Background/Objectives: At the Nuclear Metals, Inc. (NMI) Superfund Site (Concord, Massachusetts, USA), depleted uranium (DU) was used to manufacture armor penetrators for the United States (U.S.) Army from 1958 to 1985.
NMI disposed of process waste containing DU, nitric acid, and lime in an unlined basin within a property that is now bordered by residential and light commercial development. DU was released to soils and groundwater beneath and near the basin; subsequent removal of sludge from the bottom of the basin left behind residual soil contamination and a small but concentrated plume of DU in groundwater.
Geochemical data for soil and groundwater samples collected over the first five years of remedial investigation activities were used to develop a surface complexation model to predict subsurface DU transport in groundwater at the NMI Site. Concurrently, the U.S. Environmental Protection Agency (EPA) developed its guidance for monitored natural attenuation (MNA) of radionuclides which was issued in 2010. Based on the transport modelling results and the U.S. EPA MNA guidance, it was determined that an active remedy would be required to prevent DU migration in groundwater from reaching the Assabet River, which is approximately 350 meters downgradient of the basin where the DU was released.
Approach/Activities: A feasibility study (FS) conducted to assess remediation strategies for DU-impacted groundwater identified sequestration of dissolved DU via the precipitation of stable uranyl phosphate (U-P) minerals as a viable alternative. Field column studies were conducted at two locations within the DU groundwater plume to assess the use of the fish bone-derived mineral hydroxyapatite to convert dissolved DU in groundwater to U-P. Based on the results of these studies, the FS concluded that hydroxyapatite is an effective material for in situ DU remediation that can be readily implemented, requiring no redox manipulation, and that it could provide significant cost savings over more active approaches. Following the FS, the U.S. EPA selected hydroxyapatite injection within the DU plume as the preferred remedy for DU- impacted groundwater at the NMI Site.
Results/Lessons Learned: This presentation describes the conceptual design of the hydroxyapatite remedy to sequester DU in groundwater, including methods to inject powdered hydroxyapatite within the affected saturated zone soils. Predesign investigation activities to assess the effectiveness of different injection methods are about to begin pending relaxation of work restrictions associated with the COVID-19 pandemic; available data on injection method effectiveness through August 2020, and the anticipated impact of those data on the final remedial design, will be included in the presentation.
- Geosyntec Authors: Kathy Phillips, Douglas Larson, David Adilman
- All Authors: Kathy Phillips, Douglas Larson, David Adilman
- Title: EcoForum 2020 eConference
- Event or Publication: Event
- Practice Areas: Water and Natural Resources
- Citation: Geosyntec will deliver five presentations and one keynote at EcoForum 2020 eConference on September 14-18, 2020
- Date: September 14-18, 2020
- Location: eConference
- Publication Type: Platform Presentation