Evaluating drinking water treatment residuals as an in-situ capping material for metal-contaminated sediments

This study evaluated drinking water treatment residuals (DWTR) as an in-situ capping material for metal-contaminated sediments using Gust-chamber experiments. Metal release from non-capped and DWTR-capped sediments was measured under increasing shear stress (τ) from 0.05 to 0.4 Pa. Fathead minnow (FHM) juveniles (Pimephales promelas) were exposed to water from these sediments in 96-h bioassays to assess DWTR's efficacy in reducing metal toxicity. Sand was used as an inert capping material for comparison. Diffusive gradients in thin films (DGT) assessed DWTR's impact on vertical metal concentration profiles in sediment pore water and overlying water, with concentrations determined by ICP-MS. Without capping, increasing τ raised metal concentrations in the overlying water from 45 to 95 mg/L for Cd and Zn, 4-10 mg/L for Cu, and 2-4 mg/L for Pb. Sand capping reduced these levels, with Cd and Zn ranging from 4 to 21 mg/L, Cu from 0.26 to 0.63 mg/L, and Pb from 0.051 to 0.23 mg/L. DWTR capping significantly lowered metal concentrations in the overlying water, with Cd ranging from 1 to 8 μg/L, Zn from 30 to 40 μg/L, Cu from 2.5 to 5 μg/L, and Pb from 1 to 2 μg/L. Therefore, beyond the physical barrier effect, the DWTR cap immobilizes metals through other mechanisms such as sorption and precipitation. Bioassays showed that DWTR significantly decreased metal toxicity to FHM, while sand-capped and non-capped sediments caused 100% mortality. DGT confirmed DWTR reduced metal fluxes at the sediment-water interface by up to two orders of magnitude.