Estimating sediment, nutrient and mercury loads from four western Lake superior watersheds using continuous in-stream turbidity monitoring.

Abstract

Many streams along the Minnesota coast of Lake Superior have been listed as impaired from high turbidity, suspended sediment, or high fish mercury concentrations. Both total suspended sediment (TSS) and total mercury have been shown to be strongly correlated to turbidity in many disturbed watersheds. Total suspended sediments, phosphorus, and total mercury loads were estimated in four western Lake Superior watersheds from 2005-2006 using automated in-stream turbidity measurements. Regression models were developed relating this near-continuous turbidity data to grab sample measures of mercury, suspended sediments and nutrients during differing flow regimes. Suspended sediment and phosphorus loads estimated using the turbidity surrogate were compared to those made using FLUX software, a standard assessment technique based on discharge and grab sampling for TSS. Stream specific turbidity vs. TSS measures were strongly correlated (r2 = 0.6 to 0.95; p < 0.05) and total mercury also showed a close relationship with TSS (r2 = 0.82, n = 23; p < 0.05) for all four streams. Continuous turbidity monitoring appears to be a reasonable surrogate for both suspended sediment and total mercury concentration, providing information when manual sample collection is cost-prohibitive or logistically difficult, and across a wide range of flows.