By Todd Herrington
At a petroleum bulk storage facility that was in operation from 1917 to 2002, dissolved phase contamination was found to have leaked from aboveground and underground storage tanks, and contaminant had migrated off-site. Consequently, the site had been sitting idle for years.
In 2016, the property owner received a request for compliance from the regulatory authorities. They subsequently retained Patriot Engineering to design and implement a remedial plan to address the environmental impacts at the property.
Seeing gaps in the available site data, Patriot installed additional wells, replaced old wells that had been destroyed, and delineated the entire extent of the non-aqueous phase liquid (NAPL) contaminant plume.
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The challenges on this particular site were that the impacted area was quite large and had fairly high levels of dissolved phase petroleum. Previous remediation at a nearby site using a pump and treat system had actually expanded the plume even further.
Considering in-situ chemical oxidation (ISCO) combined with enhanced aerobic degradation as a potential remedial option, Stephen Sittler, a senior project geologist with Patriot Engineering, contacted REGENESIS, a provider of environmental remediation solutions, for assistance in developing a plan. The idea discussed was to perform an on-site beta test of REGENESIS’s PetroFix carbon-based remediation fluid, in lieu of more conventional approaches. The purpose of the test would be to evaluate PetroFix for possible full-scale use.
On why PetroFix was chosen for this site, Paul Erickson, a senior research scientist with REGENESIS explained: “Contaminant levels were pretty high and we knew this would be a good test of the product balance. We wanted to see how far we could push PetroFix. Also, the transport was a little bit different from other beta sites we had done, just to give us a little variety in how we were testing the product before release.”
PetroFix is a highly concentrated water-based suspension consisting of micron-scale activated carbon and biostimulating electron acceptors designed to remediate petroleum hydrocarbons (TPH-d, TPH-g, MTBE, BTEX) and provide immediate results for gas station and underground storage tank sites. The environmentally-compatible formulation of micron-scale activated carbon is combined with both slow and quick-release inorganic electron acceptors (a preferred sulfate and nitrate combination blend, or the option of a sulfate only blend) and injected together.
It works by first removing hydrocarbons from the dissolved phase by adsorbing them onto activated carbon particles and then stimulating hydrocarbon biodegradation through the addition of electron acceptors.
For years, activated carbon has been widely used in the remediation of soil and groundwater. Structurally, activated carbon is a highly porous material, made up almost entirely of elemental carbon. Due to its high surface area and unique pore structure, small organic molecules (such as environmental contaminants) are readily adsorbed from target solutions (i.e., groundwater).
While activated carbon can be found in a wide range of physical forms, corresponding to various uses, contaminants, and mediums, there is typically an optimum form and particle size for each application. For in-situ groundwater remediation applications, small-particle activated carbon is preferred due to its ability to move effectively within an aquifer.
Because of this preference, particles in PetroFix are 1 – 2 µm in size, allowing for the free flow of an aqueous suspension of activated carbon through an aquifer during an injection. As PetroFix moves through the pore space, it coats the surface of the soil particles versus occluding the pore space, allowing groundwater to flow unimpeded. The small diameter of PetroFix also increases the immediately available surface area of the carbon in it, which then results in faster than normal sorption of contaminants.
With this fast sorption capability, coupled to the ability to distribute and fully coat highly conductive transport zones, the material is an excellent way to lock contaminants in place and immobilize a plume for treatment. PetroFix also acts as a sorption barrier for those situations where persistent diffusion of contamination from tight soils into more conductive transport zones is a chronic concern.
Once contaminants are immobilized, the second step in the PetroFix process is biological destruction. The activated carbon is responsible for the initial removal of contaminants from groundwater, which can lead to rapid results. However, the contaminants, although immobilized, remain intact until they are degraded by naturally occurring bacteria in the aquifer. This is a principle that has already been successfully employed in the wastewater treatment field, with processes such as bio-GAC.
When its applied as recommended, PetroFix contains a blend of nitrate and sulfate salts that act as electron acceptors to enhance the degradation of sorbed petroleum hydrocarbons. These salts are water soluble and thus will flow with the product. Nitrate is used more rapidly, typically weeks, while the sulfate is utilized over months.
While many bioremediation approaches degrade petroleum hydrocarbons via aerobic pathways, research has shown that they can also be efficiently degraded anaerobically via syntropic processes, or a community of bacteria fulfilling different roles. These degradation processes, while enhanced by additional electron acceptors, will continue even after those electron acceptors have been depleted.
For in-situ remedial applications, it is important to achieve sufficient coverage of the impacted subsurface, as the primary factor determining the success of a project is contact between the contaminant and the remediation chemical.
For PetroFix, the goal is to not just have sufficient dosage to remediate the hydrocarbon mass, but for the total solution volume applied to theoretically fill more than half of the effective porosity present. Both those conditions must be met in order to achieve success.
PetroFix comes paired with a virtual design assistant that helps site managers determine how much product is necessary for treatment and create a design for the application. After inputting site data, the design assistant will determine the number of injection points and total volume of PetroFix required.
Shipped as a viscous liquid of approximately 1,500 – 3,500 centipoise, PetroFix has a viscosity similar to corn syrup. It is then typically diluted greater than 10:1 with water in the field so that injection into the subsurface can be performed at low pressures (typically < 30 psi). This allows PetroFix to flow outwards from the injection point through naturally occurring permeable strata, following the same transport pathway as the target hydrocarbons.
As with any remediation project, it was necessary to acquire approval from the overseeing regulatory body. Fortunately, according to Sittler, the approval process for this project was smooth. He said that “they really wanted something done on this site. The fact that anything being proposed was a plus. But it helped that REGENESIS has a good reputation. There really wasn’t any pushback at all.”
After the beta test design was finalized and approved, the injections were performed using a REGENESIS injection trailer hooked up to Patriot drill rigs. “Early post-injection groundwater monitoring results from the PetroFix pilot test look very promising,” said Sittler. “Petroleum volatile organic compounds and gasoline-range organics were essentially eliminated from the groundwater at one month post-injection and remained so at six months.”
Nitrate and sulfate levels spiked at one month post injection due to their initial addition as electron acceptors. These levels saw a sharp reduction during the three-month sampling event, indicating utilization by microbes and biodegradation.
“This conclusion was strongly supported by microbial analyses,” said Sittler. “The microbial analyses indicated a shift towards anaerobic petroleum degraders caused by the more favorable environment induced by the nitrate and sulfate injected with PetroFix.”
Based on the success of the beta test, Patriot decided to move forward with the design and implementation of full-scale remediation using PetroFix, which had proven itself to be a viable remedial option. “Concentrations were in the 30,000 – 40,000 ppb range,” said Sittler. “Now they’re gone. That was the big question.”
Sittler incorporated lessons from the beta test into the full-scale design. “In the beta test, we easily got distribution between the points, which were 1.52 m apart. We took samples between the points and saw PetroFix. For the full-scale we increased spacing to 2.1 m., and we may add more water to spread it out more without increasing costs. We should be able to scale up to 2.1 m and save a lot of money and still get the same amount of product into that area.”
The full-scale design has focused on treating several hotspots on the site. Because the area was very large, mechanical remediation was not practical and it would have been prohibitively expensive to perform a grid injection across the entire site.
“A product like PetroFix acts like a filter,” Sittler explained. “We can put it in some smaller areas and allow the contaminants to flow through them and treat the entire site. A series of grids in the hottest areas act as a big barrier. We can clean up the site and prevent off-site migration.”
The full-scale design has since been approved and implemented on the site. Monitoring is ongoing.
Todd Herrington is with REGENESIS. This article appears in ES&E Magazine’s June 2019 isssue.