Compliance Monitoring of Dredging-induced Turbidity: Defective Designs and Potential Solutions

Resuspension of sediments during dredging and dredged material disposal operations continues to be a primary concern of regulatory agencies charged with the protection of environmental resources. Consequently, almost all dredging projects incorporate some level of monitoring effort dedicated to measurement of sediment resuspension. In the majority of cases, turbidity is measured with optical sensors as expeditious indices of suspended sediment concentration. Results are interpreted with reference to specific water quality criteria, and often within the context of allowable mixing zone dimensions. When criteria are exceeded regulatory responses are triggered, ranging from collection of additional samples to modification or even cessation of the dredging operation. For numerous reasons the existing generic approach to compliance monitoring is frequently ineffective in both adaptively regulating dredging projects and ensuring true environmental protection. Turbidity is a relative index of water clarity that only has ecological relevance once it is related to an environmentally relevant parameter, such as suspended sediment concentrations, water clarity, % surface irradiance, or sedimentation rate in the water body of interest. Failing these correlations, turbidity criteria are based on speculation of thresholds that may result in negative biological impacts. Present day inconsistencies in turbidity standards across state regulatory agencies in the U.S. reflect the continued lack of specificity in knowledge of likely exposures and responses for diverse organisms. The perfunctory collection of few data at basically arbitrary spatial and temporal scales lacks sufficient statistical power to assess potential impacts in any meaningful way. Furthermore most commonly applied compliance sampling protocols seldom demonstrates any linkage between turbidity measures and detrimental biological response. Thus compliance monitoring creates a status quo in which effort is expended at many dredging projects without demonstrable protection of the environment. Wasteful, ineffective compliance monitoring designs can be enhanced by taking advantage of technological advances as well as logical, common sense-driven improvements in obtaining data that can be used adaptively. Re-invention of compliance monitoring can simply involve a combination of acoustic-based mapping of suspended sediment plume trajectories and collection of time series data at the locations of target resources of concern (e.g., hard bottom, seagrass, spawning habitats). The latter data can contribute to a project-independent database that would enable much improved management decisions regarding safe, environmentally sound dredging projects.