Solving the MTBE mistake
As the furor over methyl tertiary-butyl ether (MTBE) contamination continues to grow, the deepest concern for many U.S. citizens and legislative groups is the widespread presence of the controversial fuel additive in the nation's groundwater and its potential effect on public health. Homeowners in MTBE plume areas are worried about property values and environmentalists are troubled by the potential damage to wildlife. Cleaning up this mess, without costing an arm and a leg, is the highest priority of many state, federal and private interests.
Used as an oxygenate enhancer in gasoline since the 1970s to promote more complete burning (thereby reducing carbon monoxide and ozone emissions), MTBE has leaked from fuel storage tanks and migrated into soil and water throughout the country. Regardless of whether or not MTBE is proven to be a carcinogen or other serious health hazard, the consequences of its use are widespread and will no doubt be very costly.
A recent CBS broadcast, 60 Minutes reported that 49 states have detected MTBE in groundwater. The environmental sanctuary of Lake Tahoe has been tainted, with 12 of 34 wells in the city of South Lake Tahoe shut down due to MTBE contamination. In Maine, a statewide survey found that as many as 5,200 domestic wells contain MTBE levels above the state's drinking water standard of 35 micrograms per liter (mg/l). Three percent of shallow urban wells tested in a U.S. Geological Survey exceeded the U.S. Environmental Protection Agency's (EPA) draft health advisory level (20 mg/l).
State regulatory agencies are increasingly mandating that MTBE contamination be cleaned up. The big questions are how soon and at what cost?
A promising remediation method not only of MTBE contamination, but also "traditional" toxic gasoline constituents such as benzene and toluene is a recently developed in situ technology known as density-driven convection (DDC).
In August 1994, the Wilmington office of the North Carolina Division of Waste Management (NCDWM) Underground Storage Tank (UST) Section received a site closure report indicating that soil and groundwater samples collected from UST excavations at the site of a service station exceeded state groundwater quality standards. Due to the presence of MTBE and other contaminants, Coastal Environmental Services Company (CESCO) of Wilmington was contracted to perform a comprehensive site assessment. Two plumes were defined: a 6,000 square foot area from a pipeline leak and a 2,000 square foot area from spills and overfills. Laboratory tests indicated 11,086 parts per million (ppm) total petroleum hydrocarbons in soil at the release point and average benzene, toluene, ethyl benzene and xylene (BTEX) and MTBE concentrations of 7.15 milligrams per liter (mg/l) and 0.70 mg/l in groundwater, respectively. Groundwater was encountered at five feet and the soil consisted of medium-dense silty sand wit
h an average hydraulic conductivity of 0.17 feet per day.
A potable water well used for drinking water was located on an adjacent property. Although no dissolved petroleum hydrocarbons were detected in the potable water well, dissolved contaminant plumes were present at the service station property and the plumes were migrating. Plume migration was relatively slow, but the amount of MTBE and other contaminants warranted that the site be cleaned up.
CESCO performed pilot studies that indicated conventional forced air sparging and soil vapor extraction were workable remediation choices for the site and a corrective action plan recommending these technologies was submitted to NCDWM. The corrective action plan specified the installation of 14 air sparging/soil vapor extraction wells. However, due to the low permeability of the site soils, CESCO had reservations as to the ultimate effectiveness of the air sparging/soil vapor extraction plan and continued to evaluate alternative technologies.
One alternative plan that was explored included DDC, which employs an air-lift recirculation well technology. The remediation system design included ten standard DDC wells and two shallow soil vapor extraction components in the source areas.
The DDC system was installed and operation began in May 1997 for a total construction and oversight cost of $50,763. After six months of operation, there was a significant drop in the petroleum contaminant levels in groundwater; average total BTEX was reduced from 7.15 mg/l to 1.93 mg/l and average MTBE concentration was reduced from 0.70 mg/l to 0.20 mg/l. After 30 months of operation, average total BTEX was 0.78 mg/l, an 89 percent reduction and average MTBE was 0.002 mg/l, an almost 100 percent reduction.
DDC is a very promising technology for cost-effective remediation of many untreated sites throughout the country. As this project illustrates, the patented DDC technology has shown it can provide considerable economy and effectiveness in treating BTEX and MTBE contamination. Approximately 900 DDC wells have been installed at 62 sites in 20 different states. DDC wells have been installed to depths ranging between 10 and 205 feet, and all but a very few (real world) have been effective in treating volatile organic compound contaminants.
Generally known as groundwater recirculation wells (GRW), DDC wells are constructed with a lower screen near the bottom of the contaminated aquifer and an upper screen near the water table surface. Air injected into the DDC well rises upward within the casing. The rising air reduces the density of the water column within the DDC well, creating an upward vertical gradient that draws water into the well through the lower screen and pushes aerated groundwater out through the upper screen. This process creates a groundwater circulation cell within the aquifer around the DDC well. Groundwater remediation is achieved by air stripping within the DDC well and by circulating dissolved oxygen, which stimulates microbial growth, throughout the contaminated aquifer. Stripped compounds emitted from wells in the form of gases may be released to the atmosphere or collected at the wellhead for treatment.
DDC well systems are simple to install and require very little maintenance. DDC wells can be installed into a complete grid of wells for aggressive treatment of an entire plume area. A line of wells may also be installed across a plume to act as a barrier to plume migration by removing contaminants from the passing groundwater.
American Water Works Association technical resources www.awwa.org/asp/tech.asp
American Chemical Society MTBE water contamination: Key considerations for remediation, risk assessment and risk management www.techstuff.com/mtbe.htm
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This article originally appeared in the 05/01/2000 issue of Environmental Protection.