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Rhodhiss Lake, North Carolina: Analysis of ambient conditions and simulation of hydrodynamics, constituent transport, and water-quality characteristics, 1993-94

Water-Resources Investigations 97-4131
By M.J.Giorgino and J.D. Bales

Full Report (PDF, 68 pages, 19.7 Mb)

Abstract

From January 1993 through March 1994, the U.S. Geological Survey conducted an investigation of Rhodhiss Lake in cooperation with the Western Piedmont Council of Governments. Objectives of the investigation were to describe ambient hydrologic and water-quality conditions, to estimate loadings of nutrients and suspended solids from selected tributaries and point sources, and to simulate hydraulic circulation and water-quality characteristics in Rhodhiss Lake using a hydrodynamic computer model.

The riverine headwaters of Rhodhiss Lake were unstratified, well oxygenated, and contained relatively high concentrations of suspended solids and nutrients throughout the study period. In general, concentrations of suspended solids, nitrate, orthophosphate, and total phosphorus decreased in a downstream direction from the headwaters to the Rhodhiss Dam. However, increases in specific conductance frequently were observed downstream from a wastewater discharge near mid-reservoir.

From mid-reservoir to the dam, Rhodhiss Lake thermally stratified during the summer of 1993. In this reach, dissolved oxygen was rapidly depleted from the bottom waters beginning in May 1993, and anoxic conditions persisted in the hypolimnion through the summer. During summer stratification, concentrations of nitrite plus nitrate, ammonia, and orthophosphate were low in the epilimnion, but concentrations of ammonia, orthophosphate, and total phosphorus increased in the hypolimnion. During fall and winter, Rhodhiss Lake was characterized by alternating periods of stratification and mixing.

A maximum chlorophyll-a concentration of 52 micrograms per liter was observed at mid-reservoir on November 17, 1993, and was the only value that exceeded the North Carolina water-quality standard of 40 micrograms per liter. Concentrations of fecal coliform bacteria exceeded 200 colonies per 100 milliliters in the headwaters of Rhodhiss Lake 37 percent of the time, and at mid-reservoir and in the forebay 16 percent of the time. In Lower Creek, a tributary to Rhodhiss Lake, concentrations of fecal coliform bacteria exceeded 200 colonies per 100 milliliters in 76 percent of the samples. This stream also contained elevated concentrations of nitrite plus nitrate, phosphorus, and specific conductance.

Loading estimates showed that almost all of the suspended solids and the majority of the nitrogen and phosphorus entering the headwaters of Rhodhiss Lake originated from nonpoint sources. During the investigation, point sources accounted for less than 1 percent of the suspended solids load to the reservoir headwaters, but point sources accounted for up to 27 and 22 percent of the total nitrogen and total phosphorus loads, respectively. Additional loadings of nitrogen and phosphorus entered Rhodhiss Lake by municipal wastewater discharge near mid-reservoir.

The U.S. Army Corps of Engineers CE-QUAL-W2 model is a two-dimensional, laterally averaged model that simulates hydrodynamics and water quality. The model was applied to Rhodhiss Lake from Huffman Bridge to Rhodhiss Dam--a distance of 18.5 kilometers--and was calibrated using data collected from April 1993 through March 1994.

During the simulation period, measured water levels varied a total of 1.32 meters, and water temperatures ranged from 4 to 30 degrees Celsius. The calibrated model provided good agreement between measured and simu- lated water levels at Rhodhiss Dam. Likewise, simulated water temperatures were generally within 2 degrees Celsius of measured values; however, the model tended to overpredict temperatures near the bottom of the reservoir by 1 to 3 degrees Celsius during warm months. This suggests that the model, as calibrated, overpredicts vertical mixing. Simulated dissolved oxygen concentrations followed the same general patterns and magnitudes as measured values, and there was good agreement between simulated and measured frequency of occurrence of dissolved oxygen concentrations less than 5 milligrams per liter.

Tracer-release simulations for stratified and unstratified conditions demonstrated the effects of stratification on dilution and rate of trans- port. Simulation of increased ambient air temperatures resulted in a slightly greater amount of time that dissolved oxygen concentrations in the reservoir were less than 4 milligrams per liter, but had little effect on algal biomass. A simulated 30-percent reduction in inflow phosphate concentrations resulted in a 20-percent decrease in the maximum algal concentration, with the greatest effects in late summer and early fall. A 50-percent reduction in phosphate releases from bottom sediments resulted in only a small reduction in algal concentration, primarily in the spring. Changes in phosphate load from a municipal wastewater discharge near mid- reservoir were simulated. Algal concentrations were fairly insensitive to increases in loading, but dissolved oxygen concentrations were reduced as the loading increased. Simulated algal concentrations increased 2 to 3 times when the discharge was moved from the reservoir bottom to near the surface.

Giorgino, M.J., and Bales, J.D., 1997, Rhodhiss Lake, North Carolina-Analysis of ambient conditions and simulation of hydrodynamics, constituent transport, and water-quality characteristics, 1993-94: U.S. Geological Survey Water-Resources Investigations Report 97-4131, 62 p.