Remedy Evaluation Framework for Inorganic, Non-Volatile Contaminants in the Vadose Zone

Abstract

Contaminants in the vadose zone may be a long-term source of groundwater contamination and need to be considered in remedy evaluations. In many cases, remediation decisions for the vadose zone will need to be made all or in part based on projected impacts to groundwater. There are significant natural attenuation processes inherent in vadose zone contaminant transport. Attenuation processes include both hydrobiogeochemical processes that serve to retain contaminants within porous media and physical processes that slow the vertical rate of movement for disposed water and waste solutions. In particular, the physical processes controlling fluid flow in the vadose zone are quite different and generally have a more significant attenuation impact on contaminant transport relative to those within the groundwater system. A remedy evaluation framework is presented herein that uses an adaptation of the established EPA Monitored Natural Attenuation evaluation approach integrated into an approach based on a conceptual model focused on identifying and quantifying features and processes that control contaminant flux through the vadose zone. This report presents a framework to support evaluation of contaminant fate and transport in the vadose zone and bridges this analysis to refinement of the site conceptual model, assessment of risk, and support for baseline risk assessment and remedy selection.v

Conceptual Model of Iodine Behavior in the Subsurface at the Hanford Site

“…(Corbin et al 2005) for sites with greater than 1% of the total (the 216-U-1/2 sites associated with part of the 200-UP-1 OU plume had less than 1% of the total). Truex and Carroll (2013) examined predicted groundwater concentrations from the System Assessment Capability ). They used a DWS factor (the maximum predicted groundwater concentration divided by the DWS) to categorize sites based on their potential impact to groundwater relative to three different timeframes: up to the year 2100, up to the year 3100, and up to the year 12,000.…”

“…However, those sites with a higher pore volume estimate may have contaminants deeper in the vadose zone and therefore may affect groundwater sooner than sites with a lower pore volume estimate. Additional information relevant to evaluating the source flux is provided by Truex and Carroll (2013). Figure 7.…”

“…Vadose Zone Evaluation  Evaluation of each iodine disposal site using the approach outlined by Truex and Carroll (2013) and Truex et al (2015) is needed to evaluate the potential for continuing or future sources to groundwater. Data from 200-DV-1 (and other characterization data) can be used to support these evaluations.…”

“…Collection and analysis of characterization data to support estimating the future flux of 129 I from the vadose zone, as described by Truex and Carroll (2013) and Truex et al (2015), is also needed to as input for refining the conceptual model with respect to future plume behavior.…”

Perched-Water Evaluation for the Deep Vadose Zone Beneath the B, BX, and BY Tank Farms Area of the Hanford Site

“…The study also evaluated how measurements of transient decline in perched-water level can be used to help characterize the perched-water system and estimate flux to groundwater. The study followed the remediation assessment framework proposed by Truex and Carroll (2013).…”