Ecotoxicological Assessment of the Efficacy of Capping Materials in Reducing Zinc Bioavailability in Pit Lake Sediment

Abstract

The contamination of sediments in aquatic ecosystems is a widespread environmental issue. Sediment serves as a reservoir for contaminants and is therefore a potential sink and source of toxicants. Remediation techniques separate, destroy, stabilize, or convert contaminants to less toxic forms and include ex-situ and in-situ physical, chemical, or biological treatments. One such technique is capping, which is intended to isolate sediment-associated contaminants from surface water and benthic community interactions. Although capping shows promise in bench-scale studies, there is a need for more pilot-scale studies that incorporate multiple lines of evidence, coupled laboratory and field studies, and test organisms that can demonstrate effects through various exposure pathways. The objective of my research is to fill this gap with a weight-of-evidence approach to evaluating the performance of various capping materials in remediating zinc-contaminated sediment at East Wilson Pond (Hot Springs, Arkansas). The study consisted of both field and laboratory ecotoxicological tests using Hyalella azteca, Daphnia magna, and Chironomus dilutus. Field experiments assessed acute toxicity and water chemistry in test plots covered with three capping materials: Aquablok, limestone, and limestone-bone char. The laboratory tests involved a series of acute toxicity tests and water chemistry sampling conducted in core microcosms created from site-collected sediment. Capping materials assessed in the laboratory tests included two from the field tests (Aquablok, limestone) and two selected per literature review and site geotechnical surveys (zeolite, apatite). Overall, there were no differences in biological endpoints between treatments in both the field and laboratory tests, likely due to below-threshold dissolved zinc concentrations in the surface water. Zeolite was the most effective mitigator of zinc release, but also caused adverse organism effects. This indicates that all treatments successfully prevented zinc release from the sediment, although some may be less effective under certain hydrologic conditions. Final remedy selection will ultimately depend on results from the field and laboratory studies, as well as site conditions, cost, future uses, and ecological considerations.