Title

Big River Borrow Pit Monitoring Project Final Report

Abstract

In-channel dredging is a possible strategy for the removal of lead contaminated sediment from the Big River. This project evaluates the feasibility of dredging by testing a borrow pit strategy at two sites along the Big River located near the Desloge tailings pile. The first site is located at what is locally known as the Bone Hole where channel sediment is trapped behind a low-water bridge. The second site is located about 2 km downstream at a natural point bar complex (the Bar Site) that has formed on the inside of a large valley bend. The fieldwork for this project was carried out in the period from October 2009 to March 2010. At each site, a series of four topographic surveys were used to monitor the changes in sediment volume: (i) pre-excavation; (ii) post-excavation of approximately 382 m3 (500 yd3) of sediment; (iii) after a major flood event (>10-year recurrence interval (RI)); and (iv) after several near bankfull events (less than 1.5-year RI). Volume analysis at the Bone Hole showed the excavated pit refilled after the large flood and remained stable after the series of subsequent near bankfull events. The “skimming” of the vegetated center bar near the head of the complex at the Bar Site may have destabilized the bar and made it more sensitive to the influence of a large flood on erosion at the head and middle bar areas. This indicates low-water bridge sites, compared to bar sites, may be the preferred alternative for mine sediment excavation activities. However, this study only evaluated one bar site and more research is needed to examine other bar settings for excavation activities and geomorphic recovery. It appears “cleaner” natural sediment is replacing contaminated sediment at both locations, and Pb and Zn concentrations decreased over the monitoring period at the Bone Hole. However, Pb and Zn concentrations at the Bar Site did not change over the monitoring period. This is likely due to remobilization of stored contaminated sediment at flows required to deposit material on the bar versus the bed at the Bone Hole. The presence of heavily contaminated fine-grained “slime” deposits previously buried by chat sediment should be located and mitigated prior to excavation activities to reduce the risk of remobilization. Bedload transport modeling and field data analysis indicate that reexcavation activities should be repeated annually or immediately after high magnitude overbank flood events to maximize the rate of contaminated sediment removal from the river during the restoration period. However, a two year re-excavation cycle would be more appropriate if the goal is to maximize the amount of sediment removed per excavation event.

Document Type

Report

Comments

Big River Mining Sediment Assessment Project
Funded by U.S. Fish and Wildlife Service Cooperative Ecosystems Studies Unit

Publication Date

5-24-2012

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