SERDP Statement of Need (SON) ERSEED-15-01 called for the development of tools to identify the sources of ongoing contaminant influx to sediment sites capable of identifying ongoing contaminant sources that can be accounted for appropriately in remedy selection, design, implementation, and monitoring directly within the area of concern. The objective of this project was to demonstrate proof of concept for a remedy and recontamination assessment (RARA) array that can provide site-specific, direct measurement of recontamination potential and impact on a range of remedies while providing increased realism compared to laboratory treatability studies and reduced cost and complexity compared to large-scale field pilot studies.
The technical approach built on the broad experience with the development of in situ monitoring and assessment tools in establishing methodologies for in situ sediment treatment arrays. The intention was to leverage the project by building the prototype systems from components that were on-hand or readily available, and testing the arrays at a site where pilot-scale treatment testing was ongoing and recontamination is a potential concern. Development and testing of the RARA array focused the following research tasks: (1) Conceptual design of the array and field methodology; (2) Construction of the prototype arrays and initial pier-side testing of the methodology; (3) Proof-of-concept field deployment of the prototype RARA array, and (4) Initial evaluation of the performance and feasibility of the method. In the first task, the project team developed the conceptual design and methodology while considering the best designs and procedures for moving systems from the lab to the field and providing the capability to sustain the experiments in the field for time periods that are adequate to assess both recontamination and remedy performance. Based on the design developed in task 1, the researchers constructed a prototype array that incorporated the key design features. The methodology developed in task 1 was initially tested pier-side at the SSC Pacific test facility in San Diego Bay.
Using the prototype RARA array and methodology developed in task 2, the researchers then conducted a limited initial proof-of-concept deployment in the field. Contaminated sediments for testing in the array were collected from a nearby Navy sediment site (Naval Base San Diego Chollas Creek) that is currently being investigated under the Total Daily Maximum Loading (TMDL) program. These sediments were treated with thin-layer treatments of clean sand or clean background sediment from a reference area in San Diego Bay. Untreated controls were also included. The array was then placed back in the bay at the SSC Pacific pier and monitored for a period of about five months. In the final task, results from the pier-side and field testing were used to provide an initial assessment of the performance and feasibility of the RARA array methodology. The analysis focused on the extent to which this exploratory research could address the key questions, and the outcome of the proof-of-concept testing.
The RARA system was successfully designed and constructed based on the goal of providing an integrated technology for assessing the effectiveness of different sediment remedies when subjected to varying pressures from site conditions and recontamination loadings. The system design balances requirements for multiple treatments, controls and replication with the constraints of size, weight, deploy-ability and cost. The RARA array allows remedies to be tested in situ and on-site while reducing costs that would be associated with costly pilot-scale studies. The system incorporates standard cylindrical sediment traps around the perimeter of the array that provide adequate capture area to collect incoming depositional sediments. The prototype system also incorporated an Acoustic Doppler Current Profiler, Optical Backscatter and temperature/dissolved oxygen sensor to monitor conditions during the deployment. The system design allows for a range of measurement endpoint capabilities to provide the basis for the assessment or remedy effectiveness and recontamination.
As part of the proof-of-concept deployment, we used the RARA system to evaluate two aspects of remedy and recontamination performance for the untreated and treated Chollas Creek site sediments. Performance of two sediment treatments including a thin-layer clean sand treatment and a thin-layer clean sediment treatment was evaluated relative to untreated Chollas Creek site sediment. The deployment was also used to evaluate the concept of source influence on the remedies by removing the known source inputs at Chollas Creek by moving the RARA array to an area without significant ongoing sources.
Effects of Removing Site Associated Stressors
To determine the influence of removing localized sources, the T-Zero and T-Final concentrations in the untreated Chollas Creek sediments were compared. Comparing the T-Zero and T-Final concentrations of the untreated Chollas Creek site sediments, both the physical and chemical properties of the bulk sediment remained relatively consistent over the five-month period. Sediment traps showed moderate deposition rates and contaminant concentrations that were generally lower than the concentrations in the untreated sediments, confirming the effective removal of recontamination from site sources. Porewater trends in the untreated sediments between T-Mid and T-Final were mixed, with most metals, Total Polycyclic Aromatic Hydrocarbon (PAHs) and Total Chlordane showing downward trends, while Total Polychlorinated Biphenyl (PCBs) and Total DDXs showed increases. Over the same period, bioaccumulation of metals generally remained unchanged, Total PAHs and Total Chlordane showing decreasing uptake, and PCBs and DDXs showed increasing uptake. Benthic community health compared between T-Zero and T-Final in the untreated sediments showed that total abundance was reduced, but virtually every other metric of benthic health improved in association with moving the exposure to the undisturbed location. Based on these findings, researchers concluded that:
Overall, these results support the conclusion that removing the impacts of the creek sources and physical disturbance that are present at the Chollas Creek site resulted in some minor changes in sediment chemistry and bioavailability, but also resulted in some clear improvements in benthic community health. Because the chemical changes appear to be relatively minor, researchers suspect that the changes in benthic community health may result primarily from the removal of the physical disturbances that are known to occur at the Chollas Creek site primarily due to ship movements and associates propeller wash. The researchers conclude that the deployment demonstrated the utility of the RARA system to assess changes in source pressure and site conditions on the response of site sediments with potential practical applications to impairment assessment, source control, and the performance of monitored natural recovery remedies.
Effects of the Applied Treatments
To determine the influence of the two treatments, the untreated site sediment controls were compared to the sediments treated with thin-layer sand and thin-layer clean sediment at the TZero, T-Mid and T-Final conditions (depending on the measurement endpoint).
Comparing bulk sediment concentrations in treatments to untreated controls, researchers found reductions in a broad range of contaminant levels with the largest magnitude of reductions in the sand treatment, followed by the clean sediment treatment. Changes in bulk sediment concentrations appeared to be driven primarily by the treatment application as opposed to new deposition as indicated by the sediment traps. Comparison of trap sediment concentrations to treated sediment concentrations indicated that depositing sediments generally had contaminant concentrations that were higher than the sand treatments, but lower or comparable to the sediment treatments. These results suggest that incoming sediments would exert some upward pressure on the thin-sand treatments, but would generally have only a small downward pressure on the thin-sediment treatments. Sediment porewater concentrations measured in both treatments were generally comparable to untreated controls for metals and Total PAHs, while showing reductions in Total PCBs. Bioaccumulation results indicated that bioavailability in the sediment treatments was comparable to the untreated sediments for all contaminants except for zinc which was slightly reduced in the sand treatment. The bioaccumulation measurements generally indicate minimal effects of the treatments with respect to reduction in bioavailability. Comparing the T-Zero untreated Chollas Creek site sediment to the T-Final treated sediments, researchers found broad improvements in benthic community metrics. These improving trends were stronger for the sediment treatment compared to the sand treatment.
Overall, the treatment results support the conclusion that both the clean sediment and sand treatments were effective in reducing bulk sediment concentrations when compared to untreated sediments. However, more direct measures of bioavailability including porewater and bioaccumulation indicated minimal improvement for both treatments compared to untreated controls. In contrast, direct measurements of benthic community health showed broad improvements especially in the clean sediment treatments. Researchers conclude that the deployment demonstrated the utility of the RARA system to assess changes associated with sediment treatments using multiple lines of evidence, and that the system is effective in determining the relative performance of different sediment treatments relative to untreated controls.
The RARA system was successfully designed and constructed based on the goal of providing an integrated technology for assessing the effectiveness of different sediment remedies when subjected to varying pressures from site conditions and recontamination loadings. The system design balances requirements for multiple treatments, controls and replication with the constraints of size, weight, deploy-ability and cost. The RARA array allows remedies to be tested in situ and on-site while reducing costs that would be associated with costly pilot-scale studies. The method incorporates a broad range of measurement endpoints including surface sediment chemistry, sediment trap depositional mass and chemistry, porewater passive sampler chemistry, bioaccumulation, toxicity, benthic infauna, and sediment tracers. The system is well-suited to assess a range of remedies including thin caps, amendments, geofabrics, and natural recovery. Overall, the RARA system represents a new paradigm in cost-effective, realistic remedy performance assessment that was previously unattainable.