PT15 Contaminated Harbour and River Sediment
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(PT251) Laboratory Evaluation of In-situ Sand-Capping for Controlling the Upward Flux of PAHs from Tar-Contaminated River Sediments.

Hyun, S1, Jafvert, C1, Lee, L1, Rao, P S C1, 1 Purdue University, West Lafayette, IN, USA

ABSTRACT- River sediment that contained coal tar as a non-aqueous phase liquid (NAPL) was collected at several locations in a river adjacent to a former manufactured gas plant (MGP). The concentration distributions of 16 mono- and polycyclic aromatic hydrocarbons (MAHs and PAHs, respectively), including ethylbenzene, naphthalene, phenanthrene, and pyrene, between water and the sediment showed a consistency with a Raoult's Law-based quantitative relationship. The calculated potential PAH flux posed from the upward ground water recharge through the coal-tar contaminated sediment to the water column was significant. To protect the benthic community and to attenuate the flux of coal tar constituents from the sediment (to the river), a proposed remediation scheme is capping the sediment with a sand layer. A sand cap will be effective only if: (1) flux can be reduced via hydraulic control, or (2) flux can be reduced via utilizing the sand layer as a reactive barrier. We hypothesized that PAH flux would be reduced via microbial degradation by indigenous microorganism during mass transfer through the capping layer if sufficient oxygen and nutrients are present. In an aqueous extract of tar-sediment that was amended with oxygenated water (DO = 8 ppm) and nutrients (0.5 g/L KH2PO4, 1.5 g/L K2HPO4, 0.1 g/L NH4Cl), the concentrations of naphthalene, phenanthrene and anthracene were reduced by 99 %, 76 %, and 79 %, respectively, over a one-month period. In laboratory 1-D column studies with sand capped tar-sediment, the advective PAH flux from the imposed upward gradient through the contaminated sediment was also reduced by supplying oxygenated water to the sand layer, with decreasing PAH concentrations with distance through the sand cap. PAH flux was the highest when sediment and sand layer were sterilized by HgCl2 and deoxygenated water (DO < 1.5 mg/L) was supplied to the sand layer. A decrease in aqueous PAH concentrations in both batch and column tests can be attributed to enhanced microbial activity stimulated by supplied oxygen and nutrients. Similar studies are being conducted with a 2-D flow system in preparation for a pilot-scale field test at the site.

Key words: PAH, coal tar contaminated sediment, microbial degradation, sediment remediation strategy

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