Diminishing rates of subsurface volatile contaminant removal by soil vapor extraction (SVE) oftentimes warrants an in-depth performance assessment to guide remedy decision-making processes. Such a performance assessment must include quantitative approaches to better understand the impact of remaining vadose zone contamination on soil gas and groundwater concentrations. The spreadsheet-based Soil Vapor Extraction Endstate Tool (SVEET) software functionality has recently been expanded to facilitate quantitative performance assessments. The updated version, referred to as SVEET2, includes expansion of the input parameter ranges for describing a site (site geometry, source characteristics, etc.), an expanded list of contaminants, and incorporation of elements of the Vapor Intrusion Estimation Tool for Unsaturated-zone Sources software to provide soil gas concentration estimates for use in vapor intrusion evaluation. As part of the update, SVEET2 was used to estimate the impact of a tetrachloroethene (PCE) vadose zone source on groundwater concentrations, comparing SVEET2 results to field-observed values at an undisclosed site where SVE was recently terminated. PCE concentrations from three separate monitoring wells were estimated by SVEET2 to be within the range of 6.0-6.7 mu g/L, as compared to actual field concentrations that ranged from 3 to 11 mu g/L PCE. These data demonstrate that SVEET2 can rapidly provide representative quantitative estimates of impacts from a vadose zone contaminant source at field sites. In the context of the SVE performance assessment, such quantitative estimates provide a basis to support remedial and/or regulatory decisions regarding the continued need for vadose zone volatile organic compound remediation or technical justification for SVE termination, which can significantly reduce the cost to complete for a site.

The ability to reliably deliver and widely distribute remedial amendments through the subsurface environment is of paramount importance to achieve cleanup objectives for contaminated sediments and groundwater for protection of sensitive environmental habitats and natural resources. A wide range of amendments, delivery techniques, and subsurface access methods are available. However, the applicability of these approaches is dependent on a multitude of site-specific factors. In this review, an overview of amendments and access/distribution methods is provided, along with discussion of the maturity level, advantages, and limitations that relate to the potential effectiveness of each approach in the context of site-specific factors. Each amendment and delivery approach are further evaluated for applicability to the following subsurface target zones: saturated, unsaturated, and perched water, with specific focus on high and low permeability regions in each zone. The review highlights a critical need for field-tested approaches targeting unsaturated and perched water zones, as well as low-permeability regions within all subsurface regions. The intent of this review is to provide critical information and insight into how amendments can be delivered, emplaced, and/or distributed effectively in the subsurface environment to effectively manage subsurface contamination.

Solid-phase interactions and speciation are important to radioiodine transport in groundwater. At the Hanford Site in Southeastern Washington State, iodate (IO 3 − ) is the main aqueous species in dilute radioiodine groundwater plumes. Like other oxyanions, IO 3 − may be incorporated into and/or adsorbed onto calcite, a common mineral at Hanford, decreasing its mobility in the environment. A series of macroscale batch experiments combined with solid-phase characterization were conducted to identify variables impacting time-dependent aqueous IO 3 − removal via calcite precipitation and determine the location of IO 3 − within the calcite crystal structure. Results demonstrated 11.5–97% aqueous IO 3 − removal during initial rapid calcite precipitation. Incorporation was apparently the main removal mechanism, although later slower precipitation and/or adsorption may have also contributed to IO 3 − removal. Using a higher concentration of the calcite-forming solutions (i.e., using 1 M vs. 0.1 M concentrations) resulted in an increase in the amount of precipitated calcite and a greater percentage of IO 3 − removed; however, calcite formed with lower molarity solutions resulted in higher IO 3 − mass (µg/g) removal. Solubility testing of laboratory-produced calcites showed only small differences in solubility for calcite with and without IO 3 − incorporated into the structure. Evidence collected from SEM/FIB and TEM/SAED suggested that the IO 3 − incorporated into calcite was present in regions close to the surface (implying potential easy release upon calcite dissolution).

The ability to reliably deliver and widely distribute remedial amendments through the subsurface environment is of paramount importance to achieve cleanup objectives for contaminated sediments and groundwater for protection of sensitive environmental habitats and natural resources. A wide range of amendments, delivery techniques, and subsurface access methods are available. However, the applicability of these approaches is dependent on a multitude of site‐specific factors. In this review, an overview of amendments and access/distribution methods is provided, along with discussion of the maturity level, advantages, and limitations that relate to the potential effectiveness of each approach in the context of site‐specific factors. Each amendment and delivery approach are further evaluated for applicability to the following subsurface target zones: saturated, unsaturated, and perched water, with specific focus on high and low permeability regions in each zone. The review highlights a critical need for field‐tested approaches targeting unsaturated and perched water zones, as well as low‐permeability regions within all subsurface regions. The intent of this review is to provide critical information and insight into how amendments can be delivered, emplaced, and/or distributed effectively in the subsurface environment to effectively manage subsurface contamination.

The compendium of Hanford site sediment mineralogy data provided in this report was initiated in 2012 to support flux mitigation technologies and real-time monitoring that could limit contaminant fluxes to the groundwater in the Central Plateau. Recent efforts in advancing spectral induced polarization as a method for monitoring contaminant transformations for active biogeochemical remedies have underscored the need for a published compendium of sediment characterization data. Although the data are primarily focused on the mineralogical composition of Hanford sediments, other soil measurements such as particle size distribution such as particle size distribution and carbon content are provided. The data have been reproduced from the original reports and are provided here for information only (FIO).

Low-energy beta-emitting radionuclides that were released historically during reactor operations and plutonium separations activities at some U.S. Department of Energy sites have migrated into the groundwater, forming contaminant plumes that are subject to federally regulated remediation actions. At the Hanford Site, the low-energy beta-emitting radionuclides include iodine-129, technetium-99, chlorine-36, carbon-14, and tritium (H-3). All are highly mobile in the subsurface, and except for tritium, and have very long half-lives—thousands to millions of years. The geochemistry and transport behavior of these contaminants in the subsurface present significant challenges for remediation of groundwater to federal drinking water standards (DWS)—the appropriate or relevant and applicable requirements (ARARs) for cleanup. For some of the low-energy beta-emitter contaminants, particularly iodine-129, cleanup and restoration of groundwater to DWS may not be attainable within a reasonable timeframe using currently available treatment technologies.

Groundwater in the 200-UP-1 operable unit (OU) is contaminated with carbon tetrachloride, uranium, nitrate, chromium (total and hexavalent), iodine-129, technetium-99, and tritium, associated with past nuclear weapons production activities at Hanford. The preferred alternative described in the 2012 Record of Decision (ROD) for the 200-UP-1 OU Interim Remedial Action includes 35 years of active remediation using a combination of groundwater pump-and-treat and monitored natural attenuation (MNA) for portions of the contaminated groundwater, followed by institutional controls until cleanup levels are met for unrestricted use. As noted in the 200-UP-1 OU interim ROD, no treatment technology for iodine-129 had been found that could achieve the drinking water standard (DWS) of 1 pCi/L for the iodine-129 concentrations present in the 200-UP-1 OU groundwater. Therefore, the 200-UP-1 OU interim ROD specified hydraulic containment of the iodine-129 plume, update of the conceptual model for iodine-129, and further evaluation of potentially applicable iodine-129 treatment technologies. The 200-UP-1 OU interim ROD further stated that in the event a viable treatment technology is not available, the use of a technical impracticability (TI) waiver may need to be considered as part of the final remedy and that this information be gathered concurrently with the technology evaluation. Hence, this report provides information relevant to a TI waiver consideration on the current federal 1 pCi/L DWS for iodine-129 in the 200-UP-1 groundwater OU of the Hanford Site.

Quantitative information about in situ natural degradation mechanisms and rates for carbon tetrachloride (CT) is needed to support remedy implementation with respect to the operational lifetime of the 200 West Groundwater Treatment Facility (200W P&T system) and transition to monitored natural attenuation. Laboratory studies estimated the hydrolysis half-lives of CT and chloroform (CF) under Hanford groundwater conditions (pH 7.8, 16 °C) to be 630 and 3,400 years, respectively. This rate of CT hydrolytic degradation is too slow and will not have a consequential impact on groundwater CT remediation. Biotic and coupled biotic-abiotic mechanisms of natural CT degradation have not been evaluated or measured with respect to a natural attenuation remedy for 200 West Area aquifer (200W aquifer) conditions. Summarized in this report are biotic and abiotic reductive degradation mechanisms for CT, as well as chlorinated byproducts, along with normalized reaction rates for each mechanism based on published values from controlled laboratory studies and field trials. CT degradation mechanisms were used to identify indicators of degradation activity. 200 West Area groundwater data and the relative significance of measured trends in groundwater chemistry were evaluated, taking into account the effect of 200W P&T activities on monitoring well data.

Multiple processes affect the fate of the radioactive isotope 129I in the environment. Primary categories of these processes include electron transfer reactions mediated by minerals and microbes, adsorption to sediments, interactions with organic matter, co-precipitation, and volatilization. A description of dominant biogeochemical processes is provided to describe the interrelationship of these processes and the associated iodine chemical species. The majority of the subsurface iodine fate and transport studies in the United States have been conducted at U.S. Department of Energy (DOE) sites where radioisotopes of iodine are present in the environment and stored waste. The DOE Hanford Site and Savannah River Site (SRS) are used to illustrate how the iodine species and dominant processes at a site are controlled by the prevailing site biogeochemical conditions. These sites differ in terms of climate (arid vs. sub-tropical), major geochemical parameters (e.g., pH ~7.5 vs. 4), and mineralogy (carbonate vs. Fe/Al oxide dominated). The iodine speciation and dominant processes at a site also have implications for selection and implementation of suitable remedy approaches for 129I.

The Hanford Site 300 Area lies adjacent to the Columbia River, approximately 5 km north of Richland, Washington. Past waste disposal practices in the 300 Area resulted in vadose zone uranium contamination beneath former infiltration ponds and trenches. Stage-driven water table fluctuations and river water intrusion facilitate mobilization of uranium from contaminated sediments in regions periodically occupied by the water table (i.e., the periodically rewetted zone or PRZ), thereby raising groundwater uranium concentrations above the maximum allowable contaminant level for uranium. This report documents the ERT imaging operations and interpretation of imaging results in terms of the extent of amendment delivery (including phosphate and/or carrier fluid) within the treatment area. Based on the interpretation, amendment delivery to the LVZ and PRZ was observed to be variable between each of the three imaging clusters, as shown in Figure S.1. Amendment transport in Cluster 1 exhibited significant lateral flow in comparison to Clusters 2 and 3, resulting in a final amendment distribution throughout a large portion of the LVZ and PRZ sediment between the injection wells. Cluster 2 exhibited good amendment delivery in the LVZ only near the injection wells. In the PRZ at Cluster 2, amendment was effectively distributed through the full distance between injection wells. Cluster 3 exhibited the smallest amount of lateral flow and amendment was primarily distributed in the LVZ and PRZ very near the injection wells. Imaging results below the water table suggest that more amendment entered the groundwater in Cluster 2 and Cluster 3 than in Cluster 1.

Iodine-129 (I-129) generated at the U.S. Department of Energy Hanford Site during plutonium production was released to the subsurface, resulting in several large, dilute plumes in the groundwater, including the plume in the 200-UP-1 operable unit (OU). A requirement in the Record of Decision (ROD) for interim remedial action at the 200-UP-1 OU is to “evaluate potential treatment options for I-129 as part of the selected remedy through further technology evaluation. If one or more viable technologies are identified, treatability tests will be conducted for those technologies.” The requirements also state: “In the event a viable treatment technology is not available, the use of a technical impracticable waiver under 40 CFR 300.430(f)(l)(ii)(c) may need to be considered as part of the final remedy.” Recent work has increased the understanding of iodine behavior in the Hanford subsurface. A thorough review of potential remediation technologies has also been completed, including laboratory testing of promising technologies to evaluate their effectiveness for the iodine conditions at the 200-UP-1 OU. Using the combined information from these efforts, this report evaluates potential remediation technology options for the 200-UP-1 OU with respect to the need for conducting treatability tests and to document the technology status in support of future OU decisions for addressing the I-129 plume.

The 200-DV-1 Operable Unit (OU) is in the process of characterizing the vadose zone to support a remedial investigation and feasibility study. Contaminants disposed of at the land surface must migrate through the vadose zone before entering groundwater. Quantifying contaminant attenuation and contaminant transport processes in the vadose zone, in support of the conceptual site model (CSM) and fate and transport modeling, are important for assessing the needs for, and types of, remediation in the vadose zone and groundwater. The framework to characterize attenuation and transport processes provided in U.S. Environmental Protection Agency (EPA) guidance documents was used to guide the laboratory effort with the following objectives:

The uranium reactive gas sequestration (URGS) test described herein was conducted as an element of the Deep Vadose Zone Treatability Test Plan for the Hanford Central Plateau. The URGS technology was tested as a potential remedy to decrease the mobility of uranium in the vadose zone as a mechanism to protect groundwater. Information about URGS obtained from this test is intended for use in subsequent feasibility studies for Hanford Central Plateau waste sites with uranium contamination in the deep vadose zone.

Currently, there are large dilute 129I groundwater plumes at Hanford, including a groundwater plume in the 200-UP-1 operable unit (OU) located within the Central Plateau of the Hanford Site. The interim record of decision for the 200-UP-1 OU requires that the U.S. Department of Energy evaluate potential treatment options for 129I through further technology evaluation. The approach to the evaluation was defined in the 129I technology evaluation plan and includes an update to the conceptual model (CM) for the plume, as required by the 200 UP 1 remedial design/remedial action work plan. This report describes an updated CM of subsurface processes that affect 129I behavior in the subsurface. This CM was developed to address identified data gaps and to provide input for fate and transport modeling needed to support the remedy evaluation. A significant advancement from previous the CM for the 200-UP-1 OU is recognition of the multiple iodine species in the subsurface and the biogeochemical processes that control their fate and transport.

Target restoration cleanup levels for contaminated groundwater are established by applicable or relevant and appropriate requirements (ARARs) determined by federal and state drinking water quality standards. When these standards cannot be met within a reasonable timeframe due to limitations of available remediation technologies or other factors, the U.S. Environmental Protection Agency (EPA) may evaluate the technical impracticability (TI) of attaining the required groundwater cleanup levels, and establish alternative, protective remedial strategies. This report provides information relevant to consideration of a TI waiver on the current federal 1 pCi/L drinking water standard (DWS) for iodine-129 in the 200-UP-1 Groundwater Operable Unit (OU) of the Hanford Site if ongoing evaluations of treatment options do not identify a feasible treatment technology.

Pump‐and‐treat (P&T) is a widely applied remedy for groundwater remediation at many types of sites for multiple types of contaminants. Decisions regarding major changes in the remediation approach are an important element of environmental remediation management for a site using P&T. While existing guidance documents provide information on design, operation, and optimization for P&T systems, these documents do not provide specific technical guidance to support remedy decisions regarding when to transition to a new remedy or to initiate closure of the P&T remedy. A structured approach for P&T performance assessment was developed and is described herein, using analysis of three example P&T systems. These examples highlight key aspects of the performance assessment decision logic and represent assessment outcomes associated with optimizing the P&T system, transitioning from P&T to natural attenuation, and supplementing P&T with another technology to hasten transition to natural attenuation.

There are complexity elements to consider when applying subsurface flow and transport models to support environmental analyses. Modelers balance the benefits and costs of modeling along the spectrum of complexity, taking into account the attributes of more simple models (e.g., lower cost, faster execution, easier to explain, less mechanistic) and the attributes of more complex models (higher cost, slower execution, harder to explain, more mechanistic and technically defensible). In this report, modeling complexity is examined with respect to considering this balance. The discussion of modeling complexity is organized into three primary elements: (1) modeling approach, (2) description of process, and (3) description of heterogeneity. Three examples are used to examine these complexity elements. Two of the examples use simulations generated from a complex model to develop simpler models for efficient use in model applications. The first example is designed to support performance evaluation of soil-vapor-extraction remediation in terms of groundwater protection. The second example investigates the importance of simulating different categories of geochemical reactions for carbon sequestration and selecting appropriate simplifications for use in evaluating sequestration scenarios. In the third example, the modeling history for a uranium-contaminated site demonstrates that conservative parameter estimates were inadequate surrogates for complex, critical processes and there is discussion on the selection of more appropriate model complexity for this application. All three examples highlight how complexity considerations are essential to create scientifically defensible models that achieve a balance between model simplification and complexity.

Performance assessment of a pump-and-treat (P&T) system to support a decision about whether it is appropriate to shut down the system can be facilitated by using a structured approach that included consideration of decision elements. Decision elements involve assessment of contaminant concentrations and trends; contaminant mass discharge from source areas; the attenuation capacity of the aquifer; estimated future plume behavior; and P&T system design, operational, and cost information to identify an appropriate condition for transitioning from P&T remediation. It is also important to consider site-specific factors. Thus P&T system performance assessment for the Hanford Site should be specific to the plumes, environmental setting, and design/operational approaches for Hanford Site P&T systems. In fiscal year 2017, PNNL worked with 200-ZP-1 operable unit (OU) and 100-HR-3 OU staff in preparing OU documents associated with implementation of P&T remedies. In these efforts, elements of a structured approach to P&T decisions and Hanford-specific factors were incorporated into the documents. Because the 200-ZP-1 and 100-HR-3 OU documents are still in a draft stage at the time this report was written, the information provided here serves only as an example of how to map P&T decision elements to performance monitoring and work plans for P&T systems. Final remedy documentation for these OUs should be consulted for specific information on OU plans and activities.

Isotopes of iodine were generated during plutonium production from nine production reactors at the U.S. Department of Energy Hanford Site. The long half-life 129I generated at the Hanford Site during reactor operations was 1) stored in single-shell and double-shell tanks, 2) discharged to liquid disposal sites (e.g., cribs and trenches), 3) released to the atmosphere during fuel reprocessing operations, or 4) captured by off-gas absorbent devices (silver reactors) at chemical separations plants (PUREX, B-Plant, T-Plant, and REDOX). Releases of 129I to the subsurface have resulted in several large, though dilute, plumes in the groundwater, including the plume in the 200-UP-1 operable unit. There is also 129I remaining in the vadose zone beneath disposal or leak locations. Because 129I is an uncommon contaminant, relevant remediation experience and scientific literature are limited.