In-Situ Capping of Contaminated Sediments with Reactive Materials

Port authorities and the environmental community are seeking innovative ways to conduct remediation of contaminated sediments. The primary method of contaminated sediment remediation today is dredging. Dredging creates challenges in residual contamination and finding areas to construct confined disposal facilities. In-situ capping involves the placing of a subaqueous cover over contaminated sediments to stabilize sediments, minimize their re-suspension and transport, and reduce dissolved contaminant transport into surface waters. In past applications, in-situ capping has typically been constructed with several feet of locally available clean sediment, sand and/or gravel. In-situ capping can be limited by concerns regarding navigation, uniform cap placement, bio-intrusion and geotechnical stability. A potential solution for many in-situ capping concerns is the use of a reactive material cap. Reactive or adsorptive materials can provide treatment of the contaminants while allowing upward groundwater flow. Modeling has shown that a reactive material cap can greatly reduce the thickness required for the cap compared to conventional sand caps. Various reactive materials (e.g., activated carbon, apatite, organoclay) are used for wastewater and groundwater treatment and may be applicable to in-situ capping. Activated carbon and organoclay effectively adsorb many organic compounds. Apatite and zeolite adsorb and complex many different metals. These reactive materials may be placed in bulk. Alternatively, a reactive material mat that encapsulated reactive material between two geotextiles has additional benefits of stability, defined mass per area and reduced biointrusion. Several case studies of reactive material mat and bulk reactive material placement in waterways in Michigan, Oregon and Washington, D.C. are presented.