This study presents a long-term monitoring strategy for early risk warning of the remobilization of contaminants, mainly attenuated through an ion exchange reaction, induced by abrupt changes in geochemical conditions. The strategy aims to utilize readily in-situ measurable groundwater quality parameters in the prediction of near-future contaminant remobilization caused by cation exchange reactions. The proposed approach was demonstrated using historical monitoring data from the Department of Energy (DOE) Savannah River Site (SRS) F Area, which experienced abrupt geochemical disturbance during the pump-treat-reinjection remedy, and a reactive transport model developed through this study to understand 90Sr migration behavior in the subsurface of the SRS F Area. Both historical monitoring data analysis and reactive transport modeling results revealed a measurable temporal separation (time lag) between the arrival of background electrolyte perturbation and subsequent remobilized contaminant breakthrough. This quantified time lag provides an operational intervention window that can be utilized within an early warning framework. The results suggest that in-situ specific conductance sensors can serve as a practical early warning indicator to detect contaminant remobilization associated with cation exchange species. This strategy is expected to benefit the long-term management of the contaminated site for elements with cation exchange reactions by providing the means to detect the remobilization of contaminants prior to peak contaminant arrival.

This study aims to investigate the interplay between waste-management strategies and their environmental impacts. We focus on a highly mobile and persistent radionuclide, iodine-129 (I-129), within spent nuclear fuel (SNF), which is the dominant risk contributor from geological disposal and at existing groundwater contamination sites. Our fuel-cycle results show that the current recycling practice releases more than 90% of I-129 into the present-day biosphere, while the direct disposal of SNF in geological repositories is likely to reduce the release by eight orders of magnitude over one million years. Release from recycling can be reduced by gaseous filters, which are then disposed of in near-surface waste repositories. In addition, our data synthesis of surface-water concentrations near four nuclear facilities shows that the dilution strategy results in lower concentrations than regulatory standards, although the concentrations are significantly higher than the background over a large area and bioaccumulation has been reported. On the other hand, insufficient waste isolation in the past has resulted in locally high concentrations within one site. Our analysis suggests that (1) it is essential to consider effluents more explicitly as a part of the waste, (2) as society moves from dilution to isolation of waste (including CO2), the potential risks of waste isolation to local regions should be carefully evaluated, and (3) excessive burdens of proof could hinder/discourage waste sequestration. Comprehensive waste management strategies—considering not just volume but also mobility, isolation technologies, and ultimate fates—are needed for persistent contaminants.

Climate resilience is an emerging issue at contaminated sites and hazardous waste sites, since projected climate shifts (e.g., increased/decreased precipitation) and extreme events (e.g., flooding, drought) could affect ongoing remediation or closure strategies. In this study, we develop a reactive transport model (Amanzi) for radionuclides (uranium, tritium, and others) and evaluate how different scenarios under climate change will influence the contaminant plume conditions and groundwater well concentrations. We demonstrate our approach using a two-dimensional (2D) reactive transport model for the Savannah River Site F-Area, including mineral reaction and sorption processes. Different recharge scenarios are considered by perturbing the infiltration rate from the base case as well as considering cap-failure and climate projection scenarios. We also evaluate the uranium and nitrate concentration ratios between scenarios and the base case to isolate the sorption effects with changing recharge rates. The modeling results indicate that the competing effects of dilution and remobilization significantly influence pH, thus changing the sorption of uranium. At the maximum concentration on the breakthrough curve, higher aqueous uranium concentration implies that sorption is reduced with lower pH due to remobilization. To better evaluate the climate change impacts in the future, we develop the workflow to include the downscaled CMIP5 (Coupled Model Intercomparison Project) climate projection data in the reactive transport model and evaluate how residual contamination evolves through 2100 under four climate Representative Concentration Pathway (RCP) scenarios. The integration of climate modeling data and hydrogeochemistry models enables us to quantify the climate change impacts, assess which impacts need to be planned for, and therefore assist climate resiliency efforts and help guide site management.

In this study, we have developed a comprehensive machine learning (ML) framework for long-term groundwater contamination monitoring as the Python package PyLEnM (Python for Long-term Environmental Monitoring). PyLEnM aims to establish the seamless data-to-ML pipeline with various utility functions, such as quality assurance and quality control (QA/QC), coincident/colocated data identification, the automated ingestion and processing of publicly available spatial data layers, and novel data summarization/visualization. The key ML innovations include (1) time series/multianalyte clustering to find the well groups that have similar groundwater dynamics and to inform spatial interpolation and well optimization, (2) the automated model selection and parameter tuning, comparing multiple regression models for spatial interpolation, (3) the proxy-based spatial interpolation method by including spatial data layers or in situ measurable variables as predictors for contaminant concentrations and groundwater levels, and (4) the new well optimization algorithm to identify the most effective subset of wells for maintaining the spatial interpolation ability for long-term monitoring. We demonstrate our methodology using the monitoring data at the Savannah River Site F-Area. Through this open-source PyLEnM package, we aim to improve the transparency of data analytics at contaminated sites, empowering concerned citizens as well as improving public relations.

The objective of this report is to relate waste leaching experiments by the Savannah River National Laboratory (SRNL) to performance assessment modeling of release of several radionuclides from residual waste in closed high-level waste tanks. SRNL leached residual waste from Tanks 18F and 12H with solutions simulating the modeled evolution of pore fluids in residual waste layers of closed tanks through various stages of grout degradation. Tc-99, uranium isotopes, Np-237, plutonium isotopes, and I-129 were analyzed in the leachate multiple times over 90 days.

This study presents an effective approach to tackle the challenge of long-term monitoring of contaminated groundwater sites where remediation leaves residual contamination in the subsurface. Traditional long-term monitoring of contaminated groundwater sites focuses on measuring contaminant concentrations and is applicable to sites where contaminant mass is removed or degraded to a level below the regulatory standard. The traditional approach is less effective at sites where risk from metals or radionuclides continues to exist in the subsurface after remedial goals are achieved. We propose a long-term monitoring strategy for this type of waste site that focuses on measuring the hydrological and geochemical parameters that control attenuation or remobilization of contaminants while de-emphasizing contaminant-concentration measurements. We demonstrate how this approach would be more effective than traditional long-term monitoring, using a site in South Carolina, USA, where groundwater is contaminated by several radionuclides. A comprehensive enhanced attenuation remedy has been implemented at the site to minimize discharge of contamination to surface water. The immobilization of contaminants occurs in three locations by manipulation of hydrological and geochemical parameters, as well as by natural attenuation processes. Deployment of our proposed long-term monitoring strategy will combine subsurface and surface measurements using spectroscopic tools, geophysical tools, and sensors to monitor the parameters controlling contaminant attenuation. The advantage of this approach is that it will detect the potential for contaminant remobilization from engineered and natural attenuation zones, allowing potential adverse changes to be mitigated before contaminant attenuation is reversed.

This report provides a roadmap of applied science studies that will facilitate reaching long term monitoring end state corrective actions for groundwater contamination at the F-Area Seepage Basins and the H-Area Seepage Basins at the Savannah River Site in Aiken, SC. The F-Area and H-Area Seepage Basins are waste units on the Savannah River Site in Aiken, SC at which low-level radioactive solutions were disposed into unlined basins, resulting in groundwater contamination. The current contaminants of interest in the groundwater are tritium, iodine-129, strontium-90, uranium isotopes and nitrate. Remediation at both sites has continued since 1988, consisting of closure and capping of the basins, operation of a groundwater pump-and-treat system from 1997 to 2004, and replacement of this system with a comprehensive in situ attenuation-based remedy. This report was requested by the U.S. Department of Energy - Office of Environmental Management in consultation with the Savannah River National Laboratory and Savannah River Nuclear Solutions - Area Completions Projects to provide a scientific basis for proactively addressing groundwater issues that may need to be resolved prior to final corrective actions at the F-Area and H-Area Seepage Basins.

A Soil and Water Assistance (SWAT) team of experts was assembled to evaluate the application of a new approach to long-term monitoring of a waste unit, contaminated with metals and radionuclides, located on the U.S. Department of Energy’s (DOE) Savannah River Site. The site formally known as the F-Area Hazardous Waste Management Facility is referred to here by the more descriptive and widely used F-Area Seepage Basins.

This study investigates the potential impact of climate change on residual contaminants in vadose zones and groundwater. We assume that the effect of climate changes can be represented by perturbations in the natural recharge through the aquifer system. We perform numerical modeling of unsaturated/saturated flow and transport and consider different performance metrics: contaminant concentrations at observation wells and contaminant export at the site's boundary. We evaluate the effect of increasing and decreasing recharge as well as the impact of potential failure of surface capping structures employed to immobilize vadose zone contaminants. Our approach is demonstrated in a real case study by simulating transport of non-reactive radioactive tritium at the U.S. Department of Energy's Savannah River Site. Results show that recharge changes significantly affect well concentrations: after an initial slight dilution we identify a significant concentration increase at different observation wells some years after the recharge increase and/or the cap failure, as a consequence of contaminants' mobilization. This effect is generally emphasized and occurs earlier as the recharge increases. Under decreased aquifers' recharge the concentration could slightly increase for some years, due to a decrease of dilution, depending on the magnitude of the negative recharge shift. We identify trigger levels of recharge above which the concentration/export breakthrough curves and the time of exceedance of the Maximum Contaminant Level for tritium are remarkably affected. Moreover, we observe that the contaminant export at the control plane, identified as the risk pathway to the downgradient population, may only be minimally affected by shifts in the natural recharge regime, except for some extreme cases. We conclude that more frequent sampling and in-situ monitoring near the source zone should be adopted to better explain concentrations' anomalies under changing climatic conditions. Moreover, the maintenance of the cap is critical not only to sequester residual contaminants in the vadose zone, but also to reduce the uncertainty associated with future precipitation changes. Finally, realistic flow and transport simulations achieved through proper calibration processes, rather than conservative modeling, should be adopted to identify non-trivial trade-offs which enable better allocation of resources towards reducing uncertainty in decision making.

AbstractSustainable remediation has been increasingly regarded as a promising alternative in recent years. It considersnet environmental impactssuch as energy use, greenhouse gas emission, and waste reduction. Passivein situremediation or natural attenuation are the key component of sustainable remediation often intended to minimize the net environmental impacts. However, leaving contaminants in the subsurface requires the increased burden of proof to show that plumes are stable and residual contaminants do not pose a significant health risk. Particularly in complex geological environments and for actinide species with complex geochemical behaviors, it is difficult to ensure the system stability as well as to predict the future plume conditions. This article presents recent scientific advances to support sustainable remediation in complex geological systems, including site characterization techniques, hydrological and geochemical model developments, and numerical simulations. In particular, we highlight the recent developments in non‐invasive geophysical characterization as well as computationally efficient geochemical models for describing uranium and other reactive species. We demonstrate this approach using the extensive data and models from the Savannah River Site F‐area, which has been contaminated by low‐level radioactive waste solutions including uranium, tritium, and other radionuclides.

This study presents a Kalman filter-based framework to establish a real-time in situ monitoring system for groundwater contamination based on in situ measurable water quality variables, such as specific conductance (SC) and pH. First, this framework uses principal component analysis (PCA) to identify correlations between the contaminant concentrations of interest and in situ measurable variables. It then applies the Kalman filter to estimate contaminant concentrations continuously and in real-time by coupling data-driven concentration decay models with the previously identified data correlations. We demonstrate our approach with historical groundwater data from the Savannah River Site F-Area: We use SC and pH data to estimate tritium and uranium concentrations over time. Results show that the developed method can estimate these contaminant concentrations based on in situ measurable variables. The estimates remain reliable with less frequent or no direct measurements of the contaminant concentrations, while capturing the dynamics of short- and long-term contaminant concentration changes. In addition, we show that data mining, such as PCA, is useful to understand correlations in groundwater data and to design long-term monitoring systems. The developed in situ monitoring methodology is expected to improve long-term groundwater monitoring by continuously confirming the contaminant plume's stability and by providing an early warning system for unexpected changes in the plume's migration.

Quantifying the extent to which contaminant metals bind to subsurface soils is important for risk assessment, the tendency for a contaminant to migrate, and developing environmental remediation strategies. Unfortunately, subsurface soils vary widely in their composition, which in turn affect their tendency to bind metals. The hypothesis of this study was predicated on how a better understanding of geological facies would reduce uncertainty associated with predicting contaminant metal sorption. Facies are layers of sediment deposited in the subsurface due to similar depositional conditions, including energy of an overlying waterway. As such, facies are expected to have similar assemblages of minerals, particle size distributions, origins of organic matter, and similar microbial population structures. These are all important factors affecting contaminant metal sorption. The approach of this study was to collect 42 composite soil samples from a 5 m by 1.5 m grid outcrop in Graniteville, South Carolina and five end-member facies samples. The fraction of each of the five facies comprising the 42 composite soil samples were estimated. Particle size distribution (gravel, sand, silt, and clay fractions), pH, organic matter (OM), iron coating content, and microbial colony forming units were determined for each composite soil and the five end-member facies soils. Because hexavalent chromium (Cr) is the most common contaminant metal in the U.S. to exceed drinking water limits, this highly toxic and soluble metal was used as a model contaminant to provide a measure of contaminant sorption. Chromium distribution coefficients (Kd = Crsoil/Crwater) were measured. Significant correlations were identified between several soil chemical and microbial properties. A significant correlation (r = 0.423; p ≤ 0.05, d.f. = 47) was also determined between measured Kd values and Kd values calculated based on knowledge of facies Kd values. Importantly, the calculated values were characterized by large amount of inherent error. Additional work is needed to determine the applicability of this approach for remediation of contaminated sites and how best to identify appropriate facies for this novel application.

The present study explores a novel application of Huma-K, a commercially available, unrefined humic substance, as a promising low-cost source of organic matter for in situ remediation of contaminated acidic groundwater plumes. This can be achieved by creating a humic-rich coating on the surface of minerals which can enhance the sorption of contaminants from groundwater. Huma-K was characterized by means of scanning electron microscopy equipped with energy dispersive spectroscopy, Fourier-transform infrared analysis, and potentiometric titrations. Batch experiments were performed to investigate the sorption-desorption behavior of Huma-K and to evaluate what conditions (pH, contact time, and initial Huma-K concentration) affect these processes upon injection into aquifer sediments. As evidenced by potentiometric titrations, Huma-K possesses functional groups that have an acidic nature, with pK values in the range of 4–6 (carboxylic) and 9–10 (phenolic). Sorption, homogeneous precipitation, and surface-induced precipitation seem to be favored in the presence of sediment at pH 4, where there is less deprotonation of acidic functional groups. As the pH is increased, functional groups become negatively charged, leading to electrostatic repulsion and dissolution of Huma-K from sediment. Kinetic experiments indicate that Huma-K sorption is a slow-rate process, most likely governed by film diffusion. The enhanced sorption of Huma-K in acidic conditions suggests that it may be used to create a subsurface treatment zone in acidic aquifers for the sequestration of contaminants such as uranium. The treatment zone will persist as long as the pH does not increase sufficiently to cause soil-bound Huma-K to be released, remobilizing aqueous contaminants.

BACKGROUND: Anthropogenic activities, such as uranium mining and the nuclear industry, have resulted in groundwater contamination and the creation of uranium-affected acidic plumes. In situ immobilization through base injection is a favorable way of uranium attenuation. The present study explores the use of sodium silicate for the restoration of neutral pH of the affected zone and consequently, uranium immobilization under circumneutral conditions. RESULTS: 70 mg L-1 sodium silicate restored the pH of uranium bearing, acidic groundwater to neutral in batch experiments consisting of Savannah River Site (SRS) soil and the aqueous phase. SRS soil main components are quartz, kaolinite and goethite and the U(VI) removal was similar to 60%. Identical experiments consisting only of quartz and kaolinite showed only 19% U(VI) removal. Binding of uranium may be improved by inner-sphere complexation and is not affected by the presence of competitive cations, such as Ca2+ and Mg2+. Recovery of uranium under acidic conditions (pH 3.5) was similar to 60%, whereas sorption is not reversible under circumneutral conditions. CONCLUSION: Sodium silicate restores the pH of acidic groundwater systems to circumneutral conditions, where uranium retention by iron bearing sediments is favored. Goethite is the soil's most reactive phase and contributes to stronger binding of uranium through inner-sphere complexation. (c) 2017 Society of Chemical Industry

This white paper presents an innovative approach for sustainable and cost-effective groundwater remediation and monitoring. Our approach integrates recent advances in various technologies: (1) enhanced attenuation-based remediation technology, (2) in-situ autonomous sensors, (3) big data analytics, (4) non-invasive remote mapping, and (5) parallel high-performance computing for flow and reactive transport modeling. Bringing these new technologies is expected to transform the long-term management and closure strategies of the Department of Energy (DOE) Environmental Management (EM) sites and to result in enormous cost saving (50 – 90%).

Long-term monitoring of contaminant transport in groundwater is expected to account for a large fraction of future life-cycle cleanup costs at the DOE sites. The Attenuation-Based Remedies in the Subsurface Applied Field Research Initiative (ABRS AFRI) is developing an innovative approach for cost-effective in situ long-term monitoring. The approach is based on strategically adding the measurements of controlling variables (such as pH, redox potential, electrical conductivity, and groundwater level), which control the plume mobility and its spatial and temporal distribution. In situ measurements of these variables – supplemented with a reduced number of standard periodic groundwater samples – are expected to lead to more cost-effective monitoring, and can also serve as an early warning system for detecting unexpected plume migration. The objective of this study is to support the development of such a long-term monitoring approach through the application of advanced computational methods, including (1) statistical data mining and analysis, and (2) three-dimensional flow and reactive chemical transport modeling. We have performed the statistical data analysis of historical and current monitoring data at the Savannah River Site (SRS) F-Area to quantify the correlations between the controlling variables and radioactive contaminant concentrations. In parallel, we have used 3D modeling of flow and contaminant transport to provide the prediction of the contaminant plume evolution. The results of both data analysis and modeling confirmed that the correlations between controlling variables and contaminant concentrations are significant. Modeling results also suggest that the correlation parameters will change in the future, which is important to assess the long-term efficacy of the proposed monitoring

The Savanah River Site (SRS) was constructed during the 1950s and became one of the major producers of plutonium for the United States during the Cold War. During its production life, the F/H Area Seepage Basins received approximately 1.8 billion gallons of acidic waste containing radionuclides and dissolved heavy metals. This led to the creation of a highly contaminated groundwater plume with uranium (VI) as a key contaminant of concern within the plume. Now, as a designated hazardous waste management facility, the main activities taking place at the site are nuclear storage and remediation of contaminated soil and groundwater from radionuclides. Humic substances (HS) are major components of soil organic matter having the ability to influence migration behavior and fate of heavy metals and are being investigated for potential use in environmental remediation at SRS. Essentially, HS are polyfunctional organic macromolecules formed by the chemo-microbiological decomposition of biomass or dead organic matter. HS are able to interact with both metal ions and organic compounds based on solubility. Humic acid (HA) represents the fraction of HS soluble at pH greater than 3.5. HA carries a large number of functional groups, provides an important function in ion exchange and is a metal complexing ligand with a high complexation capacity. Silica is the term applied to solid forms with the stoichiometric composition of SiO2. The colloidal silica used in this work is amorphous and nonporous in suspension; silica’s net surface charge for pH values higher than 3 is negative, due to the isoelectric point being 1.5 2.5. Humic acid has been previously used in remediation techniques and silica is found naturally in the soil.