The objective of this project was to foster a broader understanding and acceptance of the enhanced monitored natural recovery (EMNR) remedy through demonstration and validation of performance and cost effectiveness at contaminated Department of Defense (DoD) sediment sites. The approach to the demonstration and validation of the EMNR process focused on the following key technical performance issues:
These criteria formed the basis of the Performance Objectives (POs). Data was collected in support of these POs and provided multiple lines of evidence for assessing the effectiveness of EMNR as a remedy at contaminated sediment sites.
EMNR involves the placement of a thin layer (commonly, <30 cm) of clean sand or clean sediment over contaminated sediment, coupled with ongoing natural recovery processes and a monitoring program, to achieve ecological recovery and risk reduction at contaminated sediment sites. In general, this thin-layer capping (TLC) is not designed to provide complete chemical isolation, but to provide a reasonable degree of physical isolation and reduction toward lower chemical concentrations targeting site-specific remedial action objectives and remedial goals; EMNR also reduces potential resuspension or transport of contaminated sediment particles.
The project site for implementation of this study was Site 99, the Quantico Embayment site, Quantico, Virginia. Grain sizes of the TLC were selected in the final design to be stable during both normal river flows and during periods of flood flows and storm-generated waves. Ideally, sediment sizes would be chosen to match surrounding grain sizes within the freshwater tidal systems of the Potomac River. While clean sand was used at the Quantico Site, TLC material can include a broader range of clean material, including clean dredged sediment that meets the chemical criteria for reuse. In some cases, dredged sediment may be preferable to quarried sand because it has natural organic matter to support benthic life and to help sequester and retard dissolved contaminant transport from underlying sediment.
Although conventional isolation caps have demonstrated effectiveness in the management and remediation of chemically impacted sediment, rigorous demonstration and validation of the effectiveness of EMNR remains limited (USEPA 2005). Ongoing questions regarding the application, performance, and ecological impacts of EMNR have limited its widespread implementation. To address these implementation issues, the following relevant questions were posed. Evaluation of these questions based on the literature compiled and the demonstrations conducted as part of this project are presented below.
Is artificially increased sediment deposition via TLC placement an effective strategy for enhancing MNR and accelerating natural system recovery rates?
The effectiveness of the TLC strategy for accelerating MNR appears to be a viable remediation approach depending on site conditions. From a process perspective, key aspects of the success of the TLC and the overall EMNR approach are that: (1) the TLC remain relatively stable above the sediment to be isolated; (2) any new deposition is relatively clean compared to surface sediment goals, even if the rate of deposition is low; (3) bottom-up mixing of the TLC is limited to the extent that the elevated levels of contamination in the underlying sediment do not unduly influence the exposure in the surface sediments following placement of the TLC; (4) advection rates through the cap are not so significant that they lead to a high level of porewater movement from below the TLC into the TLC; and (5) the remedy should demonstrate direct reduction in bioavailability over the short term and long term. For the Quantico embayment site, all of these conditions were documented to be satisfied. Multiple measures of cap thickness and elevation indicated that the cap material was maintaining relative stability within design guidelines. New deposition, as characterized by sediment traps and surface sediment interval samples, was generally low in DDX (the sum of dichlorodiphenyldichloroethane [DDD], dichlorodiphenyldichloroethylene [DDE], and dichlorodiphenyltrichloroethane [DDT]). Bottom-up mixing was documented to be limited. While advection rates were not directly measured, porewater measurements at critical intervals within the cap showed that advection was not significant enough to unduly influence the concentrations within the cap. Finally, direct measurements of bioavailability including uptake in organisms and porewater concentrations generally indicated significant reductions over both short and long time periods out to two years.
How sensitive is EMNR performance to the accuracy of TLC placement?
Sensitivity of the EMNR performance to the accuracy of TLC placement appears to be relatively high because the layer being applied is generally thin and on the same order of magnitude in thickness as the bioactive zone of the sediments. To be effective, the TLC must also accommodate a certain degree of bottom-up mixing that is likely to occur either during the installation or due to physical or biological disturbance over time. Thus, key aspects of the sensitivity to placement include the relative thickness of the TLC compared to the bioactive zone, the degree of bottom-up mixing that is expected based on construction methods, and site-specific likelihood of physical and biological disturbance following placement. For the demonstration at Quantico Embayment, the bioactive zone was relatively shallow because of the freshwater, riverine nature of the site. Also, it was observed that the installation of the TLC generally achieved target thickness throughout the site so that there were few areas where biological activity was likely to interact with the underlying sediments.
What are the short-term construction (risk-of-remedy) effects associated with EMNR and to what extent does TLC application influence benthic community survival?
The primary risks related to the construction of the TLC appear to be potential short- to mid-term effects on the benthic community, along with some amount of disturbance of the native sediment associated with the depositing of the TLC material. The effects on the benthic community are expected to be a function of both the initial covering of the native sediments, which can result in smothering of the existing infaunal community, as well as the potential that the community could be degraded over the mid-term as a result of the differing grain size and total organic carbon (TOC) characteristics of the TLC material. From the laboratory treatability studies, the research team observed significant smothering effects from placement of thin layers of sand over infaunal organisms. However, at the demonstration site at Quantico Embayment, researchers observed relatively rapid recovery of the benthic community following construction of the TLC. While the sand material may not have provided optimal habitat initially, it was observed that over time, top-down mixing of relatively clean sediment deposits into the surface layer tended to improve the habitat characteristics, and a general improvement in benthic community health was observed.
Under what range of physical, biological, and chemical conditions will EMNR be effective?
The range of effectiveness of EMNR was not completely explored in this project. However, general considerations for the selection of EMNR are becoming well established. From a physical perspective, the remedy should generally be applied at sites that are relatively quiescent and not subject to significant physical disturbance that would disrupt or penetrate the cap to a degree that the underlying sediments would be re-exposed or significantly mixed into the TLC. The native materials must also have the physical strength to support the TLC so that gravitational mixing does not lead to failure of the TLC. From a biological perspective, the TLC thickness should consider the nature and scale of bioactivity in the surface sediments and the expected route of exposure for the risk endpoints under consideration. From a chemical perspective, EMNR is generally viewed as being most effective at sites where MNR would be effective, but deposition rates are potentially too low to reach the desired clean-up goals in a reasonable amount of time. Most sites where EMNR has been applied have exposure levels that are near risk thresholds, as opposed to higher concentration hot spots. For the Quantico Embayment site, the results reflect these physical, biological, and chemical conditions.
Overall, review of the historical literature, coupled with the experience with the Quantico Embayment site, indicated that EMNR can be highly effective in reducing exposure in surface sediments. EMNR remedy effectiveness seems to be a function of three primary considerations: site condition for the selection of EMNR, proper design of the EMNR remedy to meet site-specific conditions, and adequate monitoring to assure remedy success and address any potential defects in the TLC. For the Quantico Embayment site, the EMNR remedy was shown to be effective in reducing exposure in surface sediments as measured by bulk sediment total DDX concentrations, porewater DDX concentrations, and direct measurement of bioaccumulation in two site-exposed benthic organisms.