During the 2015 dredging season at Ashtabula Harbor Ohio the U.S. Army Corps of Engineers Engineer Research and Development Center (ERDC) and the Buffalo District launched a study in August to determine if placing activated carbon with contaminated sediment could reduce bioaccumulation at a 50-foot open-water placement site.
Mike Asquith Corps Buffalo District dredging program manager said placement sites for dredged sediment is becoming an issue on the Great Lakes. The district manages Lake Ontario and Lake Erie – and the 14 commercial and 18 recreational harbors between the two of them. Asquith said the dredged sediment for all the harbors with the exception of Cleveland is now going to open-water sites.
Contaminated sediment is also a growing concern on the Great Lakes along with shifting standards for dredging and placement which in part led to the study Asquith said. The contamination in Ashtabula Harbor also is low making it ideal for the study so it won’t actually place harmful contaminants in the ecosystem and help to find a beneficial use for slightly contaminated sediment.
“When you have marginally contaminated sediment it may not be suitable in ecologically sensitive areas where you’re trying to create habitat marshes and wetlands used by fish and birds. So the question is how can we make these marginally contaminated sediments more acceptable for beneficial use and open-water and how do we do this in the normal context of a project using dredging equipment and how effective would it be” asked Paul Schroeder Corps ERDC sediment management team leader.
Here workers place the activated carbon with sediment already in a barge.
The project is funded with O&M funds from the Buffalo District for the field activity and ERDC will fund the work to evaluate the data.
For a long time activated carbon has been used in water air and wastewater treatment industries for purification. About 15 year ago Schroder said the first project in California added carbon to a contaminated sediment in-situ in the intertidal zone. Since then 10 to 15 projects have tested it at a demonstration level but always in-situ. The process had never been tested within the normal context of a dredging project where it would be mixed and placed separately.
Schroeder said it is known that over months and years the contaminants will bind with the activated carbon which absorbs contaminants on the surface and won’t let go The big research questions for the project Schroeder said were how to mix the carbon effectively into the sediment and how to place it and how will it stay together?
While the study would place a cap of sediment with the activated carbon atop other sediment the project was not a traditional capping project Schroeder explained. A true capping project would create isolation between contaminants and the surface. Instead this study wants to place material where the organisms reside and eat in order to study bioaccumulation. “The material that we’re placing is fine grain sediment which is different from a normal cap which is typically composed of sand” Schroeder said. This is an advantage for the process he said because the fine grains can hold the carbon in place and trap it in and make a more cohesive stable surface than fine sand could.
The barge transported the sediment mixed with activated carbon to the open-water site for regular placement. Four barges of regular sediment were placed before one mixed with the carbon.
The study will use two different types of activated carbon – granular and powder. The main difference is particle size. The granular carbon is much larger more the size of coarse sand particles and the powder carbon is the size of fine silts. The size difference affects the behavior of the carbon Schroeder explained. “The powder is much more difficult to handle and it’s hard to get wet. It gets blown away in the wind. On the plus side it gets dispersed much better and the space between the particles is very small so it’s faster acting” he said. For its larger granular counterpart it will more quickly settle to the bottom and the process loses less carbon and the granules can be more readily seen in the sediment opposed to sediment with mostly powder.
At the Ashtabula Harbor project a clamshell bucket mixed the dredged sediment and the activated carbon in a scow which then transportsed the mixture to a placement site. The scow opened normally where it was planned that sediment and carbon would fall through the water column with enough energy to hit the bottom and spread laterally around 300 feet in diameter creating a layer four to five inches Schroeder said.
Before a barge load with carbon was placed four loads with regular dredged sediment went first. “We wanted to see the layer and then also in the future we’re going to be sampling this again to see if the surface sediments have lower bioaccumulation than other material” Schroeder said.
The sediment was placed on August 15. For the study 7200 cubic yards of dredged sediment were placed in the open-lake area; 1200 cubic yards of sediment was treated with the two types of activated carbon.
For a baseline measurement the study sampled the barges without carbon. For the barge placed with carbon they sampled the bottom layer from each of the eight compartments of the dump scow and the top layer to determine the uniformity the carbon addition. Three weeks after placement samples were taken at 25 locations along the placement site analyzing the mixture for how much carbon was present and how much was granular vs. powder. Next summer the project will sample in July and August. In addition to carbon content testing they will run bioaccumulation testing. The study is expected to be complete by Spring 2017.