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Particle-bound metal transport after removal of a small dam in the Pawtuxet River, Rhode Island, USA
Katz, D., M. Cantwell, J. Sullivan, M. Perron, R. Burgess, AND K. Ho. Particle-bound metal transport after removal of a small dam in the Pawtuxet River, Rhode Island, USA. Integrated Environmental Assessment and Management. Allen Press, Inc., Lawrence, KS, 13(4):675-685, (2017).
Dam removal has become a primary step in the ecological restoration of impounded rivers, especially in the Northeastern United States due to a long history of industrialization. Many anadromous and catadromous species have been removed from historic breeding grounds for centuries in some cases. The Pawtuxet River is a prime example of such a system. The primary objective of this research was to determine whether dam removal represents an additional threat to downstream sources, not just by resuspension of impounded material but potentially from increased scouring along the river banks after the dam is removed. We collected suspended sediment in sediment traps below the dam both immediately before and 15 months after dam removal to assess potential changes in heavy metal concentrations (Ag, Cd, Cr, Cu, Ni, Pb, & Zn) and calculated the flux of metals exiting the river. We also examined a sediment core above the dam to determine concentrations in the river bed. The increasing number of dams under consideration for removal, especially in anthropogenically impacted rivers, is a major issue due to potential transport and redistribution of contaminated sediments. River to estuarine discharges are particularly important in this regard, due to their proximity to areas of high population density and their ecological value and importance. Currently, more information is needed to understand the behavior of heavy metals following dam removal in impacted systems. The information in this article will be of great interest to scientists and managers involved with assessing the risk associated with dam removal and to weigh all options before initiating similar projects.
The Pawtuxet River in Rhode Island, USA, has a long history of industrial activity and pollutant discharges. Metal contamination of the river sediments is well documented and historically exceeded toxicity thresholds for a variety of organisms. The Pawtuxet River dam, a low-head dam at the mouth of the river, was removed in August 2011. The removal of the dam was part of an effort to restore the riverine ecosystem after centuries of anthropogenic impact. Sediment traps were deployed below the dam to assess changes in metal concentrations and fluxes (Ag, Cd, Cr, Cu, Ni, Pb, and Zn) from the river system into Pawtuxet Cove. Sediment traps were deployed for an average duration of 24 days each, and deployments continued for 15 months after the dam was removed. Metal concentrations in the trapped suspended particulate matter dropped after dam removal (e.g., 460 to 276 mg/kg for Zn) and remained below preremoval levels for most of the study. However, particle-bound metal fluxes increased immediately after dam removal (e.g., 1206 to 4248 g/day for Zn). Changes in flux rates during the study period indicated that river volumetric flow rates acted as the primary mechanism controlling the flux of metals into Pawtuxet Cove and ultimately upper Narragansett Bay. Even though suspended particulate matter metal concentrations initially dropped after removal of the dam, no discernable effect on the concentration or flux of the study metals exiting the river could be associated with removal of the Pawtuxet River dam.