To evaluate the fate and transport of contaminants within fine-grained, organic-rich sediments when an in situ cap with and without a reactive layer is placed over the deposit.

Contaminated sediments represent a major pollution problem in the U.S. and abroad.  Currently, 15 to 20 percent of National Priority List (Superfund) sites and 42 of the 43 Great Lakes Areas of Concern (AOCs) contain underwater sediment beds with hazardous organic and/or inorganic contaminants.  The costs associated with this problem are significant, with recent EPA estimates for clean-up of the Great Lakes AOCs alone estimated to be in the billions of dollars.  Of the various strategies available to remediate contaminated sediments, in situ management exhibits significant promise due to the potential large cost savings, minimization of further water contamination, and little sediment disturbance.  The placement of an in situ cap over a contaminated sediment deposit to trap fine-grained particles and associated contaminants is a relatively simple procedure that induces complex changes within the underlying sediment.  These changes are poorly understood and need further study for this procedure to be confidently used.

The process of capping a sediment deposit with a layer of additional soil (often sand) alters the chemical, biological and physical conditions of the deposit. Capping a sediment deposit can isolate contaminants and prohibit the exchange of gases and nutrients with the overlying water, thereby altering the chemical and biological states in the sediment deposit.  Depending upon the properties of the sediment and contaminant of interest, such drastic changes can significantly increase contaminant solubility, and hence mobilization and bioavailability.  The additional effective stress due to the capping layer also causes consolidation of the sediment, which decreases the porosity and induces an upward flow of pore water.  This pore water can mobilize contaminants and/or suspended colloids and thus may temporarily increase contaminant flux into the cap and/or water column. This may in turn increase contaminant bioaccumulation by benthic organisms as well as those in the overlying water column.  Each of these potential release mechanisms depend upon the spatial distribution of the contaminants among the various immobile and mobile phases within the sediment.  Knowledge of the distribution of contaminants and their propensity to be mobilized is thus of paramount importance to the development of predictive tools to evaluate contaminant fate during capping. 

The project will combine experimental and computational aspects and will be conducted at The Ohio State University using sediments extracted from the Ashtabula River.  The experiments will consist of bench-scale laboratory experiments of sediment capping.  Numerical modeling will be used to predict contaminant behavior in the tests.

Natural Sediments.  Tests will be conducted using contaminated sediments provided by U.S. Steel from their consolidated disposal facility for the Calumet River in Gary, IN. Sediment samples will be fully characterized to determine presence and distribution of PCBs.  Redox potential, pH, dissolved oxygen, and ionic strength of the sediment pore water will also be measured.  Capping Materials.  Two capping materials will be used in the testing program:  1) clean coarse-grained silica sand, and 2) uncontaminated river sediments.  The sand represents a typical material that would be imported to cap a sediment deposit.  The uncontaminated river sediments would be representative of the material that would be deposited during natural recovery at the site.  We will also conduct the first controlled laboratory tests of a new prototype Reactive Core Mat (RCM) that can be placed on top of the sediment layer to adsorb contaminants before they leave the system.  RCM is a manufactured product that consists of a thin layer (~10 mm) of sorbent that is contained between two geotextiles.  The sorbent can be tailored to a specific application and might include, for example, activated carbon, organophilic clay, and/or zero-valent iron.  The RCM product is manufactured by the CETCO, Corp. of Arlington Heights, IL.  CETCO has agreed to provide RCM material and if needed can supply a letter of support. 

Contaminant Analysis.  PCBs will be monitored in the solid and aqueous phases.  The aqueous phase will be extracted following EPA method 3510C modified for small sample volumes, whereas the solid phases will be extracted using an automated Soxhlet extractor following EPA method 3541.  Following sample clean-up after EPA method 3620B, PCBs will be quantified using procedures specified in EPA method 8082 with a gas chromatograph equipped with an electron capture detector (GC-ECD). 

 Numerical simulations for PCB transport and release.  Numerical modeling will be performed to further understand and predict the transport, fate, and bioavailability of PCBs in contaminated sediments under in situ capping conditions.  This work will be used to design and interpret the experiments and to enhance the outcome of the project by simulating other conditions for which experiments are not performed.  A new numerical model will be developed for the project based on several state-of-the-art large strain consolidation and transport models that Dr. Fox has developed over the past 10 years. The capabilities of this model will surpass anything else that is currently available and represent a true leap forward in our understanding of the interaction of physical/chemical/biological process for this complex problem. Bench-Top Experiments to Evaluate PCB mass-transfer during consolidation.  Laboratory slurry consolidation tests will be performed in slurry consolidometers to evaluate the transport of PCBs during conditions designed to simulate consolidation after the application of a cap with and without a RCM.  Settlement data will be used to obtain material compressibility and hydraulic conductivity relationships.  Pore water and effluent samples will be collected for analysis of an inert tracer to determine dispersion relationships.  At the conclusion of the experiments the sediments will be extracted and analyzed for PCB distribution. Effluent waters will also be analyzed for colloid content and associated contamination levels to assess the level of facilitated transport that may occur during capping.