Formatted Title:

Bench-Scale Evaluation of the Impact of Bioturbation on Thin Layer Caps

Background/Objectives:

In situ remedies for contaminated sediments provide numerous potential advantages over dredge and disposal only remedies. These benefits include minimal disturbance, rapid risk reduction, and potential for reduced remedy costs. Thin layer adsorptive or reactive caps overlying contaminated sediments are a promising remedial technology to maintain the contaminated sediment in place and reduce the bioavailability of sediment contaminants in the biologically active zone. Bioturbation by benthic organisms results in the movement of sediments and is a design consideration in thin layer caps. It is important to estimate the potential contaminant flux resulting from the translocation of sediment particles (contaminated, clean or amendments) by bioturbation in the “habitat layer” or “biologically active zone”; this will help in addressing the impact of bioturbation on the performance of a thin layer cap. This abstract presents a laboratory study that evaluated particle translocation within and through multiple thin layer cap designs by two representative bioturbators (Neries and Macoma). The two organisms represent different modes of particle translocation and potential contaminant flux.

Approach/Activities:

Three cap designs were evaluated, each consisting of a 6-inch layer of uncontaminated sediment, with a 1-inch “reactive” layer, and three different “habitat” layers of 2, 4, and 6 inches. Two fluorescent tracers were used to examine the translocation of particles within the “reactive” layer and from the 6-inch sediment bed under the cap: a green tracer with the particle size of very coarse sand was uniformly mixed in the “reactive” layer and a red tracer with particle size of medium to fine sand was installed on the top of the uncontaminated sediment. Tanks with varying “habitat” layer thicknesses (2, 4, and 6 inches) were set up in duplicate. Forty (40) worms (Neries) and thirty (30) clams (Macoma) were added to each tank; the clams were fed weekly, and the system was monitored for 13 weeks. Weekly photos and videos were collected to estimate the rate of particle translocation to the surface for the three different caps. Following termination of the experiment, cores were collected within each cap to evaluate particle translocation within the cap and mortality was recorded.

Results/Lessons Learned:

Results of the study indicated that the tracers were translocated to the surface over time. The green tracer (i.e., “reactive” layer) was the first to appear at the surface layer of each cap followed by the red tracer (i.e., underlying sediment) after a couple of weeks. The number of each particle translocated to the surface was a function of time and cap thickness. The green tracer was the most abundant in the surficial layer for all tanks and increased on a weekly basis, though the rate of translocation decreased over time. Approximately 1 to 2 orders of magnitude less red tracer was present at the surface compared to green tracer regardless of cap thickness. Cores of each cap showed that particle translocation was from the original site (either “reactive” layer or underlying sediment) to the surface of the cap with very little being distributed within the capping media. Worms were responsible for most of the translocation of the tracer particles as they dragged these particles with them as they came to the surface to feed.

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Jesyca Jarabica