Environmental contaminants endanger lives, property, and communities. The more we learn, and share, the closer we come to discovering solutions.
More than half of the United States relies on groundwater for drinking water. However, since the 80s, high concentrations of chlorinated and recalcitrant compounds have been discovered in some communities—usually after causing irreparable harm to people, plants, and animals. Unfortunately, chemical toxicity and behavior wasn’t understood at the time of the industrial boom in the 50s and 60s. Waste material was often washed away with stormwater, stored in unlined ponds, or buried underground in steel drums. Over time, these substances have leached into groundwater, contaminating our water supply.
The Battelle 2018 International Conference on Remediation of Chlorinated and Recalcitrant Compounds, is one forum where professionals can come together to learn and share ideas. This year there were more than 1,600 remediation experts from 30 countries in attendance—many who led one or more of the 1,000 platform and poster presentations. Stantec sent 12 environmental experts from throughout North America to join the discussion. Committed to working together so we can improve the communities where we live and serve, members of our team presented discoveries, learned from others during educational workshops, and discussed strategies with fellow environmental experts.
Understanding chlorinated and recalcitrant compounds
We now understand much more about the toxic byproducts of industrial uses. Substances, categorized as dense non-aqueous phase liquids (DNAPLs), include: creosote, used in wood preservation; coal tar, a by-product of coal gasification processes to provide power; and chlorinated volatile organic compounds (CVOCs), widely used in manufacturing, aerospace, semiconductor, and transportation as a solvent to remove grease.
Because of their density, DNAPLs migrate vertically and laterally through ground surfaces, penetrate the groundwater table, and enter the saturated zone—sometimes to depths over 300 feet. In the saturated zone, they spread, dissolving into large contaminant plumes that are released into the air, soil, and groundwater. Often resistant to biodegradation and insoluble, chlorinated solvents are common contaminants at Superfund sites, Department of Defense (DOD) sites, and Department of Energy (DOE) sites.
Environmental mitigation is a unique science. Environmental laws and regulations constantly change. Engineers and regulators in other fields have proven means and methods they can base their decisions upon. Those addressing environmental conditions are always faced with uncertainty. With highly site-specific conditions and contamination, environmental site mitigation is a field where treatment methods, effectiveness, or time frames can’t be strictly regulated. With the liability associated with determining a “safe” level of contamination for various uses, environmental decision makers must answer tough questions like “how much testing or remediation is enough?” These decisions are made using the best available science, data, and outcomes available at that moment in time.
One of the panel discussions at the conference discussed how Adaptive Management Processes1 are ideal for large and complex cleanup sites—particularly when mitigating DNAPL and light non-aqueous phase liquid (LNAPL) contaminants. It was noted that despite significant remediation efforts, some of these projects cannot practically achieve stringent cleanup goals. Cost vs. benefit decisions need to be made, something regulatory agencies often recognize. A flexible process is necessary to allow pilot testing of new, possibly more effective treatment methods, even is the technology isn’t specified in an agency approved remedial action plan. The most important element in a successful long-term remediation plan is to establish trust between regulators and the responsible party (and its consultant). This can often be one of the greatest challenges we face as consultants.
In another presentation, Iain Baker of Pacific Gas & Electric explained how using an Adaptive Management Process2 has given his team a flexible approach that streamlines decision-making2 on a complex, high-profile project in Hinkley, California. For 20+ years, PG&E has been mitigating a very large, relatively dilute hexavalent chromium plume. Measures taken include agricultural application, groundwater extraction, clean water re-injection, and in-situ chemical reduction with ethanol using multiple injection/extraction well arrays. Carefully evaluating each outcome using an iterative approach to continuous process improvement has informed and improved outcomes on new projects.
Modeling what can’t be seen
Unless you are Superman, you can’t characterize a site with 100% certainty. Using new technology, however, you can make more informed decisions. Ron Falta of Clemson University introduced a Beta version of REMChlor MD3. This screening level Remediation Simulation Model enables users to simulate chemical back-diffusion from low permeability layers to higher permeability layers, reducing uncertainty when evaluating alternative remedial treatment options.
Joel Thompson, a senior professional hydrogeologist at Stantec offered a word of warning about relying upon computer simulations to model back-diffusion. He commented that modeling fate and transport of contaminants typically doesn’t include back-diffusion processes, often underestimating the required operational period of in situ remedies, natural attenuation, or enhanced natural attenuation remedies. Back-diffusion modeling is challenging because rates are determined by local concentration gradients at 0.1-ft or less scale, making the numerical simulation very computationally intensive. New analytical solutions using discrete quantities coupled with analytical and numerical solutions will improve the simulations.
Sequence stratigraphy and high-resolution site characterization can be used to identify contaminant flux. Use of adaptive transects and detailed soils analysis can help us better understand stratigraphic flux based on soil types and hydraulic conductivity. In one study, a 3D heat map was generated using soil type and hydraulic conductivity data to identify migration pathways on a complex site. Environmental Visualization System (EVS)4 software is one program available for analyzing data and creating 3D visualization tools for geophysical analysis.
Tools like these enable users to better characterize the subsurface. They also help with client and stakeholder communication. Stakeholders have strong opinions on whether the need for projects overrides potential negative environmental impacts. Lawsuits are a tool used to delay or prevent development projects. To overcome these challenges, teams need to develop a rapport with stakeholders. Visual models are very helpful when explaining technical data (obtained on publicly accessible environmental impact reports), site issues, and remediation methods to a non-technical audience.
Radial diagrams illustrate the degree of chemical degradation and/or the relative concentrations of selected compounds in groundwater. Visual Bio5 is a DOS executable program for creating output files that can be imported into Surfer for use in creating figures for reports. It is useful for visually presenting a tremendous amount of data and conveying what the data means relative to biodegradation parameters and compound distribution ratios.
Discovering new strategies and compound solutions
Chlorinated solvents in dissolved plumes can be removed by natural attenuation processes including bioremediation. Adding engineered compounds that modify environmental conditions—physical, chemical, biochemical, or microbiological—encourage microorganisms to destroy or detoxify organic and inorganic contaminants in the environment. In situ bioremediation of groundwater is the most widely used method for treating contaminated sites because their cost is relatively low, they can adapt to site-specific conditions, and are highly effective when used properly.
When evaluating LNAPL recovery/remediation options for sites in the later stages of the remediation cycle, natural source zone depletion (NSZD)6 is a practical alternative. This approach uses naturally occurring biodegradation processes to reduce contaminants in the subsurface. Thomas Sale from Colorado State University provided an overview of the state of NSZD practice and the importance of understanding NSZD in selecting and designing active LNAPL recovery/remediation technologies. NSZD studies in combination with updated transmissivity tests and long-term groundwater monitoring data can also present a rationale for ceasing active remediation.
To treat contaminated soils and liquid organic wastes, STARx (ex situ smoldering) has been proven effective.7 API separator sludge was treated using heaters and circulated air to create smoldering conditions in the pile of sludge, oxidizing hydrocarbons and treating soil.
When designing a multilevel groundwater monitoring system, it is important to choose the best technology that fits specific site parameters. One presentation described the pros and cons of four systems.8 In a nested well, 2-3 screens can be used in multiple field positions for purge sampling. The Waterloo FLUTe system can be used to install 6-15 screens in fixed positions in a single well, offering high-resolution vertical profiling; however, to be effective the water level within the FLUTe must be maintained (or weighted mud must be used), and an open space is required for installation to lay out all the wires and tubing. Westbay installs 20 screens per 100 feet that can be adjusted for grab sampling using a wireline (no purge sampling); a drawback is that it can be difficult to obtain good dissolved oxygen (DO) readings and to maintain valves which are susceptible to clogging.
A case study on the Portland Harbor Superfund site demonstrated that petroleum hydrocarbons discharged from groundwater into the ocean can be remediated using an oleophilic bio-barrier9 (OBB) for sheen control. First, the shoreline was regraded, then five layers of material were placed over top to form a barrier. The bottom of the 5 layers is gravel to level the site, followed by a sorbent geosynthetic layer to absorb free product, then a sand and GAC layer for the dissolved phase, on top of that a geotextile layer separates and anchors the layers, and finally the surface is armored using a riprap. Contaminant fluxes are absorbed then biodegraded using natural oxygenation processes through freshwater inflow and aeration. Despite the high initial capital cost, preliminary results are promising and appear effective at a significantly lower lifecycle cost.
- Yuncu and Borden from Solutions-IES, with funding from the Environmental Security Technology Certification Program (ESTCP), developed a spreadsheet that calculates the buffer demand of various carbon substrates and estimates the soil demand if soil buffer capacity has been measured.10 This spreadsheet is available at ESTCP’s website or send an email to email@example.com and we will forward it to you. ESTCP is DoD’s environmental technology demonstration and validation program.
- Total petroleum hydrocarbon (TPH) remediation strategies come with varying levels of risk and effectiveness. As TPH degrades to CO2 and H2O, intermediary metabolites include alcohols, aldehydes, ketones, and acids. New methods are evolving and at the same time, the toxicity risk of metabolites is being researched. Chevron has funded several studies looking at the toxicity of polar organics in response to the San Francisco Regional Water Quality Control Board’s decision to regulate diesel range organics without silica gel cleanup11, which removes the polar organics that have previously been presumed to be less toxic than the parent compounds.
Zero-valent iron (ZVI) hasn’t been very effective in degrading 1,2-DCA (Ethylene dichloride-EDC), a chlorinated hydrocarbon used to produce of PVC pipes is highly toxic, flammable, possibly carcinogenic, and very difficult to treat. Adding sulfidated iron appears to improve ZVI’s effectiveness, extending its longevity and making it less reactive with water.12
In the presence of multiple electron acceptors, some contaminants are degraded sequentially resulting in longer cleanup times.13 For example, chromate must be degraded before nitrate, nitrate before chlorate, and chlorate before perchlorate. If introduced together, it can take up to 6 months before perchlorate begins to degrade (after the sequential degradation of the other electron acceptors.)
To increase treatment effectiveness for biodegrading chlorinated ethenes and 1,4-dioxane, one can introduce the isolated aerobic microbe CB1190 sequentially with KB-1.13 CB1190 survived in reducing conditions as TCE was anaerobically biodegraded, and then subsequently flourished to biodegrade chlorinated ethane and 1,4-dioxane as aerobic aquifer conditions returned.
- Electrokinetic (EK) transport mechanisms14 involve inducing a magnetic field to transport in situ remediation, bioremediation, and oxidation amendments into fine-grained strata. This strategy has been used successfully to migrate electron donor and dehalorespiring bacteria through low-K soils. Transport rates range from 2-5 cm/day in clay soils. Results are more variable for oxidation, though EK does promote persulfate migration. There is potential to use EK to address back-diffusion of residual source mass from fine-grained saturated materials.
- When treating contaminants with calcium polysulfide (CPS), sulfur precipitation is an issue. By buffering to a higher pH prior to injection this effect can be mitigated; however, a pH of about 11 must be maintained throughout the injection process.15
Identifying the source
Often the key to remediation is identifying the source of contamination. In a study conducted by Mariam Wahid from Orbicon, compound-specific isotope analysis for carbon-13, hydrogen-2, and chlorine 37 was used to identify the origin of VOCs in indoor spaces.16 Her team demonstrated that the isotopic signature can be used to determine if contamination in a building is caused by a biodegraded source in groundwater or an unbiodegraded source within the building. Vapor sourced from undegraded material has a lighter isotopic signature than TCE in soil vapor sourced by biodegraded groundwater. This is an excellent tool to determine whether a groundwater source is contributing to contamination in a building and subslab vapor mitigation is required or whether the source within the building and has to be mitigated by other means.
For the link to the conference site and more information on Stantec at the 2018 Battelle Conference on Remediation of Chlorinated and Recalcitrant Compounds go to ideas.stantec.com/battelle
- PANEL: Building a Remedy with the End in Mind: Advances in Adaptive Management for Efficient Cleanup of Complex Sites, Tamzen Macbeth (Moderator)
- Remediation of a 3-Mile Hexavalent Chromium Plume in Hinkley, California. Iain Baker et al (Pacific Gas and Electric)
- REMChlor-MD: A Screening Level Remediation Simulation Model that Considers Matrix Diffusion, Ron Falta, et al (Clemson University)
- Stratigraphic Flux: A Method for Determining Migration Pathways at Complex Sites - Joseph Quinnan, Patrick Curry, Nicklaus Welty (Arcadis)
- Link to Visual Bio
- NSZD State of the Practice, Tom Sale (Colorado State University); Integrating Theory and Practice to Better Understand and Apply NSZD at Field Sites, Sanjay Garg, et al (Shell Global Solutions, US) ; New Developments in Thermal Monitoring Methods for Continuous NSZD Measurement: Application at an LNAPL Site, Charles Newell, et al (GSI Environmental Inc.); Temperature Effects on Petroleum NSZD Processes: Lessons from Coupled Heat Transfer and Heat Generation Modeling, Julio Zimbron et al (E-Flux)
- STARx (Ex Situ Smoldering) for the Treatment of Contaminated Soils and Liquid Organic Wastes: Results from a Full-Scale Application - Grant Scholes (Savron)
- Selection, Design, and Construction of a Multilevel Groundwater Monitoring System. John Dougherty (CDM Smith)
- Shoreline Remediation of Petroleum Hydrocarbons Using an Oleophilic Biobarrer for Sheen Control on the Portland Harbor Superfund Site. Jeff Gentry et al (Jacobs)
- How Much Buffer do you Need to Adjust Aquifer pH? – B Yincu and RC Borden (Solutions-IES a Div of Draper Arden)
- Potential Human and Aquatic Toxicity of Petroleum Biodegradation Metabolites in Groundwater at Fuel Release Sites. Renae Magaw et al (Chevron/USA)
- Controlled Sulfidation to Optimize the Remediation Performance of Zero-Valent Iron and Related Materials. Ying Lan et al (Oregon Health & Science University)
- Optimizing a Mixed Microbial Community to Biodegrade Chlorinated Ethenes and 1,4-Dioxane. Alexandra Polasko et al. (UCLA)
- From Laboratory to Full-Scale Implementation: Electrokinetically-Enhanced Delivery of Amendments for In Situ Remediation. David Gent et al (US Army Corps of Engineers)
- Injection of pH-Adjusted Calcium Polysulfide to Treat Groundwater Plume Commingled with Cr(VI) and TCE. Jim Leu et al (Parsons)
- Compound Specific Isotope Analysis used to Identify the Origin of VOCs in the Indoor Environment: Internal Sources versus Subsurface Contamination – Trine Skov Jepson et al. (Orbicon/Denmark)