Greater than 90% of the water in the South Platte River below the largest wastewater treatment plant in Denver, Colorado consists of treated sewage effluent during periods of low flow. Under these conditions, dissolved-oxygen concentrations in the river frequently fall below the regulatory limit of 4.5 mg/L. Twenty-four hour measurements of seepage across the bed-sediment/water interface showed that daily changes in river stage caused by fluctuations in effluent-discharge rates cycled surface water in and out of bed sediments at high (up to 100 cfs per mile) but transient rates, resulting in sediment oxygen demands that were 10 to 100 times higher than rates estimated using diffusion-based sediment-oxygen-demand chambers. Segments of the river having the most active water cycling also had the lowest dissolved-oxygen concentrations. Dissolved-oxygen uptake rates measured in laboratory incubations of bed sediment indicate that sediment oxygen demand also was dependent on the availability of dissolved ammonia, sedimentary organic carbon, and fine-grained sediments. Sediments with higher contents of these constituents had the highest dissolved-oxygen uptake rates and corresponded to river reaches having the lowest dissolved-oxygen concentrations. These results indicate that efforts to improve dissolved-oxygen dynamics in the South Platte River might include stabilizing daily fluctuations in river stage and lowering dissolved-ammonia concentrations in effluent discharge.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225