Formatted Title
Later Stage Remedial Design Refinement Based on Depositional Environment and Stratigraphic Evaluation
Background/Objectives
The pathway to site remediation is often decades-long with varying quality and detail in site data collected over time. The synthesis of these data into an accurate conceptual site model (CSM) for use in remedial design can be challenging and data gaps are often identified upon reinterpretation. A re-evaluation of site stratigraphic data in respect to published studies and the collection of additional detailed stratigraphic data can be used in later stages of remediation to refine the remedial approach and target delivery based on depositional geometry. A Formerly Used Defense Site (FUDS) located in Waldorf, Maryland was historically utilized as a Nike missile launch area between 1955 and 1965 for the purpose of the assembly, launch, and control of guided missiles for defense against hostile aircraft. Historical operations resulted in the release of trichloroethene (TCE) and carbon tetrachloride to groundwater. The site has undergone numerous environmental investigations following the CERCLA process between 1986 and 2012. A Decision Document issued in 2015 with a chosen remedy of in situ chemical reduction (ISCR). A pilot study utilizing zero valent iron was conducted and a remedial design was subsequently developed based on the pilot study with injection locations chosen based on groundwater data collected to date. A full-scale injection of Provect-IR (zero valent iron with organic substrate and nutrients) was conducted in 2021.
Approach/Activities
A thorough understanding of site geology is important with respect to understanding the sequence stratigraphy at the site in relation to preferential pathways and for contaminant transport and sources of matrix diffusion. Prior to the implementation of the remedial design the CSM was refined by re-evaluating the historically generalized description of site geology. The remedial investigation through to the remedial design reused the same generalized description of Pliocene-aged coarse-grained deposits underlain by a confining layer. The CSM was refined to interpret the Pliocene-aged fluvial deposits as scouring into underlying the Miocene-aged sand and silt that were deposited in a shallow shelf environment. Additional post-ISCR soil borings conducted to evaluate the distribution of injectate served a dual purpose to also collect additional detailed stratigraphic data. Interpretation of these data identified an additional distinct Miocene unit present in areas of lesser scour, this unit like is a source of matrix diffusion. Each unit has general distinct color and sediment type associated; historical borings were re-evaluated to map general depths to the bottom of the fluvial unit which has identified the general orientation of coarse-grained channel scours.
Results/Lessons Learned
Performance monitoring collected between 2022 and 2023 indicated decreases of contaminants within the initial injection area with four areas beyond radius of influence of the initial injection area remaining above the remedial goals. Refinements to the CSM identified the general orientation of channel deposits and finer-grained sources of matrix diffusion. This information was used to better target injection locations and depths for a planned additional ISCR injection event to address the remaining areas exceeding remedial goals while accounting for the located finer grained materials that are sources of matrix back diffusion.