Contaminants at the sediment-water interface: implications for environmental impact assessment and effects monitoring

Many contaminants in aquatic environments are associated with loosely packed aggregates of particulate material called flocs. Flocculation allows contaminants to accumulate at the sediment-water interface and it packages them in a form that is readily available for ingestion by filter feeding organisms. Unfortunately, most samplers being used for environmental assessment and monitoring suspend this material on impact and fail to sample this critical component of the seabed. In this study we use a slo-corer to collect seabed samples with an undisturbed surface layer and a Gust microcosm erosion chamber to erode the surface of the cores at increasing shear stresses. Results from two different sites, one impacted by tailings from historic gold mining and the other by open-pen salmon aquaculture, showed the levels of metals suspended at stresses below 0.24 Pa were greater than in the underlying sediment. Sampling this highly mobile surface layer is critical for determining the total contaminant load in bottom sediments and, more importantly, this layer represents the most readily available material for suspension. The loss of this layer during sampling could lead to inaccurate measurements of contaminant levels during environmental assessment and effects monitoring. A re-evaluation of the ISO standard for bottom sediment sampling is recommended.