In situ capping frequently has been used to physically separate contaminated sediments from the aquatic environment above the cap and, in some cases, to act as an impermeable barrier to groundwater flux. Sequestration based on physical separation alone, however, is not desirable because it does not ensure that dissolved phase contaminant flux is eliminated as a transport pathway either through the cap or around it. More recently, in situ capping with chemically reactive materials has been explored to provide a physical barrier to remobilization of sediment-bound contaminants and, at the same time, to sequester dissolved contaminants as they out-flux through the cap via groundwater flow. To date, these studies have largely focused on applying one type of reactive material to treat one class of contaminant and have typically been deployed as relatively thick layers of the material (6 to 12 inches) over the bottom. These approaches may not be applied at sites that have physically challenging site conditions, multiple classes of contaminants, concerns over contaminant remobilization, or are prohibitively large relative to the costs of using coarsely applied reactive materials.

The objective of this project was to develop and test a mixture of chemically reactive materials suitable for incorporation within an engineered geotextile mat to create a composite active capping system capable of deployment in a wide variety of environmental settings in order to effectively sequester both metal and organic contaminants in sediments.

Reactive Mat Deployment

Laboratory tests were designed and performed to identify the mixture of amendment materials to be incorporated into the reactive mats that most effectively sequester contaminants of interest. The results of these experiments were used to design various small-scale test mats used for preliminary evaluation as well as to construct the prototype mats used for long-term monitoring and evaluation. Desktop audits were performed to identify a project location (water body) that could be used as a pilot site for in situ testing of various reactive mat and amendment arrangements. A subsequent geophysical investigation was then performed at this pilot site to select a particular area to serve as the target location for long-term field testing of the prototype mat system constructed to the specifications of the composite material testing results. All field efforts for this project were performed at Cottonwood Bay in Grand Prairie, Texas.

Small-scale test mats featuring different types of geotextile materials were tested under controlled laboratory conditions as well as at the selected pilot site and recovered after two predetermined soak times to assess the potential effects of biofouling, biofilm formation, and weathering on final mat design and efficacy. The geotextile type found to be most resistant to biofouling while still maintaining proper integrity and porosity was ultimately used to construct the prototype mat system used for long-term monitoring and evaluation. Variations on a prototype mat system were constructed at the pilot site to include various treatments (e.g., single mat, double mat, mat with sand cap) of a reactive mat featuring the most resistant geotextile and the optimum amendment mixture. This mat system was monitored and evaluated over a period of two years to determine the effectiveness of the proposed technology in achieving project goals.

The results of this project show that the reactive core mat technology is effective at sequestering metals and polycyclic aromatic hydrocarbon (PAH) compounds in fine-grained sediments at a quiescent site with low groundwater flow. Therefore, it would be suitable to use the system for full-scale demonstration/validation under similar conditions. The combined results of the laboratory chemical and geotechnical testing, field mini-mat testing, and final mat prototype testing involving different reactive mat arrangements provide a solid foundation to support further expansion of testing in a pilot-scale demonstration (e.g., increase mat size tested from 400 ft² to 10,000 ft²).

Promoting the use of reactive mats as a more environmentally sustainable remedy relative to traditional dredging would be achieved by a pilot-scale demonstration. Reactive mat capping (assuming sand capping alone would be insufficient) when used as a remedy would largely eliminate greenhouse gas emissions otherwise released during excavation by dredge barge and trucking equipment and would increase the life expectancy of landfills not otherwise depleted with dredged material. Lastly, the use of reactive mats may provide a starting point for monitored natural attenuation (MNA), wherein the initial benthic recolonization made possible by the mats would jump-start further sediment deposition and therefore eventual re-establishment of infaunal communities.