Development of an in situ Remediation Strategy for a Metals-Contaminated, Alkaline Groundwater

Abstract

Groundwater beneath a former industrial site in Tukwila, WA contains dissolved metals and organic material at high levels, as well as elevated pH (10-12). Contaminated groundwater discharge to site-adjacent waterways must be controlled to minimize impact to potential receptors. In a preliminary study, the efficacy of five amendments (chitosan, apatite, granular activated carbon (GAC), Thiol-SAMMS®, and limestone) was examined for the removal of copper, lead, vanadium, and arsenic for both unaltered and pH-adjusted (pH = 8) site groundwater in laboratory batch studies. Dissolved metals and dissolved organic carbon (DOC) concentrations in site groundwater were measured both before and after pH adjustment, and after application of amendments. pH adjustment generally improved amendment performance. The greatest reductions in dissolved copper (70%), lead (62%), and vanadium (62%) concentrations were observed under pH-adjusted conditions in the presence of apatite. Thiol-SAMMS® also performed well under pH-adjusted conditions (Cu: 69% reduction; Pb: 46% reduction; V: 24% reduction), and also removed both copper (55% reduction) and lead (31% reduction) at the original pH. GAC was somewhat effective at removing lead under all conditions (15-30% reduction) and copper (31% reduction) when the pH was adjusted, and also reduced DOC concentrations under all conditions (24-27% reduction). These results suggest that a strategy using pH adjustment and some combination of apatite, Thiol-SAMMS®, and/or GAC may be best suited for remediation at this site. Further studies testing combinations of bone char (in place of apatite), GAC, and Thiol-SAMMS® were performed. In addition, the impacts of air sparging and pH adjustment through addition of hydrochloric acid or ferrous sulfate heptahydrate (FeSO4:7H2O), a coagulant commonly used in conventional water treatment, were evaluated. The introduction of FeSO4:7H2O resulted in the formation of coagulation solids, further decreases in pH after the coagulation solids were removed, and the largest significant reductions of dissolved copper (at most 81%), arsenic (72%), vanadium (80%), and DOC (88%) of any treatment evaluated. Sparging with air increased reductions of lead concentrations for all pH adjustment strategies and slightly increased reduction of vanadium concentrations only when the pH was adjusted with FeSO4:7H2O. Combinations of bone char, GAC, and Thiol-SAMMS® generally did not confer a substantial advantage over single amendment treatments. Bone char, though, acted as a buffer to curb further decreases in pH after coagulation solids were removed. The drops in pH after the coagulation solids were removed, either with or without air sparging may be related to the oxidation of residual ferrous iron, but it is unclear if this single mechanism can explain the magnitude of the observed pH declines. Using the ferrous sulfate treatment strategy potentially combined with passive barrier or cap of bone char, flow-through column experiments will be designed to show how implementation of the strategy would impact the subsurface hydrology and to determine whether this strategy can achieve applicable or relevant and appropriate requirements (ARARs).