A number of sites around the United States have used activated carbon (AC) amendments to remedy contaminated sediments. Variation in site-specific characteristics likely influences the long-term fate and efficacy of AC treatment. The long-term effectiveness of an AC amendment to sediment is largely unknown, as the field performance has not been monitored for more than three years. As a consequence, the focus of this research effort was to evaluate AC’s long-term (6–10 yr) performance and efficacy at two sites where sediments have been treated with AC, representing a range of physical environments. Specifically, the individual objectives comprised the following:
The data collected enabled comparison of AC distribution, polychlorinated biphenyl (PCB) concentrations, and bioaccumulation measured over the short and long term (months to years).
Assessments were performed at two pilot-scale demonstration sites, Grasse River, Massena, New York and Canal Creek, Aberdeen Proving Ground (APG), Aberdeen, Maryland, representing two distinct physical environments. Sediment core samples were collected after 6 and 10 years of remedy implementation at APG and Grasse River, respectively.
At the Canal Creek pilot study site, two commercially available technologies designed to deliver AC in a pelletized form into sediments were evaluated: SediMite, produced and marketed by Sediment Solutions, and AquaGate, manufactured and marketed by AquaBlok.
In the Grasse River pilot study, mixed and layered carbon treatments were evaluated. In the mixed treatment, a tiller-like device mechanically mixed bituminous coal–based AC (Carbsorb, Calgon Carbon) into surface sediments, while in the layered treatment a coconut shell–based AC (055C-CNS-V000, Calgon Carbon) was layered by broadcasting of the material over the water surface and allowing it to settle onto the sediment surface and mix into the sediments via natural processes (for example, bioturbation).
Data collected at each of the demonstration sites enabled a comparison of AC distribution, PCB concentrations, and bioaccumulation measured over the short and long term under differing hydrologic regimes, AC amendment types, and application techniques.
Overall, the long-term monitoring at the two pilot-scale studies revealed that AC is stable in the sediment environment at two very distinct sites (that is, vegetated estuarine wetlands [Canal Creek, Maryland] and a riverine system [Grasse River, New York]) with different AC amendments deployed using different application technologies. Results from the present study provide the following key insights into the long-term performance of AC amendment to river sediments:
The data collected enabled comparison of AC distribution, PCB concentrations, and bioaccumulation measured over the short- and long-term (months to years). The study found an important issue with performing ex situ measurements for sediments from a tidally influenced marsh site. Intact cores in the laboratory do not appear to accurately reflect the field exposure conditions after treatment for passive sampling or organism exposure when there is very active groundwater movement due to diurnal tidal pumping. This can be especially true where a major source of the pollutants lies deeper in the sediments and there is an active tidal pumping process as seen at the Canal Creek site.