Manufactured gas plants (MGPs) emerged in the early 1800s, converting coal, coke (a coal product) and oil into a combustible gas used for lighting, heating and cooking. Additionally, these plants produced a range of byproducts, including tars, oils, ammonias and lampblack, to name a few. Byproducts that weren’t sold were either disposed of or remained on-site as the properties were redeveloped.

In the early 1920s, natural gas, a cleaner alternative, began taking over, bringing the 150-year era of manufactured gas to a close in the 1960s. It wasn’t until the 1970s that federal regulations encouraged the environmental industry to take a closer look at decommissioned MGP sites. Through those investigations, they found that residual waste — with chemical components hazardous to human health — still existed on many of these sites.

In the late 1980s, states began to develop their own programs to clean things up. With the advent of these voluntary programs came the development of risk-based approaches, which allowed property owners to control costs and meet goals for site remediation/restoration without requiring the removal of every impact. These approaches offer more flexibility to owners by providing cleanup levels for varying exposure pathways, risk management plans, engineered barriers and institutional controls for property usage.

Combined with evolving developments in regulatory frameworks, advances in technology are offering owners more alternatives to achieve closure on these sites. These new technologies are providing an easier, more efficient way of sharing what’s at the site with an owner, as well as where it is and how it's interacting with the subsurface. By better understanding the whole picture, owners can confidently make the right decision on which remedy, and its associated costs, would be right for them.

Today’s advanced investigative field techniques and better practices for data analysis and evaluating contaminated media are improving conceptual site models (CSMs). Some of these techniques include high-resolution site characterization tools, 3D visualization tools and molecular biological tools. In addition to these tools, technological advances have enabled laboratories to drastically increase precision and accuracy and deliver expedited results.

Here’s a breakdown of some of the advancements making a difference in the remediation field.

High-resolution site characterization (HRSC) tools can be used by a wide range of less invasive equipment platforms and can generate a significant amount of subsurface data over large areas — without having to physically dig up the soil and dump it in a landfill.

Historically, hollow stem auger drilling was a common method, and the larger pieces of equipment with these capabilities commonly had limited mobility and presented access issues. Today, multiple types of continuous or discrete soil sampling methods are available, and the vehicles used to advance this tooling have become more mobile and more powerful. HRSC can provide both qualitative and quantitative information on contaminant distribution and physical properties (soil types, permeability, etc.) of aquifer materials. Geophysical tools can even be deployed down existing monitoring wells or at the surface without any intrusive activity. Results can be exported as electronic or digital data for use in models or simulations, offering a more powerful way to visually present data to stakeholders.

By developing investigation programs that combine the use of HRSC tools with more conventional soil sampling and laboratory analysis/borehole logging/slug or pumping test activities, a significant amount of data can be collected in a short period of time that provides a higher level of confidence in subsurface conditions and the preferential pathways for contaminant transport. The ability to increase data density within the investigation areas minimizes the uncertainty of remedy scope and increases potential remedy effectiveness.

Enhanced visualization tools allow site data to be presented in a more visually appealing way to project stakeholders who may possess varying degrees of experience with environmental investigation and remediation reports. Aerial imagery, contaminant distribution data, groundwater elevation data, excavation extents, surface topography, subsurface stratigraphy, and former and existing structures both above and below ground are some of the data streams that can be viewed from multiple directions or manipulated by the viewer.   

Environmental Sequence Stratigraphy (ESS) is another method that is becoming a best practice in the industry for developing more confidence in the subsurface. With deep knowledge of stratigraphic principles for corresponding depositional environments, our sequence stratigraphers review existing site data and re-format existing logs to reveal vertical and lateral trends in grain size. We then analyze the trends in the context of the depositional history of a site and assess subsurface heterogeneity. Contaminant pathways are better understood and critical uncertainties with respect to the subsurface are clearly identified. Additional data collection, if necessary, is much more focused and efficient. The ESS workflow and practitioners have a proven track record of adding tremendous value to projects through remedy and monitoring optimization, reduction in uncertainty and risk, and regulatory communication and negotiation.”

Molecular biological tools can quantify microbial populations that might be present and their ability to degrade certain compounds, provide information on contaminant sources, and demonstrate that active degradation is occurring prior to or as a result of a selected remedy.

Population quantification analyses provide a defensible line of evidence that contaminant destruction via microbial degradation is possible by confirming the populations that are present at the site and the ability that their functional genes present to degrade certain contaminants under different degradation pathways. This type of data is valuable when conducting a monitored natural attenuation (MNA) evaluation and selecting potential remedies. 

Compound-specific isotope analyses (CSIA) at MGP sites are most applicable to benzene, toluene, ethylbenzene and xylenes (BTEX) and naphthalene. This analysis can provide information on different potential sources of material at the site. It also can demonstrate whether active degradation is occurring by comparing isotopic carbon signatures and their spatial and temporal trends in conjunction with contaminant concentrations.

The stable isotope probing is a tool offered by various laboratories. It combines principles of CSIA and population quantification analyses to provide evidence that microbes are actively degrading a contaminant by measuring stable isotope concentrations taken up by microbial populations from enriched substrates.

Aside from techniques and technologies that are enhancing CSMs, there also have been advances in remedies over the years to treat impacts at sites.

In situ remedies present the opportunity to address contaminants located within discrete intervals in the subsurface without having to disturb overlying deposits.

In situ chemical oxidation (ISCO) and surfactant usage are some approaches that have been successful in certain scenarios. ISCO is a process capable of destroying a wide range of organic contaminants, which is why it has been implemented for several years at complex sites. Surfactant usage has been employed to decrease nonaqueous phase liquids (NAPL) surface tension and viscosity to enhance recovery, encourage NAPL desorption from aquifer matrices and, in some cases, enhance an oxidant’s effectiveness for contaminant destruction.  Estimating an accurate oxidant demand and achieving adequate amendment distribution/contaminant contact are common hurdles for effective remedies. Delivery strategies might include direct-push injection, slurry emplacement or injection/recovery wells.

In situ stabilization (ISS) is a soil-mixing method that binds up contaminants and minimizes their leachability to groundwater, affectively creating a subsurface monolith. This approach might be effective when significant vertical intervals are impacted, and other individual exposure pathways can be eliminated with other remedies or pathway exclusions. Soil mixing can be accomplished by conventional earthwork equipment at shallower depths, large diameter auger mixing for deeper applications where access is not an issue, or jet grouting in less accessible areas. ISS can be used in conjunction with excavation remedies as a potential alternative to conventional excavation support systems.

In situ thermal remediation (ISTR) of differing energy input and cost is available for achieving different levels of treatment, and selection of thermal technology requires a cost-benefit analysis. The most commonly used methods include electrical resistance heating (ERH) and thermal conduction heating (TCH). In MGP remediation scenarios, ERH involves uniformly heating the subsurface to the boil point of water, typically for targeting volatile organic compounds, while TCH could increase subsurface temperatures to several hundred degrees Celsius, typically for targeting semi-volatile organic compounds.  

So what happens next?   

Much progress has been made in the past few decades, but there’s still more to come — better, more efficient techniques and evolving technology. For now, the progression of regulatory frameworks and the technological advances for CSM refinement and potential remedies are making the cleanup of complex MGP sites safer, less disturbing to surrounding communities, more efficient and more cost-effective.


Get a deeper look into the overall remediation process from an owner’s perspective.

Download the Presentation

Ben Clement has extensive experience in site characterization activities (soil/groundwater/vapor intrusion) with an emphasis in hydrogeologic interpretations. While familiar with a wide range of conventional drilling and sampling approaches, he also has experience collecting and analyzing data using high-resolution site characterization technologies for enhancing conceptual site models. Additionally, Ben specializes in planning and implementing remediation strategies, including soil excavation, hydraulic containment, LNAPL recovery, AS/SVE, and in-situ remedies utilizing both chemical oxidation and enhanced aerobic bioremediation.