WATERSHED MODELING TO EXAMINE THE EFFECTS OF GROWTH ON WATER QUALITY IN THE McDOWELL CREEK BASIN

Water quality modeling was conducted to examine the effects of growth in the McDowell Creek Basin on the water quality in the McDowell Creek Cove of Mountain Island Lake. The water quality modeling included the use of both a watershed and a lake model. The watershed model was developed and used to predict the pollutant loadings from nonpoint sources in the basin. The lake model was used to calculate the reservoir response to inputs from non point and point sources of nutrients. This study investigated the effects of effluent discharge ranging from theexisting 6 mgd up to 24 mgd associated with development of the basin. Nonpoint source loadings were estimated ateach level of development and were included in the lake modeling.Results of the modeling showed that as the treatment plant is expanded, a reduction in effluent nutrient concentrations will eventually be required to maintain acceptable water quality in the McDowell Creek cove. The model was also used to show the relative impacts to water quality from point and nonpoint sources. Because of the conservative assumptions made, the water quality that would actually occur in the lake is likely to be better thanthat estimated by the model. However, even with the conservative assumptions, the results of the model showed that an expansion of the treatment plant eventually to 24 mgd combined with some reductions in nutrient concentrations would not degrade water quality in the cove.