By Leslie H. Pennington

Summary: A new density-driven convection technology is projected to save a manufacturer hundreds of thousands of dollars on toxic site clean-up in Denver, Colo. The technology is cited as faster, more efficient and more thorough in handling TCE (tetrachloroethene) and MTBE.

After analyzing “pump-and-treat” and high pressure sparging remediation processes, an environmental consulting firm in Colorado discovered that both systems were too costly, time-consuming, and inefficient to clean up a computer component manufacturing site near Denver, Colo.—especially one contaminated with TCE.

Cordilleran Environmental Consultants, based in Golden, Colo., has handled environmental remediation projects for commercial, industrial and railroad clients. Their work includes handling environmental impact assessment, remedial feasibility studies and various types of remediation projects, as well as providing expert courtroom testimony for environmental impact issues.

TCE spill at electronics plant
The toxic chlorinated solvent TCE spilled by the microprocessor manufacturer near Denver posed complex problems, resulting in pollution of groundwater at the site. The remedial solution had to clean up the site quickly and efficiently to state government standards. The client wanted a solution that was economical, but needed to work as quickly as possible before government intervention.

“We had to consider many factors, both physical and financial, when selecting the best system to decontaminate this specific site,” said Brad Stephenson, principal consultant for Cordilleran Environmental. “The soil sediments were relatively dense, the pollutant was a chlorinated solvent and we had a budget to meet.”

After initial analysis of the soil surface, it was clear further studies were needed underground. Sixty monitoring wells were drilled to determine the extent of contaminant in the aquifer, and to investigate the most effective method for remediation. With data from the monitoring wells, the remediation firm delineated a TCE plume covering an area approximately 720,000 square feet (sq. ft.), about 30 feet underground in tightly packed soil. The highest concentration of volatile organic compounds (VOCs)—70 percent of the total mass—was located in an 1,800-sq.-ft. area.

Evaluating options
Old-style pump-and-treat technologies are the most time consuming and costly of the variety of remediation methods available to meet cleanup standards. With these systems, millions of gallons of groundwater must be pumped to the surface for cleaning, filtration, treatment and storage prior to discharge to a city wastewater treatment system (or reinjection to control plume movement). Pump and treat also requires a large electrical budget, costly holding tanks and water treatment equipment and piping, as well as constant maintenance of the equipment and resulting operational expenses. Plus, substantial costs and delays are necessary when handling the permit process and compliance needed to discharge water back to a treatment plant. Thus, Cordilleran eliminated this as a viable option.

Several different types of forced air sparging systems were evaluated, but none appeared to be cost effective or efficient. High-pressure, forced air sparging involves pumping air at 100 pounds per square inch (psi) pressure through pipes or wells below the water table. The three-dimensional coverage area is generally very small, requiring a large number of wells to be drilled, especially with channeling problems that may occur in fine, dense, silty and clay soil.

“From my past experience with many different systems, I know that both pump and treat and traditional high-pressure air sparging methods would not work efficiently or be cost effective for this site, especially since we have a chlorinated contaminant and a low-yield aquifer,” stated Stephenson. Instead, its evaluation indicated that a remediation system involving a “density-driven convection” process would be the best remedial method for this site.

Groundwater recirculating well
Incorporating the principal of continuous aeration, this technology is used for remediation of VOCs including sites contaminated with the controversial fuel oxygenate MTBE and “traditional” toxic gasoline constituents such as benzene and toluene (BTEX). It’s been used to remove volatile, semi-volatile and biodegradable pollutants, with more than 900 such systems installed to date.

A remediation technique using groundwater recirculation wells (GRWs) creates a convection cell beneath the water table. Air is injected inside the well, pulling contaminated water into the bottom of the well, where it’s air-stripped in situ. The contaminant gas then travels up the well casing into the open air if the level of VOCs is below required government standards or to a vapor treatment system if required. In the Denver site case, the wells are connected by underground piping to one common extraction pipe that vents VOCs to the open air.

“With this density-driven system, the well design allows for VOC treatment of a greater section of the formation,” Stephenson explains. “A large convection cell surrounds the wellbore, which creates a negative groundwater gradient at the bottom of the well and a positive gradient at the top of the well, separated by a grout seal in the middle.”

At one site, this technology reduced groundwater contaminants significantly after six months. Average total BTEX was cut from 7.15 milligrams per liter (mg/L) to 1.93 mg/L and MTBE from 0.70 mg/L to 0.20 mg/L. After 30 months, average total BTEX was 0.78 mg/L, an 89 percent reduction, and average total MTBE was 0.002 mg/L, almost a 100 percent reduction.

Cordilleran decided to test the system on a pilot well for nearly a full year to determine if it was suitable for the TCE site. In October 1999, after a 10-month test, the consultant was convinced that density-driven convection technology was the best process to remove the VOCs from the tainted groundwater plume. They immediately began installing 37 additional related wells.

“This is the first time such a system was used in Colorado, so the state government was not sure it would be effective,” said Stephenson. “But after studying our analysis and statistics from the pilot test well, they were comfortable it would work efficiently.”

System criteria
Because of their simplicity (see Figure 1), density-driven convection systems are easy to install and operate, and require very little regular maintenance due to few moving parts and off-the-shelf equipment. Site personnel can be trained to maintain the system in less than one day.

The systems are not off the shelf—they must be custom designed and engineered to meet specific requirements of a particular site. However, one can be designed as a complete grid of wells to aggressively treat an entire plume area, or as a line of wells placed across a plume to act as a barrier to contaminant migration. Since each well decontaminates a large section of the plume area, fewer are required to clean the site.

While this technology has been implemented at sites requiring over 60 wells, it has also been implemented at sites requiring only one to six. In fact, it’s well-suited to small plume remediation because of its simplicity, requiring only a small air compressor for operation. Wells for small, shallow groundwater sites can be installed for less than $2,000 each. Also, several small treatment sites have been remediated in less than a year.

“The system is a very efficient way to pull VOCs out of groundwater, since the dirty water keeps moving around the convection cell over and over again,” Stephenson says. “All the while it is circulating, it is further vaporizing contaminants out of the water.”

Fine soil decontamination
Forced air sparging and pump-and-treat methods aren’t effective in fine soil materials, due to the limitations of both systems, according to Stephenson. Density-driven convection, however, with its large recirculation cell, is able to decontaminate soil and groundwater within its radius of influence.

“No matter what the substrate is, whether it’s composed of fine- or coarse-grained materials, it works efficiently,” Stephenson says. “This site is mostly silt and fine sand, which is very difficult to process, but we’ve already pulled a large quantity of VOCs out of the aquifer.”

Because the groundwater is treated inside the well, channeling does not occur. In certain soils, conventional forced air sparging isn’t effective because bedding planes or fissures channel air thereby reducing air/water mixing. This results in negligible air/water interaction thus minimizing effectiveness of the remediation.

“We have a very tight formation of soil materials at this site, that’s very fine-grained, so ordinary sparging wouldn’t have worked here very well,” Stephenson says. “We get a lot of air/water mixing with the density-driven convection system, so it decontaminates the formations more readily.”

Unlike most petroleum hydrocarbons, where bioremediation plays a crucial role, chlorinated solvents are difficult to bioremediate.

“With the heavier-than-water chlorinated solvents, we need a high volume of vaporization and recirculation of the air/water mixture to decontaminate the groundwater,” Stephenson emphasizes. “So far, the system has proven to be very successful. In the initial test area, we calculated that we had about one million ppb (parts per billion) of VOCs when we started; we’ve taken it down to about 100 ppb in only six months.”

According to Cordilleran’s estimations, about 110 gallons of TCE leached into the groundwater—the equivalent of two full 55-gallon drums. They expect to pull about 400 pounds of VOCs per year out of the site. Thus, the entire groundwater remediation project is estimated to take 3-to-5 years to complete.

“When the project is finally completed, we’ll have saved our client hundreds of thousands of dollars with this system,” Stephenson commented. “We’re going to finish cleaning up this site much more quickly with density-driven convection and the site will be cleaned more thoroughly.”

About the author
Leslie H. Pennington is founder and principal engineer of Wasatch Environmental Inc. A practicing geotechnical engineer for 23 years, he’s been involved in more than 600 environmental assessments and 200 remedial actions, has patented technologies for groundwater re-circulation wells and soil sampling equipment, and has implemented innovative techniques utilizing in-well aeration, groundwater sparging, bioventing and horizontal vapor extraction. Pennington can be reached at (801) 972-8400, (801) 972-8459 (fax) or email: [email protected]

Figure 1. Density-driven convection systems need little regular maintenance since they have fewer moving parts and off-the-shelf equipment. Systems, however, must be custom designed and engineered for specific site requirements.
Sources for Small Systems

    National Drinking Water Clearinghouse: Includes a Registry of Equipment Suppliers of Treatment Technologies for Small Systems (RESULTS) and newsletters “On Tap” and “Water Sense.”
    National Small Flows Clearinghouse: Sister to NDWC (both based at University of West Virginia), this focuses on wastewater instead of drinking water and includes “Pipeline” and “Small Flows” newsletters, recently merged into Small Flows Quarterly magazine.
    National Rural Water Association: Small systems for water and wastewater applications of rural utility services. It includes a “Query Professor Faucet” section and articles from Rural Water Magazine, with authors like USEPA Office of Groundwater and Drinking Water director Cynthia Dougherty, and a variety of links for small systems.
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    National Rural Community Assistance Program: A federally supported network of non-profit organizations working to improve water and waste disposal services in rural communities with funding and technical assistance for lower-income areas.
    NSF/EPA Environmental Technology Verification (ETV) Program: From “package drinking water treatment” to “source water protection,” this has drawn major players in point-of-use/point-of-entry market to streamline state approval of products for small system applications.

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