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As shown in Highlight 8-4, a variety of monitoring equipment and methods can be used for capping projects during both remedial action and long-term monitoring. The extent of any necessary monitoring should be a site-specific decision and also may depend on decision and enforcement document requirements. In general, bathymetric surveys to determine cap thickness and stability over time, sediment core chemistry (including surface sediment and upper portion of cap) to confirm physical and chemical isolation and test for recontamination, and some form of biological monitoring are useful for most capping projects. Specialized equipment, such as seepage meters, diffusion samplers (e.g., peepers and semi-permeable membrane devices), sediment profile cameras, sediment traps, or use of caged organisms, may also be useful in some cases.
Construction monitoring for capping normally is designed to measure whether design plans and specifications are followed in the placement of the cap and to monitor the extent of any contaminant releases during cap placement. During construction, monitoring results can be used to identify modifications to design or construction techniques needed to meet unavoidable field constraints.
Construction monitoring frequently includes interim and post-construction cap material placement surveys. Appropriate methods for monitoring cap placement include bathymetric surveys, sediment cores, sediment profiling camera, and chemical resuspension monitoring for contaminants. For some sites, visual observation in shallow waters or surface visual aids, such as viewing tube or diver observations, can also be useful.
Biological monitoring in the initial period following cap construction may include monitoring of the benthic community that may recolonize the capped site and the bioturbation behavior of bottomdwelling organisms. Where contaminants are bioaccumulative, fish or other biota edible tissue or whole body monitoring are also likely to be needed.
Long-term monitoring of in-situ capping sites typically is important to ensure that the cap is not being eroded or significantly compromised (e.g., penetrated by submerged aquatic vegetation, ground water recharge, or bioturbation) and that chemical contaminant fluxes that ultimately do move through the cap to surface water do so at the low projected rate and concentration. It may be also desirable to include ongoing monitoring for recontamination of the cap surface and non-capped areas from other sources.
For areas that may be subject to cap disruption, more extensive monitoring should be triggered when specified disruptive events (e.g., storms, flow stages, or earthquakes of a specified recurrence interval or magnitude) occur, to evaluate whether the cap was disturbed and whether any disturbance caused a significant release of contaminants and increased risk. Additional monitoring for the effects of tidal and wave pumping and boat propeller wash is also recommended where these are expected to be important factors. In general, the project manager should monitor cap integrity both routinely and following storm/flood events that approach the design storm magnitude envisioned by the cap’s engineers. As for other types of sediment remedies, the project manager should design the monitoring plan to handle the relatively quick turnaround times needed to effectively monitor disruptive events.
Cap maintenance is generally limited to the repair and replenishment of the erosion protection layer in potentially high erosion areas where this is necessary. Project managers should consider the ability to detect and respond quickly to a loss of the erosion protection layer when evaluating a capping alternative. Seasonal limitations, such as ice formation or closure of navigation structures (locks), can affect the ability to monitor and maintain in-situ caps and should be accounted for in monitoring plans.
Capping remedies frequently include provisions for actions to be taken in the case that one or more cap functions are not being met. Options for modifying the cap design may or may not be available.
If monitoring shows that the stabilization component is being eroded by events of lesser magnitude than planned, or the erosive energy at the capping site was underestimated, then eroded material can be replaced with more erosion-resistant cap material. If monitoring indicates that bottom-dwelling organisms are penetrating the cap and causing unacceptable releases of contaminants, then project managers should consider placing additional cap material on top of the cap to maintain isolation of the contaminated sediment. These types of management options are usually feasible where additional cap thickness, and the resulting decrease in water depths at the site, does not conflict with other waterway uses. Where a cap has been closely designed to a thickness that will not limit waterway use (i.e., recreational or commercial navigation), the options for modifying a cap design after construction can be limited.
8.4.3 Monitoring Dredging or Excavation
Monitoring for dredging or excavation remedies generally includes construction and operational monitoring of the dredging or excavation, transport, dewatering, any treatment, transport, and any on-site disposal placement. Following dredging or excavation, the residual sediment contamination should also be monitored. Additional monitoring following sediment removal may include monitoring of sediment toxicity or benthic community recovery or, for bioaccumulative contaminants, tissue concentrations in fish or shellfish, as well as continued monitoring of any on-site disposal facilities and monitoring sediment and/or biota for recontamination.
Depending on the levels of contamination and the selected methods of dredging/excavation, transport, treatment or disposal, potential construction and operational monitoring may include the
• Surface water monitoring at the dredging site and any in-water disposal sites (e.g., total suspended solids, total and dissolved contaminant concentrations, caged fish toxicity, caged mussel intake);
8-16 Chapter 8: Remedial Action and Long-Term Monitoring
• Effluent quality monitoring after sediment dewatering and/or treatment;
• Air monitoring at the dredge, transport, on-site disposal, and treatment sites; and • On-site disposal monitoring of dredged sediment or treatment residuals.
A thorough monitoring plan will normally enable project managers to make design or construction changes to ensure that the spread of contamination to uncontaminated areas of the water body, sensitive habitats, or adjacent human populations is minimized during dredging, transport, treatment, or disposal. Depending on the contaminants present and their tendency to volatilize or bioaccumulate, the project manager should consider water, air, and biological sampling in the monitoring plan.
Generally, a monitoring plan for dredging should include collecting data to test the effectiveness of silt curtains, dredge operating practices, and any other measures used to control sediment resuspension or sediment or contaminant transport. In most cases the project manager should include sampling upgradient of the dredging operation and both inside and outside of any containment structures.
Generally this sampling should also include dissolved compounds in the water column, although in some cases it may be a appropriate to use a tiered approach with analysis of dissolved compounds triggered by exceedances of threshold criteria for total compounds or for suspended solids. Also, where contaminants may be volatile, project managers should consider the need for air sampling. At highly contaminated sites, it may be necessary for the project manager to conduct a pilot study on a small area to determine if the sediment can be removed without causing unacceptable risks to adjacent human populations or adjacent benthic habitat. This information can help to determine what containment barriers or dredging methods work best and what performance standards are achievable at the site. The project manager should compare monitoring results with baseline data for contaminant concentrations in water and, where appropriate, in air. This should ensure that effects due to dredging may be separated and evaluated from natural perturbations caused by tides and storms. The project manager should develop contingency plans to guide changes in operation where performance standards are not met.
Following dredging, it is usually essential for project managers to conduct monitoring to determine whether cleanup levels in sediment are achieved. Initial sampling should be analyzed rapidly, so that contingency actions, such as additional dredging, excavation, or backfilling, can be implemented quickly if cleanup levels have not been met.
Following sediment removal, it is usually necessary for the project manager to conduct long-term monitoring to ensure that the dredged or excavated area is not recontaminated by additional sources or by disturbance of any residuals that remain above cleanup levels. Long-term monitoring is usually necessary to provide data to determine whether RAOs are met, and may be necessary for a period of time following remedial action to provide confidence that the objectives will remain met.
cap remains intact to ensure protection from infiltration. Depending on the type of disposal site and the
nature of the contamination, long-term disposal site monitoring may include the following:
• Seepage from the CDF containment cells to surrounding surface water;
• Revegetation or recolonization by plant and animal communities monitoring, and their potential uptake of contaminants.
Highlight 8-5 lists important points to remember related to monitoring sediment sites.
Highlight 8-5: Some Key Points to Remember About Monitoring Sediment Sites • Presentation of a monitoring plan is important for all types of sediment remedies, both during and following any physical construction, to ensure that exposure pathways and risks have been adequately managed • Development of monitoring plans should follow a systematic planning process that identifies monitoring objectives, decision criteria, endpoints, and data collection, and data interpretation methods • Before implementing a remedial action, project managers should determine if data adequate baseline data exists for comparison to future monitoring data and, if not, collect additional data • Where background conditions may be changing or where uncertainty exists concerning continuing off-site contaminant contributions to a site, it may be necessary to continue collecting data from upstream or other reference areas for comparison to site monitoring data • Monitoring needs include both monitoring of construction and operation and monitoring intended to measure whether cleanup levels in sediment and remedial action objectives for biota or other media have been met • Monitoring plans should be designed to evaluate whether performance standards of the remedial action are being met and should be flexible enough to allow revision if operating procedures are revised • Field measurement methods and quick turnaround analysis methods with real-time feedback are especially useful during capping and dredging operations to identify potential problems which may be corrected as the work progresses • After completion of remedial action, long-term monitoring should be used to identify recontamination, to assess continued containment of buried or capped contaminants, and to monitor dredging residuals and on-site disposal facilities 8-18 Contaminated Sediment Remediation Guidance for Hazardous Waste Sites Abramowicz, D.A., and D.R. Olsen. 1995. Accelerated Biodegradation of PCBs. Chemtech 24:36–41.
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