Developing Sediment Load Thresholds Protective of Aquatic Life

Abstract

Abstract. A method was developed for setting target Total Maximum Daily Load (TMDL) sediment loads in non-tidal watersheds of Virginia that directly relates biological conditions with sediment load levels. The new method is based on a modification of methodology developed by the state of Maryland which simulated sediment loads using Chesapeake Bay Watershed Model procedures. The modified method is proposed for use in several case study watersheds in Virginia using the GWLF model. The biological condition is represented by the average Virginia Stream Condition Index (VSCI), while sediment is represented as the existing sediment load normalized by the corresponding load under an all-forest condition. The existing sediment load in any given watershed divided by the corresponding sediment load simulated under an all-forest condition, results in an all-forest load multiplier, AllForX. When AllForX is regressed against VSCI for a number of healthy watersheds surrounding a particular impaired watershed, the developed relationship can be used to quantify the value of AllForX for the biological health threshold (VSCI = 60) used to assess aquatic life use impairments in Virginia. The TMDL is then calculated as the value of AllForX at the VSCI threshold times the all-forest sediment load of the impaired watershed. Since a number of watersheds are used to set the regression, a confidence interval around the threshold can also be quantified and used to calculate the margin of safety in the TMDL equation. The relationship between AllForX and the biological condition is validated with plots and regressions between AllForX and various independent sediment-related habitat metrics.