Evaluation of a novel zero-valent iron based reactive amendment for wetland sediment contaminated with historical gold mine tailings using Chironomus dilutus

Numerous wetlands in Nova Scotia (NS) have been contaminated by mercury (Hg) and geogenic arsenic (As) from gold mine tailings deposited between the mid-1800s and mid-1900s. These two potentially toxic elements have been shown to bioaccumulate in benthic invertebrate species in wetlands impacted by legacy gold mine tailings. We evaluated the efficacy of a Thin Reactive amendment and Protective capping (TRaP) treatment composed of silica sand, zeolite, zero-valent iron (ZVI), and bentonite as a potential remediation strategy for these wetlands. We assessed whether TRaP could limit Hg and As toxicity and bioavailability to Chironomus dilutus larvae. Overlying water from TRaP-treated sediments had dissolved methylmercury (MeHg) and As concentrations which were 78 % and 99 % lower respectively compared to untreated contaminated sediments. Chironomid survival improved from 45 % in the contaminated sediment to 90 % when treated, while growth increased by 36.5 %. Additionally, TRaP reduced MeHg levels and improved chironomid growth and survival beyond that observed in sediments treated only with ZVI. However, total Hg bioaccumulation in the chironomids remained similar across contaminated and treated sediments at the end of the test. This laboratory experiment demonstrates that TRaP holds considerable promise as a practical remediation strategy for legacy-contaminated wetland ecosystems. Further research is needed to assess its effectiveness under real-world field conditions.