A Sediment Quality Assessment and Management Framework for Dam Removal Projects

Sediment trapped behind a dam may be chemically contaminated, with the potential to cause toxicity and contaminant bioaccumulation in biota and also to gready complicate dam removal. However, sediment quality has been assessed in only a small fraction of the more than 80,000 reservoirs in the U.S. Therefore, efficient screening and assessment techniques are essential to determine the level of sediment management necessary for dam removals and to avoid unnecessary impacts to the river system. The Massachusetts Riverways Program is working with the USGS to develop the Regional Impounded Sediment Quality Assessment (RISQA), a GIS-based model that assesses the likelihood of contamination in impoundment sediment. The model combines land-level data, such as historic land use and known contamination sites, with the drainage network in the watershed upstream of an impoundment. Reservoirs identified by landscape-level screening as likely to contain contaminated sediment should be individually assessed using a sampling design appropriate for the local situation, a set of sediment quality criteria against which to judge the magnitude of the problem, and a decision framework for selecting the appropriate management action. Survey design and sampling techniques are well established, and range from a quick, qualitative reconnaissance survey to a statistically-rigorous coring survey. Sediment quality criteria vary from state to state, and are typically either narrative standards or are based on biological/toxicological assessments and/or bulk sediment chemistry. In addition to sediment quality criteria assessments, the proposed sediment management decision framework considers sediment transport capacity of the stream, downstream sensitivity to sedimentation, and contaminant bioavailability. Available management options include natural erosion and deposition downstream, staged dam removal, full or partial dredging, isolation/stabilization, or a combination. Choosing among the management options requires consideration of site-specific social concerns and financial limitations as well as technical issues. This paper outlines a process for sediment assessment as well as a potential management framework when dealing with sediment on a dam removal project.