Feasibility evaluation of a blended cover with activated carbon for in-situ stabilization of DDT in sediment

Abstract

Activated carbon (AC) sediment amendment is an in-situ remediation technology in which the applied AC immobilizes organic contaminant flux from sediments, thereby reducing contaminant bioavailability and associated risks. While various studies have demonstrated the feasibility of in-situ AC treatment, hesitation to apply this technology exists due to limited experience under field-specific scour conditions and hydrodynamic forces. To address this concern, we conducted a feasibility study for an AC-blended cover at the Lauritzen Channel of the United Heckathorn Superfund Site in Richmond, California, United States, which was contaminated with dichlorodiphenyltrichloroethane and its metabolites (DDx) as well as dieldrin. Vessel activities causing sediment disturbance were identified as key factors for remedy selection. A blended cover with AC and coarse materials was designed to withstand varied hydrodynamic conditions and AC stability was tested in a current flume. The cover comprised medium-size gravel (D₅₀ = 15 mm, D₉₀ = 19 mm) with 4 % granular AC by weight. Flume erosion studies showed minimal AC loss (1–2 % of total AC) under shear forces of 9–31 Pa, which was equivalent to or exceeded the estimated worst-case erosional conditions in the channel induced by a hypothetical, stationary tugboat propelling at high power thrust. The treatability performance of the engineered blended cover design was evaluated through mesocosm studies using site sediment and various cover options. Post-treatment assessments on days 5 and 145 showed rapid reductions in freely dissolved (C_free) DDx and dieldrin in the blended cover layers and surface water. For example, by day 145, C_free DDx was reduced by over 98 %, meeting US EPA remedial goals for the site. It is concluded that the combination of both stability and performance testing demonstrates that an engineered blended cover-AC design would be a feasible remedial option at the site, and that this testing approach can be applied to evaluate in-situ treatment in other sediment cleanup activities.