Estimating mercury concentrations and loads from four western Lake Superior watersheds using continuous in-stream turbidity monitoring

Many streams along the Minnesota coast of Lake Superior have been listed as impaired from either high turbidity or high fish mercury concentrations or both. Both turbidity and total mercury have been shown to be strongly correlated to total suspended sediment in many disturbed watersheds. Turbidity and total mercury concentrations and 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 during differing flow regimes. Total mercury values ranged from 1 to 28 ng l⁻¹ throughout the open water season and showed a close relationship to total suspended sediment (r² = 0.85, n = 23; p < 0.001) and a less robust but still significant relationship with turbidity (r² = 0.40, n = 34; p < 0.001) for all four streams. Mercury loads to Lake Superior were estimated to range from 8 to 97 g yr⁻¹ with watershed yields ranging from 0.5 to 4.3 μg m⁻² yr⁻¹. 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.