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by Peter B. McMahon, Kevin F. Dennehy, and Ken Lull 1

ABSTRACT

Groundwater discharge is an important source of water to the South Platte River during periods of low flow. For example, surface-water discharge measurements taken in August, 1992 showed that there was a 20 percent increase in the volume of streamflow between Brighton and Ft. Lupton as a result of groundwater discharge. The groundwater discharge rate in this reach was 15 cubic feet per second per mile. Although concentrations of dissolved oxygen range from about 2.0 to 6.0 mg/L in groundwater adjacent to the river, concentrations are < 0.2 mg/L in groundwater immediately underlying the river. These data show that streambed sediments are a sink for dissolved oxygen transported by groundwater, and imply that groundwater discharge dilutes dissolved-oxygen concentrations in the river. As an example, if the concentration of dissolved oxygen in anaerobic groundwater could reduce the surface-water dissolved-oxygen concentration at Ft. Lupton to 4.8 mg/L, a concentration that is less than the current regulatory limit of 5.0 mg/L. Large inputs of anaerobic groundwater to the river near Ft. Lupton are consistent with observed low concentrations of dissolved oxygen (< 5.0 mg/L) in surface water in this area. These results demonstrate the importance of accounting for groundwater discharge in water-quality studies of the South Platte River.

1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225

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