Complicated Spill Tough to Clean Up
September 20, 2011
Editor’s note: Ten years ago last month, the water supply in the village of Pascoag, in the town of Burrillville, was contaminated by the now-banned gasoline additive methyl tert-butyl ether (MTBE). MTBE is a petroleum byproduct that replaced lead in gasoline as an oxygenating agent. Many gasoline companies termed it an “anti-knocking agent.” The petrochemical has been shown to cause cancer in rodents. As of 2007, it had been banned — partially or fully — in 24 states, including Rhode Island.
PASCOAG, R.I. — In order to understand the devastating impact of the now-decade-old MTBE contamination on the bedrock aquifer under the village of Pascoag, one must understand how gasoline and its constituent chemicals behave when leaked into soil.
When gasoline is spilled or leaks out of underground storage tanks, it almost immediately begins to break down into the various additives, stabilizers and sub-chemicals that are used to refine petroleum products. MTBE’s extremely potent taste and odor mean that, even at very low concentrations, it can render large quantities of groundwater non-potable. The chemical is much more soluble in water than most petroleum byproducts.
This solubility causes it to “dive” into bedrock aquifers and to travel faster and farther than many other components of gasoline when released into aquifers. To make MTBE matters worse, the chemical can be released into groundwater in “pulses” that can cause radical upticks in contamination levels, due in most part to heavy rain events and rising groundwater.
The Environmental Protection Agency (EPA) reports that because MTBE behaves differently in soil, air and water than other petroleum constituents, the choice of remediation technology may be different when MTBE is present. Benzene is most often the contaminant of concern in gasoline because of its relatively high solubility and its known carcinogenicity.
When MTBE is in the soil as the result of a petroleum release, it may separate from the rest of the petroleum, reaching the groundwater first and dissolving rapidly. Once in the groundwater, MTBE travels at nearly the same rate as the groundwater. Benzene and other petroleum constituents tend to biodegrade and adsorb into soil particles.
According to Mike Cote, remediation specialist for the state Department of Environmental Management (DEM), in addition to the natural qualities of MTBE that make cleanups difficult, there were numerous site-specific issues that made the original assessment of the Pascoag spill and subsequent remediation extremely difficult.
“It was found in a weird way,” he said. “It was found initially in the public well. We had to make sure of the source, so we had to evaluate several properties around the well to determine the source of the spill.”
The problem was so complicated that DEM had to have several managers deal with the spill, and after the source was determined to be the Main Street Mobil station, the responsible parties were charged, and not long after that, they disappeared — leaving DEM the sole responsibility of cleaning up the mess.
“The spill was a big problem not only in size, but in depth,” Cote said. “On top of that, the entire town’s water supply was being affected because they had no other source.”
The problems just snowballed after the initial spill. Gasoline and benzene vapors caused the evacuation of some homes and an assisted-living facility. DEM responded by quickly installing a carbon filter on the wellhead so the people could use the water, at least for bathing, while the assessment was ongoing.
The emergency remediation plan consisted of separating the affected area into five zones that were prioritized by the level of contamination, and the areas with the highest levels of MTBE were addressed first. The geological features of the area, namely the extremely shallow bedrock, allowed the spill to move quickly toward the village’s wellheads, which, at the time, had only been online for a few months. The hybrid design of the wellhead itself made the problem worse. The new well was a hybrid that extracted water from the bedrock as well as from the soil.
“After the well was shut off, the contamination stabilized, but at a very high level,” Cote said. “Normally, gasoline spills of this type never leave the initial site, but in this case we had several testing wells that had standing gasoline in them. Anytime you find contamination to that degree, you’re talking anywhere from a quarter-million to a half-million dollars per site to clean up. It was almost like six or seven problems instead of just one.”
The filters that were deployed by DEM can only handle about a half-acre of contaminated land at a time. Currently, there are five different remediation systems in place, in addition to a couple of mobile filtration systems, to deal with contamination wherever it may pop up.
Large production wells like those in Pascoag actually pull contamination toward the well, so the natural movement of contaminants and biodegradation processes are accelerated and hindered, respectively. Then, when the well was shut off in early 2002, instead of the contamination seeping back to whence it came, it moved toward the nearest water source, a stream that empties into the Pascoag Reservoir — a frustrating development for the DEM because, as Cote said, “It made for a very weird shaped plume.”
When dealing with these remediations, the law of diminishing returns applies. “It’s easy to get out the first 60 percent, 70 percent,” Cote said. “Trying to get out the last 30 percent, the costs go up astronomically and it takes years and years. You start to get to the point that it’s either economically or technically infeasible. What we’d like to do — and I’m not saying that that’s what’s going to happen — is to clean up the worst of it and let Mother Nature, through processes like dilution or bacteria that eat it, take care of the rest.”
Oddly, the water isn’t the problem; it’s the soil. Soil acts like a sponge for contaminants. “Think of a soapy sponge,” Cote said, “you can rinse and rinse that sponge and never get all of the soap out of it.”
Pumping water out of the ground is really just a measure to control the movement of the contamination. Lowering the levels of contamination in soil takes soil vapor extraction (SVE). SVE is basically using giant vacuums that suck vapors out of the soil.
“That (SVE) is very effective and cost-effective,” Cote said. “The vapor extraction performed here removed most of the contamination from the soil.”
Unfortunately, soil venting only works on unsaturated material. If the contamination is deep under the water, SVE is ineffective.
DEM currently has about 120 monitoring wells in Pascoag, a small set of which are sampled every three months. Once a year, DEM samples all of the monitoring wells. Cote said that while the contamination was really transient initially, it has become much more predictable over time.
In a best-case scenario, the contamination is reduced to below the 40 parts per million threshold set by the EPA. “That’s my goal,” Cote said, “but is that good enough to turn the well back on? That’s Department of Health. There are a lot of different standards for what is protective.”
In January 2002, local residents asked the Rhode Island judiciary to close the well, which they did, and the well remains closed to this day.
The Rhode Island Department of Health (DOH) maintains that, even if the MTBE is cleaned up to EPA standards, it will never reopen the village’s public well. DOH claims that this is due not to the existing contamination, but the potential for future contamination.
ecoRI News has asked the DOH why — given that there has been almost no major development in the area since 2001, and in fact, a potential major source of contamination was eliminated when the leaking gas tanks were removed from the Main Street Mobil location — that determination wasn’t made prior to the Pascoag Utility District spending big money to drill the well in the first place? A response has yet to be received.
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