Many underwater sites contain concentrations of metals, PCBs and PAH contaminants that could continue to cause injury to the biota in the sediments and overlaying water. There are increasing concerns regarding the potential environmental impact of dredging PAH-, PCBs- or metals-contaminated sediments and the resulting resuspended sediments. The concern with dredging resuspension is not just the increase of suspended solids in the water column but also the release of contaminants from pore water and desorption of the contaminants from the resuspended sediment particles into the water. The migration of the resuspended solids and the dissolved contaminants typically cause an increase of the impacted sediment and water area. There is a need for in-situ treatments that can be used to minimize the impacts of dredging-related resuspended sediment on the benthic community and surface water. In addition, if the treatments can also be used to render sand-based isolation capping more stable and help enhance the benthic community in the cap, it would make capping applications more effective and increase the applicability of capping technology. The present study discusses an innovative approach to stabilizing sand caps and minimizing sediment resuspension using protein polysaccharide biopolymers (PPBs).
Polymers have been used to stabilize agricultural soils that are prone to erosion due to poor soil structure. Soil/sediment properties, polymer type, and water chemistry affect adsorption and aggregation, determining which biopolymer product to use can be challenging since polymers vary in molecular weight, size, type of surface charge (anionic, cationic and nonionic), and structure; soils/sediment vary in particle surface area, structure, and pore size distribution; and water chemistry in soil/sediment varies in pH, electrolyte concentration, and ionic strength (Seybold, 1994; Letey, 1994). A study showed rapid degradation of the polymers and no evidence of increased aggregation or consolidation. Certain biopolymers are also highly susceptible to water, and cycles of drying and wetting can have a significant effect on soil cohesion. Proprietary protein polysaccharide biopolymer formulations (PPBs) have been developed by Dr. Dahmani. The product formulations are composed of polysaccharides and vegetable protein extracts that are generally Recognized as Safe (GRAS) by the FDA. When a solution of PPBs is placed in sand, sediment or soil, PPBs abiotically coat the surfaces, thereby enhancing shear strength and cohesion.
Direct Shear Testing and Unconfined Compressive Strength Test (UCS)
In order to evaluate the impact of PPB treatment on cohesion, direct shear and unconfined compressive strength (UCS) tests were conducted on PPB-treated sand and soil. The effect of salinity on SED1 PPB-treated sand was evaluated because beach erosion occurs in a saline environment and potential sediment sand cap applications occur under brackish and saline conditions. When saline water was used in the sand sample preparation, cohesion was significantly increase of approximately 50% to that of a treated sand with tap water. This indicates the SED1 PPB treatment has a significantly higher impact on sand cohesion in a saline environment. As was the case with the direct shear (cohesion) testing, the data indicates that the SED1 PPB treatment had a significantly higher impact on sand compressive strength in a saline environment, which may translate to a more positive impact on sand erosion mitigation and sand cap stabilization.
Moisture retention curves show that SED1 PPB-treated sand retains a much higher water content as compared to the control sand. The data also indicates an increasing water holding capacity of the sand with increasing SED1 PPB dosage. The implication of increased moisture retention by SED1 PPB-treated sand is that the sand may be less prone to aeolian erosion on beaches. Treated sediment may also be less prone to erosion during drying cycles. Increased water retention in sand may also mean improved dune vegetation.
Sediment Resuspension Mitigation Tests
Sediment samples from a tidally influenced shallow creek system were used to assess PPB treatment effectiveness in reducing the turbidity and total suspended solids (TSS) of sediment/water mixtures. The results indicate a reduction in turbidity of 76% for the SED1 PPB treated sediment as compared to the control tests. Similarly, a reduction in TSS of 87% was achieved for the SED1 PPB treated sediment. The results indicate that the PPB SED1 treatment can potentially minimize sediment resuspension during dredging operations.
Aquatic Toxicity Testing
Aquatic toxicity testing was conducted at a certified laboratory, New England Bioassay (NEB), in Manchester, CT, to assess toxicity of the SED1 PPB amendment. Duration of the tests was 48 hours for the C.dubia and 96 hours for the fathead minnow, mysid shrimp and Menidia. Each test consisted of three control replicates (which did not contain sand, to verify organism health without the interference of sand) and five replicates containing sand and overlying water, each with 10 organisms per replicate. The 7.5 g/kg (0.75%) dosage passed with an ending survival of greater than 90%. Based on the results obtained from the sand, soil and sediment stabilization tests conducted in this study, dosages of < 1% are likely to be used in-situ.
The applicability of PPB treatment for erosion mitigation of beaches, sediment and sand caps will be site- and treated material-specific. Designing the proper formulation will be important in order to optimize effectiveness and cost. Laboratory evaluations based on UCS and direct shear testing provide valuable insight into PPB treatment effects. Testing at various or field water saturation levels is recommended.
Project: Innovative PPB Testing
Services: Environmental Engineering