- An in situ amendment of 3.4 wt% activated carbon (AC) was mixed into the upper sediment layer using large-scale mixing technologies.
- Two commercial mixers were tested: the Aquamog Rototiller and Compass Environmental, Inc. (CEI) slurry injector.
- Four test plots of 370 square feet each were used in the field study.
- Various treatments were applied to the four plots.
- Plot E served as a reference plot (a non-mixed control).
- Plot C was treated by mixing the sediment using the Aquamog Rototiller, but without applying AC.
- Plots D and F were treated by applying an approximate 3.4 wt% AC and mixing it into the sediment with the Aquamog and CEI slurry injector system, respectively. The AC dose was applied into the test plots to a 1 ft depth.
Richard G. Luthy
Bioaccumulation Studies Leader
U.S. Army Engineering Research and Development Center
3909 Halls Ferry Road
Vicksburg, MS 39180
Yang & Yamazaki Environment & Energy
Building, Room M03
473 Via Ortega
Stanford CA 94305-4020
Mixing Technology A Leader
Aquatic Environments, Inc.
Mixing Technology B Leader
Mark A. Fleri
Compass Environmental, Inc.
BRAC Environmental Coordinator, Hunters Point Shipyard
Base Realignment and Closure
1455 Frazee Road, Suite 900
San Diego, CA 92108-4310
Parcel F Remedial Project Manager
Dane C. Jensen
Base Realignment and Closure
1455 Frazee Road, Suite 900 San Diego, CA 92108-4310
Polychlorinated Biphenyls (PCB)
Historical site activities associated with Ship Repair and Maintenance at Hunter Point Shipyard (HPS) from 1945 to 1991 resulted in the release of chemicals to the environment, including offshore sediments in Parcel F.
Type/Quantity of Media Treated:
Sediment (quantity not documented)
Purpose/Significance of Application:
The overarching goal of this project was to demonstrate that AC sorbent mixed with sediment is a cost-effective, in situ, nonremoval, management strategy for reducing risk and the bioavailability of PCBs in offshore sediments at the HPS site. Specific demonstration objectives included:
- Demonstrate and compare the effectiveness, in terms of AC application and ease of use, of two commercially available, large-scale mixing technologies.
- Through field testing, demonstrate that AC treatment reduces PCB bioaccumulation.
- Demonstrate that no significant sediment resuspension and PCB release occurs after the large-scale mixing technologies are used.
Regulatory Requirements/Cleanup Goals:
The primary performance objective of this demonstration was to reduce the bioavailability of PCBs in sediments and thereby reduce the bioaccumulation of PCBs in indigenous organisms in the demonstration area.
Sampling was conducted once before and three times after the application of AC. Samples were taken from five different sampling locations in each of the four plots. Results from these samples indicated the following.
- Ex situ bioassay results indicated significant reduction in PCB biouptake for M. nasuta exposed to AC-amended plots (Plots D and F) compared to the control plots (Plots C and E).
- Using a one-time, approximately 30-minute mixing event, AC amendment was able to reduce PCB bioaccumulation in marine clams (M. nasuta) by 30-50% over the 3 year study period.
- In situ bioassay results were sometimes confounded by field conditions resulting from newly deposited sediment and heat stress. To overcome these factors, ex situ bioassays with M. nasuta were conducted with field sediment in the laboratory, which showed about 50% reduction in PCB bioaccumulation with a 2% AC dose.
- Sampling results indicated no significant difference in PCB tissue concentrations of indigenous organisms, the PCB levels in indigenous amphipods responded to overlying water rather than the under-lying sediment layer.
Based on these results, this three-year project successfully demonstrated that the top layer of sediments in a PCB-contaminated tidal mudflat could be amended with AC using large-scale commercial mixing equipment. Field-scale AC-amendment reduced the availability of PCBs to water without adversely impacting the natural benthic community of macroinvertebrates or releasing PCBs into overlying water.
The final cost for the three year field scale study including site preparation, site characterization, AC amendment application, and monitoring was approximately $174,500. A breakdown of these costs is provided below:
- Treatability Study and Baseline Characterization (includes materials and lab fees): $71,100
- Mobilization/Demobilization of Aquamog Mixer: $10,000
- Mobilization/Demobilization of CEI Injector: $10,000
- AC Amendment (350 lb/100ft2): $7,000 (The cost of AC for this field project was approximately $2.9/lb)
- Operation and Maintenance Costs (includes materials and lab fees): $76,400
HPS is a former Navy installation located on a peninsula in the southeast corner of San Francisco, CA. The site is approximately 928 acres, with approximately 432 acres of offshore sediment. Historical site activities associated with ship repair and maintenance (1945 to 1991) released chemicals such as Polychlorinated Biphenyls (PCB) to the environment, including offshore sediment located near the South Basin. Because PCBs tend to adsorb to fine-grained sediment particles and organic matter, sediment resuspension and deposition are major contaminant transport pathways at the site. In 1989, HPS was placed on the National Priorities List (NPL). The Navy closed the Base in 1991 under the Defense Base Realignment and Closure (BRAC) Act. The base is being remediated and will be converted to nonmilitary use.
A site characterization study was conducted in 1991 to evaluate the presence of contaminants in offshore areas of the HPS. Results of the study indicated that PCBs were a major risk driver for HPS Parcel F located at the HPS tidal mudflat within the South Basin. Within this area of the site, PCB concentrations were identified as approximately 2 parts per million (ppm) from a depth of 0 to 12 inch. As a result, Parcel F of the site was selected for the in situ remedial technology demonstration study.
A three year field-scale project was conducted at the site to demonstrate that Activated Carbon (AC) sorbent mixed with sediment is a cost-effective, in situ, nonremoval, management strategy for reducing risk and the bioavailability of PCBs in offshore sediments. To compare mixing technologies and evaluate the AC treatment technology, four test plots with areas of 370 square feet each were studied. Two test plots were amended with AC in January 2006 using two different mixing devices (Aquamog rototiller system and CEI slurry injector system), respectively. At each of these plots, AC was added to a nominal 1 ft depth. The other two test plots served as controls; one test plot served as a mixing control, and the other served as a non-mixing control. The four plots were analyzed using a combination of sampling and analysis/statistical tests. The plots were sampled once before and three times after treatment. This three-year field scale project successfully demonstrated that the top layer of sediments in a PCB- contaminated tidal mudflat could be amended with AC using large-scale commercial mixing equipment. Field-scale AC-amendment reduced the availability of PCBs to water without adversely impacting the natural benthic community of macroinvertebrates or releasing PCBs into overlying water.