A former mining site has been the subject of extensive remediation and restoration, with a significant focus on disconnecting mine spoils from groundwater and managing the quantity and quality of runoff. A remaining task is ensuring concentrations of zinc (Zn) in the stream outflow of a pit lake are reduced below water quality standards. The efficacy of multiple capping materials for decreasing Zn dissolution from sediments was conducted under natural and reasonable worstcase conditions (pH = 5.5). Capping materials included AquaBlok™, limestone, and limestone-bone char. Field exposures were conducted in limnocorrals that isolated overlying water columns above the sediment and capping treatments. Simultaneous in situ and ex situ toxicity tests were conducted using Daphnia magna, Hyalella azteca, and Chironomus dilutus. In situ caged organisms were protected from temperature shock (warm epilimnetic waters) by deploying within a Toxicity Assessment Container System (TACS). Organisms were exposed to surficial sediments, caps, and hypolimnetic overlying waters for 4 d. Ex situ testing was conducted in core tube mesocosms containing sediments and caps at similar temperatures (15-19°C). Results demonstrated the usefulness of TACS deployment in stratified lake systems. There were no differences in responses between treatments involving sediment capping materials in both in situ and ex situ tests. The lack of differences was likely due to dissolved Zn in surface water being below the hardness-adjusted threshold effects levels (164 μg L -1 ). This fieldand laboratory-based weight-of-evidence study provided site-specific data to support the selection of an effective remedy, with reduced uncertainty compared to laboratory and chemistry-only approaches. Environ Toxicol Chem 2020;39:240-249. © 201939:240-249. © SETAC . 2018. Enhancement of active thin-layer capping with natural zeolite to simultaneously inhibit nutrient and heavy metal release from sediments. Ecol Eng 119:64-72. Zhuang Y, Allen HE, Fu G. 1994. Effect of aeration of sediment on cadmium binding. Environ Toxicol Chem 13:717-724. wileyonlinelibrary.com/ETC © 2019 SETAC Laboratory and field assessment of sediment capping on Zn flux-Environmental Toxicology and Chemistry , 2020;39:240-249