Reservoir Sediment-Water Microcosm Simulation: Pre-Impoundment Predictions vs.Observed Water Quality at Ridgway Reservoir, Ridgway, Colorado. Science and Technology Program Research Report

摘要

Sediment-water microcosm simulation was used to predict post-impoundmenthypolimnetic water quality of Ridgway Reservoir, Ridgway, Colorado, U.S.A. Reservoir basin soils and one river sediment were combined with Uncompahgre River water in 55-gal tanks, and water was sampled and analyzed before and after bacterially mediated reducing conditions developed. After completion of Ridgway Dam, the inflow streams, the reservoir, and outflow were sampled and analyzed on a monthly schedule from march 1987 to October 1996 for major ions, nutrients, and trace elements. Ridgway Reservoir contains 59,400 acre-ft. (2,094 m), and is approximately 200 ft (61 m) deep behind the dam. Despite extensive mine wastes and acidic drainage in the upper elevation reaches of the watershed, Ridgway Reservoir is an oligotrophic lake that has very low algal productivity and very good water quality. Reservoir median pH was 8.14 and dissolved solids ranged from 200 to 500 mg/L. Generally low total phosphorous concentrations (median=8.0 ug/L) and elevated total nitrogen:total phosphorus ratios (median ratios for unfiltered sample ranging from 26 to 52) suggest that the lake is phosphorus-limited. The post impoundment data also suggest that the reservoir improve downstream water quality by reducing inflow concentrations of trace elements and nutrients, with total nitrogen loading reduced by 38 percent and total phosphorus loading by 77 percent. Because of a lake bottom outlet in the dam and hyplimnion mixing from outlet operations, anoxic and reducing conditions at depth were not commonly observed during summer sampling. A comparison of simulation data with post impoundment chemistry revealed that microcosm simulation reasonably predicted median reservoir behavior. Both simulation and post-impoundment chemistry data are presented, and modifications of simulation methodology and considerations regarding differences between soils and sediments are discussed.