Suspended sediment releases in rivers: Toward establishing a safe sediment dose for construction projects

The harmful effects of suspended sediment (SS) exposures on aquatic ecosystems have been well documented. Integrating this knowledge into the management plans of in-stream construction projects that cause SS releases remains challenging. Commonly, these projects have fixed scopes that require decisions about trade-offs between elevated SS concentrations (SSC, mg∙L^-1) and duration of exposure (DoE, h). Constraining SSC to levels below a regulatory guideline may lead to an increase in project duration, while extended SS exposure times have uncertain environmental impacts and reduce project efficiency. This paper evaluates an alternative limit of SS dose (SSD, mg∙h∙L^-1), which is defined herein as the product of SSC and DoE, as a more useful regulatory guideline compared to maximum SSC for fixed scope projects. Managing SSD may lead to improved outcomes for project efficiency without jeopardizing environmental health. A dose-response model for salmonids is developed to contrast this approach with more common regulatory limits applied to fixed scope projects that focus on managing SS releases using maximum SSC values within a given time interval. Results demonstrate that the latter constrain the management operating space to a subset of the acceptable range of exposures, which may reduce project efficiency and increase environmental risk. Based on simulated predictions for mean salmonid probabilities of major physiological and lethal effects, an SSD limit of 600 mg∙h∙L^-1 may offer a conservative upper range of SSC and DoE values when managing salmonid populations, according to a common regulatory guideline for short-term exposures to not exceed 25 mg∙L^-1 over 24 h. This SSD limit would increase the available operating space to enhance project efficiency by not overly constraining SSC. The methods developed in this study may be used to evaluate other SS management limits by undertaking Monte Carlo simulations using dose-response models fit to available datasets to assess acceptable operating ranges.