Water Quality Modeling Approach to Investigate Nutrient Criteria on the South Fork Humboldt Reservoir, Nevada

Abstract

The development of nutrient criteria for reservoir-river systems requires theconsideration of downstream impacts, a difficult variable to quantify in arid environmentswith highly productive periphyton conditions. The South Fork Humboldt Reservoir(Elko County, Nevada) represents a river impoundment operated by bottom release andobserved to increase nutrient loadings to highly eutrophic tail-waters (benthic Chl-avalues >500 mg/m2). The USEPA mechanistic model AQUATOX 3.1 has been used tosimulate the South Fork Reservoir and River algal nutrient loading functional response,and to determine coupled system changes for nutrient criteria development. Thecalibrated and validated reservoir model was used to simulate scenarios of 10 percentincremental changes to reservoir phosphorous loadings, with the equivalent covariancerelationship of phosphorous to nitrogen and sediment loadings. The results indicatehypothetical ranges of eutrophic conditions for nutrient-criteria variables of phosphorous,nitrogen, secchi depth and chlorophyll-a (Chl-a). The reservoir scenario of 40 percentincrease of observed loadings resulted in a decrease of average secchi depth, from 5.27 to4.20 meters, and an increase of average Chl-a values from 2.11 to 2.29 ìg/L respectively.Reservoir scenario outflows were used as model boundary conditions for the calibratedand validated downstream river model. River scenarios results of 20 reservoir outflowsindicate the immediate tailwater reach is somewhat insensitive to changes in reservoirloadings, demonstrating outflows conditions are potentially dominated by long-term in-reservoir sediment diagenesis processes.