Measurement and Modeling of Ecosystem Risk and Recovery for In Situ Treatment of Contaminated Sediments
Many Department of Defense (DoD) sediment sites are contaminated with polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). PCBs and PAHs are persistent and toxic, and PCBs are bioaccumulative. Over the past decade, various laboratory and field trials have shown that in situ sediment treatment technologies using activated carbon (AC) sorbents will reduce ecological and health risk from PCBs and PAHs. While several lines of evidence have demonstrated the AC treatment effectiveness, there is a need for further investigation of ecosystem recovery after AC sorbent amendment, assessment of secondary effects of AC on ecosystem health, development of mechanistic mass transfer modeling frameworks, and the design and testing of rapid, yet reliable, performance monitoring tools. The overarching objective of this project was to advance sediment in situ AC treatment technologies by studying these considerations. The project was comprised of three phases.
Phase I: Conventional ecosystem health determinants, such as benthic organism bioassays and community surveys, are time-consuming and expensive. No fast, inexpensive field methods currently exist to predict the bio-uptake of contaminants and explain post-treatment ecological effects. The objectives of the first phase were to incorporate rapid, inexpensive assessment tools to measure contaminant concentrations in the sediment pore water, to use a biodynamic modeling approach to predict contaminant burden at the base of the food web, and to develop a general model to predict the ecological characteristics of recovery.
Phase II: With the current understanding, some concerns exist as to whether the AC itself may cause adverse effects in benthic invertebrates inhabiting the remediated area. Comprehensive studies targeting the causes and effects of carbon on organisms’ health and habitat quality have to be investigated. Furthermore, predictive models are needed to assess long-term trends in PCB-pore water concentrations and (bio)availability under field conditions. The project team’s previous ESTCP field project (ER-200510) showed the need for predictive models to assess the long-term performance of AC amendment under quiescent field conditions with slow mass transfer compared to well-mixed conditions in the laboratory. The objectives of Phase II were to evaluate possible adverse effects of AC amendment on local invertebrates, to develop a mass transfer model to predict PCB mass transfer under conditions relevant to field application of AC amendment, and to assess the long-term sequestration ability of field-aged AC.
Phase III: This phase further expanded the scope of the project by enhancing the usability of the project’s outcomes. The objectives were to investigate the potential repartitioning of contaminants in sediment following the removal of AC after stabilization treatment, to standardize field monitoring methods using polyethylene passive samplers, and to develop a user-friendly, standalone program for the hydrophobic organic contaminant (HOC) mass transfer model to predict sequestration and pore water concentrations.
Phase I: Two rapid assessment tools were tested to measure PCB sediment pore water concentrations: polyethylene devices (PED) and a PCB immunoassay. The tools were tested in the laboratory and validated in the field, and the results were correlated with those obtained using conventional methods. The measured sediment pore water PCB concentrations also served as inputs to a biodynamic model, which were used to predict contaminant uptake by native benthic organisms. This approach used measured uptake, elimination, and metabolism rates as a basis for predictive modeling under a variety of conditions. Building on the biodynamic modeling for individual species, a general model was constructed to predict the ecological characteristics of recovery at a contaminated site post-treatment. In contrast to the conventional benthic community survey method, this modeling approach required only basic information about taxa-specific biodynamics, which needs to be established only once for each species, combined with data on the species available for community recruitment.
Phase II: A PCB mass transfer model in AC-amended sediment with heterogeneous AC distribution and advective pore-water movement was developed. The initial model successfully interpreted the long-term performance of field-amended AC to sediment at an inter-tidal mudflat adjacent to Hunters Point Shipyard, California. To further enhance and validate the model, comprehensive laboratory column studies were designed to study the effect of advective pore-water movement, AC mixing regime, AC particle size, and time after the mixing. Other laboratory experiments were conducted to assess the model input parameters that reflect mass transfer kinetics and partitioning of PCBs. To evaluate possible adverse effects of AC amendment on local invertebrates, AC amendments to sediments were tested for effects on survival, weight change, and energetic biomarkers of the deposit feeder Neanthes arenaceodentata. The tests employed silica sand, reference sediments, and contaminated sediments.
Phase III: A series of sediment slurry experiments simulated the field situation where the maximum benefit from in situ AC amendment was obtained, then the AC particles were removed, as might happen by erosion or intentional retrieval from the site. Tenax beads were selected as a surrogate of AC to mimic this field situation, and PEDs were used as a monitoring device for contaminant availability. For the PED method, the use of performance reference compounds was evaluated to consider kinetic effects of HOC sampling, to standardize the method, and to investigate possible anisotropic exchange kinetics in the field. Lastly, the current HOC mass transfer model developed in Phase II was modified for an executable version without the need to purchase MATLAB software. The beta-version program was tested and evaluated by non-expert users for user-friendliness. A booklet with case study modeling results was prepared and submitted together with the program.
Phase I: The utility of PEDs for sampling pore water in the field showed that pore water concentrations decreased up to 60% within 18 months after AC amendment. Results of this study illustrate that PEDs provide an inexpensive, in situ method to measure total PCB contamination in sediment pore water. The immunoassay method is semi-quantitative and useful only for assessing range values of total PCBs in sediment. Analysis based on functional traits of the benthic community showed that the community at Hunters Point is deprived of species that may be stressed by the contaminated sediment due to their feeding mode, reproductive mode, or position in the sediment. The biodynamic model was able to predict observed bioaccumulation and captures the effect of AC amendment and field observations. The biodynamic model improved the mechanistic understanding of bioaccumulation and how AC amendment reduces PCB uptake.
The results of this study can be found in the Phase I Final Report.
Phase II: The results from long-term monitoring of field-scale AC amendment to sediment confirmed the progressive benefits achieved by AC over time, which was anticipated to occur according to the PCB mass transfer model. The PCB mass transfer model was further advanced by considering various field conditions and engineering options and equipped with robust model parameters determined from independent tests. The PCB mass transfer model successfully reproduced the experimental results observed in the column studies. Adverse effects of AC amendment were not observed on the Neanthes survival regardless of the sediment type, the AC dose (20% versus 5%), or the AC particle size. Without additional food supply, no significant differences on growth were observed. When feeding on fish food, there were some effects of AC on these deposit feeders, but absolute effects of AC amendments on growth and energy reserves were not significant.
The results of this study can be found in the Phase II Final Report.
Phase III: The experimental results showed that when sorbent is selectively lost from the sorbent-treated sediment by winnowing, the repartitioning of contaminants may occur to some degree, but the repartitioning process is neither prolonged nor substantial to cause significant loss of the treatment effectiveness. Even with an extensive period of mixing of sorbent-deprived sediment in a slurry phase following sorbent treatment and removal, the available fraction of PCBs in the sediment was substantially lower than that for the untreated sediment. The HOC mass transfer model MATLAB codes were successfully developed into a standalone program. The standalone program is equipped with an I/O excel file, GUI, and detailed user manual to enhance its user-friendliness. An additional standalone program for sediment desorption kinetic modeling was developed to easily determine model parameters. Furthermore, the modeling case study booklet will serve as an exemplary and introductory material of in situ AC amendment and its effectiveness to DoD users.
The results of this study can be found in the Phase III Final Report.
Phase I: Establishing correlations between conventional and alternative measurement tools in the field allows members of the scientific community and DoD users to confidently use the rapid, inexpensive tools in the future to assess the ecological recovery of a contaminated sediment site after treatment or during monitored natural recovery. Biodynamic modeling results in a general predictive ecosystem recovery model that is directly applicable to any contaminated sediment site, given some knowledge of the species inhabiting the local community and basic information about taxa-specific biodynamics for the contaminants of interest. Ideally, future researchers and DoD users will need only to measure contaminant pore water concentrations and collect information on the benthic organism recruitment pool to predict the extent of ecosystem recovery following remediation.
Phase II: The PCB mass transfer model predicts the variable effectiveness of AC amendment with different AC application scenarios. The model certainly will be useful for site managers and DoD users who may be considering the in situ AC amendment technology. The model provides site managers with the ability to conduct screening assessments for the selection and optimization of engineering parameters for the treatment (e.g., AC particle size, mixing duration, mode of application, etc.). This study on possible adverse effects of AC amendment on local invertebrates concludes that the AC does not significantly impact the organism’s survival, growth, energy reserves, or behavior for the deposit feeder tested. Although future work is needed to better understand the effects on benthic communities for large-scale field deployments, these results increase confidence for site managers or DoD users to consider this technology as a remedial option.
Phase III: A situation that might result in complete removal of AC amendment after treatment is not likely to occur, but regulators, site managers, and DoD users want assurance that risk reduction is still maintained even if the AC were removed. This study addressed these concerns and raised the confidence level for in situ AC amendment technology. The effort with PED field method standardization helps to advance the understanding of the PED passive sampling technique as a reliable monitoring tool for site investigation and remedy assessment. Development of the standalone program for the HOC mass transfer modeling improves the usability and accessibility of the model for remedial project managers and DoD users.
Points of Contact
Dr. Richard Luthy
SERDP and ESTCP