PyLEnM: A Machine Learning Framework for Long-Term Groundwater Contamination Monitoring Strategies
Not Available
54 ENVIRONMENTAL SCIENCES↗
Engineering topics
Denham, Miles
Publications and source records attributed to Denham, Miles.
Reactive transport modeling for supporting climate resilience at groundwater contamination sites
Abstract. 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.
54 ENVIRONMENTAL SCIENCES↗
The Challenge of Achieving Site Closure of a Complex Groundwater Plume - 20531
Over the last three decades, DOE has made substantial progress in nearly every area of nuclear waste cleanup and has closed and transitioned 91 of the 107 sites in the EM complex. Several of the remaining groundwater plumes at Hanford, Los Alamos, Paducah, Oak Ridge, and Savannah River are complex and tend to have unique site or contaminant characteristics which provide an almost impossible challenge to remediate to regulatory levels. As a result, these complex sites will require a large investment to remediate. In order for DOE to achieve regulatory closure, a technically sound case must be developed and presented to regulators and stakeholders that clearly demonstrates that remedial goals have been or will be irreversibly achieved or that there is an alternate acceptable end-state. Five lines of evidence are required: - A conceptual site model that is as complete as possible, with the knowledge that it may continue to evolve with time; - Demonstration that trends in contaminant concentrations, or flux, versus time indicate that remedial goals will be achieved on schedule or within a reasonable time thereafter and that trends that deviate from this path are explainable by the conceptual model; - Demonstration that remobilization of attenuated contaminants is unlikely to occur in the future; - Explicit consideration of remedial contingencies, should deviations in the path toward remedial goals occur; - Development of an effective long-term monitoring plan to detect any conditions that might alter the path toward remedial goals. SRS has made significant progress in the remediation of contamination associated with the F-area and H-Area Seepage Basins, and it is currently developing a strategic closure path that addresses each of these lines of evidence and identifies both basic and applied scientific issues that will need to be addressed in order to obtain regulatory closure. (authors)
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Big Data and AI at DoE's Legacy Sites - 20546
More than 30 years have passed since DOE started the decommissioning of nuclear weapon complexes and the clean-up of soil and groundwater. All the sites have been collecting and archiving soil and groundwater monitoring datasets; particularly contaminant concentration time-series. These datasets provide unparalleled opportunities to understand the system behavior (including more fundamental hydrological and geochemical processes, the response to various perturbations, the long-term trend and environmental decay rate towards the regulatory limit). This understanding is critical for providing multiple lines of evidences that can support site closure. In this study, we explore the machine learning (ML) and artificial intelligence (AI) applications to the long-term soil and groundwater management at DoE's legacy sites. ML can improve our understanding of the subsurface systems, which is critical for long-term monitoring and management of the sites, while AI can automate or support some of decision-making processes (e.g., anomaly detection, monitoring well placements). The particular focuses are to develop general algorithms to: (1) to identify distinct spatiotemporal patterns and to identify several groups that have similar temporal behaviors, using unsupervised clustering methods, (2) identify the different temporal scales of hydrological responses to climate perturbations by time-series analysis, and (3) reduce the number of monitoring wells by identifying the minimum sufficient number of wells to capture the heterogeneity of the groundwater contaminant plume and concentration distribution, using the Gaussian Process model. We demonstrate our methodology at the Savannah River Site F-Area. (authors)
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Hydrological Controls on Residual Contaminants Under Sustainable Remediation - 19287
This paper investigates the impact of natural aquifer's recharge shifts induced by precipitation changes on residual contaminants located in soil and groundwater, subject to sustainable remediation techniques. We illustrate five climate change scenarios which mimic shifts in the natural recharge regime. We evaluate the impact on different performance metrics that include contaminant concentrations at observation wells and contaminant export to a creek. The approach is demonstrated by simulating the tritium plume migration within a U.S. Department of Energy's nuclear reservations: the Savannah River Site F-Area. Results show that increasing the aquifers' recharge initially causes a decrease in the well concentrations due to dilution, and subsequently an increase due to the mobilization of contaminants from the vadose zone. As the recharge shift increases, the peak concentration is higher and may occur at an earlier stage. The mobilized contaminants reach the wells or control planes several years or decades later than the precipitation events thus monitoring near the source zone is important for early detecting mobilized contaminants. Moreover, we observe that, even when the well concentrations are significantly affected, the export at the control planes may not be affected by shifts in the natural recharge regime therefore the risk of downgradient population may be under control even under significant hydrological changes. (authors)
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Climate change impact on residual contaminants under sustainable remediation
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.
54 ENVIRONMENTAL SCIENCES↗
In Situ Monitoring of Groundwater Contamination for Sustainable Remediation Using the Kalman Filter - 18423
Groundwater monitoring is critical for nuclear-related facilities as well as radioactively contaminated sites. Recent advances in sensing, data analytics and simulation capabilities provide substantial opportunities to improve and change current monitoring procedures. In particular, we need a paradigm shift from contaminant-only or compliance-only monitoring programs to comprehensive monitoring, i.e., including other ancillary environmental variables for a holistic understanding of the groundwater systems and plume behaviors. Such new approaches are needed for the effective implementation of sustainable remediation (enhanced and natural attenuation), and they have great potential for cost reduction in site monitoring. This paper reviews recent developments for improving long-term monitoring, with focus on the effective use of in situ sensors, data analytics and simulation capabilities. In particular, we highlight a recent application of the Kalman filter algorithm to estimate contaminant concentrations continuously in real time, based on in-situ measured water quality parameters (e.g. electrical conductance and pH) and sparse groundwater sampling. The results show that we can reduce the groundwater sampling frequency significantly, while capturing the dynamics of short- and long-term contaminant concentration changes. Such advanced and integrated monitoring systems will be beneficial for local residents (e.g., to ensure the safety by detecting adverse events early, enabling a swift response) as well as for site operators (e.g., to gain/improve public acceptance, to be better prepared for litigations and false claims, and to reduce overall monitoring cost). (authors)
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Technical Advances for Sustainable Remediation: In Situ Monitoring and Climate Resiliency - 18436
Recent advances in sustainable remediation techniques show promise as solutions for contaminant removal in soil and groundwater media. Although passive remediation (or natural attenuation) has gained momentum as an acceptable technique, there are still challenges in regulatory frameworks as well as in public acceptance. Leaving contaminants in subsurface requires increased burden of proof to show that plumes are stable and residual contaminants do not pose a significant health risk. Climate changes and frequent extreme climate events contribute another level of complexity and make it difficult to demonstrate system stability and predict future plume conditions. In this study, we show two coupled approaches to tackle this challenge: (1) in situ groundwater monitoring based on advanced sensor and data analytics technologies (i.e., Big Data), and (2) state-of-art modeling for optimizing monitoring and site closure strategies. In the new monitoring approach, in situ sensors are used to continuously measure the key variables that control contaminant plume mobility and its spatial and temporal distribution (such as pH, redox potential, electrical conductivity, and groundwater level). Coupled with a limited number of groundwater sampling, the data analytics methods - data mining and machine learning - allow us to estimate contaminant concentrations, and also to detect any significant changes associated with the plume mobility. In parallel, the state-of-art numerical flow and reactive transport model - ASCEM software under DOE Office of Environmental Management - provides a physical and mechanistic understanding of the system behavior, and predicts the long-term plume migration for optimizing the monitoring and closure strategies. In particular, we focus on the effects of extreme precipitation or drought on the site closure as well as monitoring strategies. We demonstrate this approach at the Savannah River Site (SRS) F-Area, where groundwater is contaminated by various radionuclides, including uranium, tritium and technetium. (authors)
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Study of Synergetic Interactions between Uranium, Humic Acid, Silica Colloids and SRS Sediments - 18581
The Savannah River Site's (SRS) 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. 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. HS are polyfunctional, organic macromolecules able to carry a large number of functional groups which is an important function in ion exchange and is a metal complexing ligand with a high complexation capacity. Humic acid (HA) represents the fraction of HS soluble at pH greater than 3.5. Silica, naturally found in soil, is the term applied to solid forms with the stoichiometric composition of SiO{sub 2}. The colloidal silica used in this work is amorphous and non-porous in suspension. This investigation studied the synergistic interactions between four key components: U (VI), humic acid (HA), colloidal silica and SRS sediment; the interactions were studied under pH 3 - 8. Multi-component batch systems were constructed to effectively analyze the U (VI) removal. The results show that filtered and unfiltered samples depicted similar trends; while, filtered samples had more overall removal than the unfiltered samples at the varying pH levels. Colloidal silica does not have a significant effect on the removal of U (VI) due to U (VI) bound to colloidal silica remaining in suspension. (authors)
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Climatic Controls on Residual Contaminants Under Sustainable Remediation - 18699
We investigate the impact of climate change on residual contaminants in soil and groundwater under sustainable remediation. We illustrate seven climate change scenarios which mimic shifts in the precipitation regime as well as the failure of the source-zone cap. We evaluate different performance metrics including contaminant concentrations at multiple wells and export to a creek. The approach is demonstrated by simulating the tritium plume migration at the Savannah River Site F-Area, USA. The cap integrity is critical not only to reduce the well concentrations and export but also to reduce the uncertainty associated with the climatic variability. Sampling or in situ monitoring near the source zone is important for the early detection of plume mobility changes. Realistic hydrological models are necessary to evaluate the trade-offs (mobility vs dilution)
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In Situ Monitoring of Groundwater Contamination Using the Kalman Filter
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. As a result, 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.
54 ENVIRONMENTAL SCIENCES↗
Unrefined humic substances as a potential low-cost amendment for the management of acidic groundwater contamination
Not Available
Gonzalez-Raymat, Hansell↗
Influences on Effective Decay Rates of Radionuclides in Groundwater: F-Area Seepage Basins, Savannah River Site - 17149
Effective decay rates for radionuclides incorporate all attenuation mechanisms and are useful for assessing the progress of natural or enhanced attenuation. If a concentration versus time trend at a given monitoring well behaves according to first-order decay, then an effective decay constant can be calculated from the slope of the linear trend. It is important to view time trends in terms of all possible processes that can affect them prior to evaluating effective decay rates, because changes to groundwater flow and attenuation processes influence the rate at which contaminant concentrations decrease. In addition, other factors can lead to apparent changes in the effective decay rate. Groundwater associated with the F-Area Seepage Basins at the Savannah River Site in South Carolina provides an excellent case study for examining processes that affect concentration versus time trends and some of the pitfalls in evaluating effective decay rates. The site has an extensive groundwater monitoring database and its history is well known. The objective of this case study is to highlight the importance of viewing a site holistically in any evaluation of effective decay rates, because what is obvious at the F-Area Seepage Basins may not be obvious at sites with sparser monitoring data and more obscure histories. (authors)
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Coupling Big Data Analytics and Reactive Transport Modeling for Cost-effective Groundwater Monitoring - 17163
This study presents an innovative approach for sustainable and cost-effective groundwater monitoring. This approach takes advantage of recent advances in various technologies: (1) in situ autonomous sensors, (2) big data analytics, and (3) parallel high-performance computing for flow and reactive transport modeling. In situ sensors are used to periodically measure the key variables (such as pH, redox potential, electrical conductivity, and groundwater level), which control contaminant mobility and the plume spatial and temporal distribution. Based on a limited number of groundwater sampling, the data analytics methods-data mining and machine learning-allow us to identify and quantify the correlations between the in situ-measured variables and contaminant concentrations, and also to detect significant changes associated with the plume mobility. In addition, a state-of-the-art parallel numerical flow and reactive transport simulator Amanzi, and uncertainty quantification software Agni are used to provide an improved physical and mechanistic understanding of the contaminated groundwater system behavior, and to predict the long-term plume distribution for optimizing and adapting the monitoring strategy. Amanzi and Agni were developed as part of the Advanced Simulation Capability for Environmental Management (ASCEM) program of the DOE Office of Environmental Management. Such modeling is critical, particularly, for assessing the impact of climate change and associated hydrological shifts. The developed approach is expected to significantly reduce the groundwater sampling frequency and associated cost. In addition, the real-time information on plume mobility serves as an early warning system, improving the resiliency of contaminated or potentially contaminated sites. We demonstrate this approach using as an example the Savannah River Site (SRS) F-Area, where groundwater is contaminated by various radionuclides, including uranium, tritium and technetium. (authors)
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Technetium Management Program Plan - 17376
Technetium management is a high-priority activity for the U.S. Department of Energy (DOE) Office of Environmental Management (EM) complex. Waste management and remediation issues involving technetium represent environmental risks and challenges that are unique to DOE. This plan lays out the technology development activities that are underway as well as future technology needs to address technetium issues across DOE EM sites and across programmatic areas of tank waste, groundwater and soils, and facility decommissioning and deactivation in order to be comprehensive and to allow integration of similar activities. The program was developed in close coordination with site representatives and program offices. Activities and needs that were identified share common themes and were grouped into areas of characterization, treatment, and disposition. Characterization includes understanding the inventory at particular locations, the chemical species, and chemical and physical behavior, whether in a treatment process flowsheet or a geological setting that displays periodic contaminant releases. The tools for characterization in these different environments might use the same sensors or protocols. Treatment options include methods to capture technetium such as ion-exchange or manipulation of oxidation states, which also may be applied across program areas. Disposition includes end-points of the processes and environmental risk, including waste form behavior, mobility of technetium once it is treated or remediated, and long-term monitoring. Disposition represents integration of all cleanup actions at a site. (authors)
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Iron Behavior in Microcosms Simulating Bioreduction in Savannah River Site Sediments - 17389
An estimated 1.8 billion gallons of acidic low-activity waste solutions originating from the processing of uranium slugs and irradiated fuel at separation facilities at the Savannah River Site (SRS) were discharged to the unlined seepage basins located in the F/H-area. The acidic solutions of the basin influent (pH from 3.2 to 5.5) contaminated with a variety of radionuclides and dissolved metals caused groundwater contamination. In 2010, DOE funded a demonstration of the Enhanced Anaerobic Reductive Precipitation (EARP) process at the SRS F-Area that consisted of in-situ injections of a carbohydrate substrate to establish anaerobic reactive zones for metal and radionuclide remediation. The addition of the molasses substrate solution to groundwater was done to produce anaerobic conditions conducive to uranium reduction and then precipitation as uranium (IV). The SRS soil is highly weathered and features very low natural alkalinity. A microcosm study, prepared with sieved SRS sediments and augmented with a solution mixture containing molasses and sulfate, was designed to provide evidence of the capabilities of this remediation technology under SRS environmental conditions. The objective of these microcosm experiments was to replicate the anaerobic conditions created as a result of injections of molasses combined with sulfate ions, similar to the EARP process that was performed at SRS, and investigate if any mineralogical changes could occur in the soil due to the addition of molasses. Specifically, the study aimed to determine if solid phases of reduced iron such as siderite and pyrite would be formed, as this would indicate the potential to form a long-lasting bioreductive zone. An understanding of this technology will be useful in determining if it is a viable option for remediation. In the experiments, the media solution was amended with molasses and sulfate to stimulate sulfate-reducing bacteria. Sulfate reduction occurs extensively under the redox conditions occurring after iron reduction and before methanogenic conditions. These conditions are considered the second most reducing condition in natural groundwater systems. Microcosm tubes prepared in triplicate were kept inside an anaerobic glove box, which was continuously monitored to ensure that conditions remained anaerobic. The initial X-ray diffraction (XRD) analyses on the background sediment samples indicated the presence of quartz, kaolinite, montmorillonite, and goethite. In the molasses-treated samples there were no visible peaks for reduced forms of iron such as siderite and pyrite. The pH measurements suggest that all samples in either batch, including those that were brought to a neutral pH before the addition of the sediments, have followed a similar trend with a decline in the pH value to between 4 and 4.7. This can be attributed to the fermentation of molasses and the natural acidity of the SRS sediments used for the microcosm study. In the acidic iron-rich sediment, the microbial reduction of Fe (III) was the predominant electron-accepting process for oxidation of the organic substrate. The maximum iron concentration detected during the experiments was 13 mg/L. Analytical results showed that there was no sulfate reduction in any of the samples augmented with sulfate and the concentration remained level at 500 ppm as originally added to the initial solutions (518 - 542 mg/L). This is consistent with the absence of any indication of iron sulfide formation. Under the experimental conditions in these microcosms, the abundance of biologically available Fe (III) allows Fe (III)-reducers to out-compete sulfate-reducing bacteria using molasses as an electron donor. (authors)
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