A.2 Anacostia River, Washington, D.C.

A.2.1 Contacts

Danny Reible

Texas Tech University


[email protected]

A.2.2 Summary




Pilot (field scale)

Contaminants of Concern:

PAHs, metals

Final Remedy:

Amended cappingTechnology which covers contaminated sediment with material to isolate the contaminants from the surrounding environment.

A.2.3 Site Description

A demonstration of amended capping was conducted at the Anacostia River in Washington D.C. as described by Reible et al. (2006)

A.2.4 Remedial Approach

Final selected remedy: Amended capping

Four caps were placed in the spring of 2004 to assess the potential effectiveness of capping on a field scale. The four capping materials used were AquaBlok, coke in a Reactive Core Mat, apatiteName given to a group of phosphate minerals, usually referring to hydroxylapatite distributed widely in igneous, metamorphic, and sedimentary rocks, often in the form of cryptocrystalline fragments. Hydroxylapatite is used in chromatographic techniques to purify proteins and other chemicals., and sand. AquaBlok is a manufactured clay-like material for permeability1) Characteristic of a material or membrane that allows liquids or gases to pass through it; 2) The rate of flow of a liquid or gas through a porous material. control. Coke is a by-product of coal manufacturing and is an inexpensive, moderately strong sorbent of organic compounds. The use of coke in the demonstration primarily served to evaluate placement of AC or other low-density material in a thin mat. Apatite has been proposed for the sequestrationThe act of segregation. In environmental terms this usually refers to separation of materials by use of various technologies. Carbon sequestration refers to the capture and removal of of CO2 from the atmosphere through biological or physical processes. of heavy metals. 

All bulk caps were successfully placed using a clamshell bucket. The caps were constructed with a nominal thickness of six inches overlain by a six-inch sand layer. The coke was placed in a reactive core mat with a thickness of less than one inch overlain by a six-inch sand layer.

A.2.5 Monitoring

The AquaBlok capA covering over material (contaminated sediment) used to isolate the contaminants from the surrounding environment. effectively eliminated groundwater upwelling in the area capped and to divert groundwater towards the center of the river (Reible et al. 2006). The AquaBlok cap was monitored with an inclinometer, which provided unique information on vertical cap movements. Because of the interaction of tides with the hydrostatics of the groundwater, the AquaBlok cap uplifted 0.1-0.5 mm every tidal cycle (up at low tide and down at high tide). In addition, continued gas formation in the river sediments led to an accumulation of gas beneath the low-permeability layer. After approximately 1½ months, the lower end of the AquaBlok lifted approximately 0.75 m to release gas, and this behavior continued approximately every six weeks throughout the summer. This behavior was not noted in the summer of 2005, although it is unclear if that was due to failures of the inclinometer, a cessation of significant  gas ebullitionThe act, process, or state of bubbling up usually in a violent or sudden display. due to  the consumption of labileEasily altered. organic matter, or normal migration through a compromised AquaBlok layer.

The coke layer was also monitored extensively using bulk-solid monitoring, bioaccumulationThe accumulation of substances, such as pesticides, or other organic chemicals in an organism. Bioaccumulation occurs when an organism absorbs a toxic substance at a rate greater than that at which the substance is lost. Thus, the longer the biological half-life of the substance the greater the risk of chronic poisoning, even if environmental levels of the toxin are not very high. monitoring, and passive sampling for PAHs in the interstitial space of the sediment and cap layers. Bulk solid monitoring showed the accumulation of new sediment on the top of the cap due to nearby stormwater drains and, potentially, surficial sediment migration (Reible et al. 2006). This result demonstrated the potential for recontamination due to stormwater inputs and confirmed methods for evaluating its influence. Bioaccumulation monitoring in caged organisms and passive interstitial pore-water sampling (using polydimethylsiloxane partitioning samplers) showed a good correlation between pore-water concentrations and bioaccumulation of PAHs and also indicated the performance of the cap (Lampert, Lu, and Reible 2013). The coke layer was not designed to significantly retard contaminant migration through the cap but instead to demonstrate the ability to place light sorbents in a reactive core mat (this was the first application of a reactive core mat for sediment remediationThe act or process of abating, cleaning up, containing, or removing a substance (usually hazardous or infectious) from an environment.).

As expected, the monitoring results three to five years after placement showed that the caps were at steady state relative to organic contaminant migration through the cap. Even at steady state with upwelling velocities of the order of 1 cm/day, the concentrations in the near-surface biologically active zone were 70-80% lower than concentrations prior to capping.

A.2.6 References

Reible, D., Lampert, D., Constant, D., Mutch Jr, R. D., & Zhu, Y. 2006. Active capping demonstration in the Anacostia River, Washington, DC. Remediation Journal 17(1), 39-53.

Lampert, D. J. X. Lu, D. D. Reible. 2013. Long-term PAH Monitoring Results from the Anacostia River Active Capping Demonstration Using Polydimethylsiloxane (PDMS) Fibers. Environmental Science: Processes & Impacts.



Publication Date: August 2014

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