Application of a Two-Dimensional Reservoir Water-Quality Model of Beaver Lake, Arkansas, for the Evaluation of Simulated Changes in Input Water Quality 2001-2003
Beaver Lake is considered a primary watershed of concern in the State of Arkansas. As such, information is needed to assess water quality, especially nutrient enrichment, nutrient-algal relations, turbidity, and sediment issues within the system. A previously calibrated two-dimensional, laterally averaged model of hydrodynamics and water quality was used for the evaluation of changes in input nutrient and sediment concentrations on the water quality of the reservoir for the period of April 2001 to April 2003. Nitrogen and phosphorus concentrations were increased and decreased and tested independently and simultaneously to examine the nutrient concentrations and algal response in the reservoir. Suspended-solids concentrations were increased and decreased to identify how solids are distributed in the reservoir, which can contribute to decreased water clarity. The Beaver Lake model also was evaluated using a conservative tracer. A conservative tracer was applied at various locations in the reservoir model to observe the fate and transport and how the reservoir might react to the introduction of a conservative substance, or a worst-case spill scenario. In particular, tracer concentrations were evaluated at the locations of the four public water-supply intakes in Beaver Lake. Nutrient concentrations in Beaver Lake increased proportionally with increases in loads from the three main tributaries. An increase of 10 times the calibrated daily input nitrogen and phosphorus in the three main tributaries resulted in daily mean total nitrogen concentrations in the epilimnion that were nearly 4 times greater than the calibration concentrations at site L2 and more than 2 times greater than the calibrated concentrations at site L5. Increases in daily input nitrogen in the three main tributaries independently did not correspond in substantial increases in concentrations of nitrogen in Beaver Lake. The greatest proportional increase in phosphorus occurred in the epilimnion at sites L3 and L4 and the least increase occurred at sites L2 and L5 when calibrated daily input phosphorus concentrations were increased. When orthophosphorus was increased in all three tributaries simultaneously by a factor of 10, daily mean orthophosphorus concentrations in the epilimnion of the reservoir were almost 11 times greater than the calibrated concentrations at sites L2 and L5, and 15 times greater in the epilimnion of the reservoir at sites L3 and L4. Phosphorus concentrations in Beaver Lake increased less when nitrogen and phosphorus were increased simultaneously than when phosphorus was increased independently. The greatest simulated increase in algal biomass (represented as chlorophyll a) occurred when nitrogen and phosphorus were increased simultaneously in the three main tributaries. On average, the chlorophyll a values only increased less than 1 microgram per liter when concentrations of nitrogen or phosphorous were increased independently by a factor of 10 at all three tributaries. In comparison, when nitrogen and phosphorus were increased simultaneously by a factor of 10 for all three tributaries, the chlorophyll a concentration increased by about 10 micrograms per liter on average, with a maximum increase of about 57 micrograms per liter in the epilimnion at site L3 in Beaver Lake. Changes in algal biomass with changes in input nitrogen and phosphorus were variable through time in the Beaver Lake model from April 2001 to April 2003. When calibrated daily input nitrogen and phosphorus concentrations were increased simultaneously for the three main tributaries, the increase in chlorophyll a concentration was the greatest in late spring and summer of 2002. Changes in calibrated daily input inorganic suspended solids concentrations were examined because of the effect they may have on water clarity in Beaver Lake. The increase in total suspended solids was greatest in the hypolimnion at the upstream end of Beaver Lake, and negligible changes were observed at the downstream end of the reservoir for all of the scenarios. An increase of 10 times the calibrated daily input concentration of inorganic suspended solids in all three tributaries resulted in an increase in daily mean total suspended solids concentration of more than 5 times the calibrated condition in the epilimnion and more than 10 times the calibrated condition in the hypolimnion at site L2. Concentrations were similar to the calibrated condition in the epilimnion and more than 11 times the calibrated condition in the hypolimnion at site L3. A conservative tracer was introduced into the model at eight locations in the reservoir during high-flow (March 17, 2002) and low-flow (August 7, 2001) conditions. In general, the duration of high tracer concentrations at the four water-supply intakes was relatively short when tracers were released at high-flow conditions compared to releases during low-flow conditions. When tracers were placed in the more riverine portion of the reservoir, the tracer was rapidly transported into the hypolimnion in the upstream portion of the reservoir and into the epilimnion and metalimnion further downstream in the reservoir during high-flow conditions. In comparison, when tracers were introduced during low-flow conditions, most of the tracer remained in the upstream portion of the reservoir until mid-December when a storm event flushed most of the tracer downstream into the hypolimnion, mainly at greater depths than the downstream water-supply intakes.