Approaches to quantify the reduction in chemical availability in sediments amended with solid materials (e.g., sand, dredged material (DM), activated carbon, etc.) can involve complicated, expensive, and time-consuming bench- or pilot-scale evaluations.
These evaluations often do not (or cannot) evaluate the long-term conditions in which the solid amendment has been fully or partially mixed with underlying contaminated sediment through bioturbation or other natural processes. In response to these challenges, we devised and tested a rapid method to assess the long-term reduction of chemical availability gained from solid amendments: the Ex Situ Chemical Availability Recontamination Grab Observation (ESCARGO). The ESCARGO approach combines a small sample of unamended site sediment (~10-50 g, dry weight), the sediment amendment(s) of interest, and a polyethylene (PE) passive sampler under agitated conditions for 28 days. Following the equilibration period, the PE is analyzed to measure concentrations of freely-dissolved organic chemicals. Three treatments are assessed: 1) an unamended sediment; 2) a 10% sediment/90% amendment mixture to represent "short term" remedy performance; and 3) a 40% sediment/60% amendment mixture to represent "long term" remedy performance.
Approach/Activities. The ESCARGO approach was applied to evaluate the reduction in PCB availability for a sediment from Pearl Harbor amended with 3 amendments: 1) sand; 2) a DM with 0.4% Total Organic Carbon (TOC) content; and a DM with 1.0% TOC content. These treatments were also applied in an in situ mesocosm experiment using the Navy's Remedy and Recontamination Assessment (RARA) arrays, deployed in Pearl Harbor. The mesocosm used the same sediment and amendments, which were allowed to equilibrate with site conditions for 1 month and then evaluated with a 28-day in situ PE passive sampler deployment to measure PCB availability.
Results/Lessons Learned. ESCARGO short term results indicated that the higher carbon DM reduced available PCBs by an average (SD) of 81% (± 5%), performing better than the lower carbon DM and sand (reduction in PCB availabilities of 51% (± 6%) and 45% (± 9%), respectively). ESCARGO long-term results also indicated superior performance of the higher carbon DM, but indicated slightly lower reductions in PCB availability, as compared to the short term results (reduction in PCB availabilities of 66%, 39%, and 37% for higher carbon DM, lower carbon DM, and sand, respectively). Results suggest additional PCB binding capacity of the higher carbon DM compared to the sand and lower carbon DM, and project long-term reduction in PCB availability. ESCARGO short-term results agreed with the in situ mesocosm results within an average factor of 1.3, and the mesocosm results also indicated the superior performance of the higher carbon DM (reduction in PCB availability of 86% (± 2%)). The ESCARGO approach is a rapid and inexpensive method to compare the site-specific performance of multiple contaminated sediment amendments and provide a prediction of their long-term performance.