Addressing Complex Remediation Sites

With the onset of improved treatment technologies and a better understanding of remediation processes, sites once considered too challenging or complex are becoming viable candidates for remediation. These sites can be successfully remediated through innovative remedial design where multiple in situ remediation technologies are integrated to optimize treatment, increase overall project efficacy, and minimize project costs.

These combined approaches may take the form of combining multiple in situ thermal remediation (ISTR) technologies or combining ISTR with other in situ remediation technologies such as in situ bioremediation (ISB), in situ chemical reduction (ISCR) or in situ chemical oxidation (ISCO) to take advantage of thermally enhanced reactions and remediation mechanisms.

Want to learn more? Register for our webinar, Can Thermal Remediation Technologies be Combined at a Site?

What is a Complex Remediation Site?

The environmental industry has reached a stage of maturity where a great fraction of the sites that have yet to achieve regulatory closure are considered “complex”. Various remedies may have been implemented at these sites, without reaching the desired remedial goals for many reasons. As an industry, we have learned that there is no “silver bullet” remedy to address complex sites; often, a combination of remedies is required to achieve remediation goals.

Some factors that may contribute to making a site complex include:

• Multiple contaminated zones,
• Hard-to-treat geologies such as bedrock or thick clays,
• Hydrogeological conditions limiting or preventing the use of certain remediation strategies,
• Source area too deep for excavation or soil mixing,
• Hard-to-treat chemicals and mixed waste, and,
• Access limitations.

Can you Combine Multiple In Situ Thermal Remediation Technologies?

Oftentimes one ISTR technology stands out as the best fit. However, on many sites, choosing between technologies can be a close call, and in some cases, site conditions or project goals require multiple ISTR technologies to get the job done. Keeping the selected ISTR heating approach open allows the thermal design team to mix and match multiple ISTR technologies. These designs offer effective solutions for more complex sites.

In today’s marketplace, the three main ISTR technologies are Thermal Conduction Heating (TCH), Electrical Resistance Heating (ERH) and Steam Enhanced Extraction (SEE). Combined thermal remedies may include:

• TCH – SEE
• ERH – SEE
• ERH – TCH
• ERH – TCH – SEE

Both TCH and ERH can be combined with SEE to effectively treat sites that exhibit a low porosity, low hydraulic conductivity interval (TCH or ERH) and a highly transmissive aquifer featuring groundwater flow rates from 1 ft/day to tens of ft/day. On sites where portions of the treatment zone are best accessed by angled or horizontal installations, TCH heaters, ERH electrodes, and vapor and liquid extraction wells can be installed at an angle or horizontally.

Addressing site and project complexities with the optimal combination of thermal remediation technologies and installation techniques can effectively overcome challenging site conditions. Combined thermal remedies will allow you to efficiently achieve your project goals in a shorter time frame and with lower overall cost.

Can you Combine Thermal with other In Situ Remediation Technologies?

In many cases, the best solution for a site is to combine technologies spatially and/or temporally. Choosing the best technology at the right time and place for each zone (e.g., source zone vs. dissolved phase plume) or stage of the project represents the most cost-effective and efficient approach.  

Can you Combine In Situ Remediation and In Situ Bioremediation?

During traditional applications of ISTR, temperatures of 100°C or higher are reached throughout a treatment volume. While these high temperatures would seemingly limit biodegradation and inhibit microbial populations, collective experience and research surrounding this topic over the past few decades suggest that not only do ISTR projects not “sterilize” microbial populations from a site, many contaminant degrading populations recolonize and flourish at ISTR sites as temperatures begin to cool below 40°C.

More moderate heating (i.e., 30 to 40°C) can effectively accelerate dissolution/desorption rates and enhance naturally-occurring biotic degradation at a site.

For petroleum hydrocarbons, BTEX biodegradation has been shown to triple from 10 to 20°C, and petroleum hydrocarbon biodegradation rates can peak between 30 and 40°C.

For chlorinated solvents, many studies indicate that optimal temperatures for maximum bacteria population growth and biodegradation rates peak at around 30 to 35°C. Up to approximately 35°C, dechlorination rates are expected to double with every 10°C increase in subsurface temperature due to increases in:

• Microbial metabolic rates
• Electron availability (release from organic material)
• Population growth
• Bioavailability of contaminants

These lower temperature applications can beneficially reuse the residual heat from a traditional ISTR project or be specifically designed to achieve lower temperatures and enhance biodegradation rates. These combined ISTR-ISB projects take on three forms:

• Bio-Polishing – Residual heat from completed, ISTR areas is used to “polish” off source area contamination through enhanced biodegradation. Often, carbon from the soil is liberated during thermal treatment and can stimulate indigenous microbes; however, existing ISTR wellfield infrastructure can also be used to introduce amendments or oxygen to further stimulate microbial populations and make use of residual heat.
• Low-Temp Heat Enhanced Bio Application – A low temperature ISTR system is installed specifically with the operational strategy of achieving 30° to 40°C temperatures (depending on target contaminants).
  • This system looks a lot different than a traditional ISTR system because it does not generally require extraction and above ground treatment, does not generate steam, and requires much less energy and power delivery equipment.  
  • The temperature range accelerates biodegradation reaction rates, but also can promote degradation through abiotic processes like hydrolysis or iron-sulfide mediated reactions.
  • Gentle heating supports a higher transfer of mass into the aqueous phase where contamination is bioavailable, a common rate limiting step at injection sites.
  • Amendment can be delivered in the same borehole as the heater is installed, if biostimulation is required, which is the case for most sites with chlorinated ethenes. For petroleum sites, an electron acceptor can be delivered in the form of oxygen, an oxidant, or sulfate source such as gypsum.  
• Coupled ISTR Source and Heat Enhanced Biodegradation Applied to Downgradient Plume – A traditional ISTR system is designed to achieve 100°C in the source area. Warm, treated water then migrates beyond the source zone to support biodegradation in the downgradient plume or transition zone between the source area and diffuse plume. This warm water can migrate under natural flow conditions or can be directed through recirculation. Low-temperature heaters can also be deployed and operated in tandem with the traditional ISTR system.

Can you Combine In Situ Thermal Remediation and In Situ Chemical Oxidation?

Exposing a solution of sodium persulfate to elevated temperatures (e.g., 40°C) activates the persulfate anion which increase the effectiveness and rate of reaction. Unlike other activators like iron, pH and peroxide, heat is less susceptible to attenuating in situ. Thermal activation is thought to proceed where heat decomposes persulfate into two sulfate radicals responsible for the in situ destruction of HVOC contaminants in groundwater and sorbed to the soil matrix.

When Should I consider Alternatives to Full Temperature Thermal?

During operation of a traditional ISTR system, contaminant removal rates increase until a peak rate is achieved. Following this peak, mass removal rates often decrease along a first-order (exponential) decay curve until reaching an asymptotic lower limit.

Historical data from sites completed by TerraTherm show that the average energy requirement per unit of contaminant mass removed is approximately 7- 10 times higher during post-asymptote operations, compared to during pre-asymptote operations.

Continuing ISTR system operations once the asymptotic recovery rate has been reached is neither economically nor environmentally responsible. Turning off the power to an ISTR project once these conditions are met and implementing an alternative polishing strategy in the treatment area will take advantage of less energy and resource intensive thermally enhanced degradation processes. Knowing when to transition between technologies and phases of treatment can lead to significant overall project cost savings.

Complex sites require smarter and more up-to-date remediation strategies, often in the form of combined remedy approaches. With a greater understanding of the types of ISTR and injection-based technologies that can be combined to deliver successful remediation, you’ll be better prepared for your next project.

If you’re considering ISTR technology or simply want to learn more, register for our webinar Can Thermal Remediation Technologies be Combined at a Site? orcontact our team of thermal remediation experts today.