Last week, the Arkansas Department of Environmental Quality (ADEQ) released preliminary findings from Exxon’s testing of soil and sediment data in Mayflower. The agency has also been posting the raw data on its website for public consumption; on Friday, ADEQ posted Exxon’s final report of its testing so far. ADEQ’s full review is expected soon. Wilma Subra, a MacArthur “Genius” Award-winning environmental chemist based in New Iberia, La., looked at the data for the Times.
She was unable to provide a full analysis, but after her initial inspection, she expressed skepticism about ADEQ’s findings. She said that the data indicates more sampling must be done in order to determine the extent of the contamination, and she isn’t ready to rule out the possible presence of oil in the main body of Lake Conway.
When sampling soil and sediment in the wake of an environmental disaster, it’s necessary to take “background” samples from a nearby area that’s not been impacted. This provides a baseline picture of what environmental conditions were like before the event occurred. But some of the sites used for background samples near Mayflower themselves indicate the presence of petroleum contamination. Other sites do not. The background samples in question were too far away to be affected by the Pegasus spill, but do they provide a useful baseline?
“The issue is when you have values that range as much as their, quote, background, it’s very clear that a lot of the areas they sample have other sources of contamination impacting them, to have that kind of high concentration,” Subra said. “I don’t think their selection of background locations was very appropriate.”Exxon’s sampling plan takes dozens of soil and sediment samples from along the path of the spilled oil — “sediment” in this context refers to land that is submerged below the water line, such as the lakebed — but it samples only six spots in the main body of Lake Conway beyond the mouth of the cove. ADEQ officials have said they are “confident” no oil spread into the lake beyond the cove. While there are petroleum hydrocarbons found in those six sites, ADEQ notes that such chemicals are present everywhere gas and oil are burned and they may have came from outboard motors in Lake Conway or runoff from I-40.
But Subra said her initial inspection shows a consistency in the specific chemicals detected in the few lake sample sites that’s not present in the background sites. Such a pattern may indicate the compounds in the lake samples come from Exxon’s oil. Also, in many of the sites sampled, contaminants were present in the lowest depth of sediment examined (the depth of the samples varied from site to site). Subra said this indicates that samples should be taken deeper into the sediment and soil to establish the vertical limit of the contamination.
Ryan Benefield, ADEQ deputy director, said that more sampling is likely. “In most of our investigations of this nature, you have to go back out and collect additional data because you realize ‘we didn’t get to the edge [of the contamination.’”
That could entail both going deeper down and sampling further afield. As for the presence of contaminants in the main lake, Benefield reiterated that ADEQ has seen no evidence of oil in the lake beyond the cove, nor any other environmental impact on Lake Conway resulting from the spill. However, he acknowledged that some water from the contaminated cove entered the lake after the spill, including water pumped by the response team to keep the sealed-off cove from flooding. Certainly, some unpleasant molecules traveled from the lake to the cove in the wake of the spill; the question is just how many.
“We’re not seeing levels of constituent [chemicals] that we could connect to the oil spill,” Benefield said. He noted that hydrocarbons in Lake Conway come from many sources — outboard motors, highway runoff — and insisted that “free oil did not progress past our first couple booms and weirs,” which the response team used to seal the cove from the rest of Lake Conway. Benefield also said ADEQ was continuing to look for any evidence that the Pegasus spill had affected Lake Conway, including the sort of chemical signature that Subra referenced above.
The agency and Subra agree that the cove itself is contaminated. Each of the constituent chemicals tested by the sampling process has what’s called an “ecological screening value,” or ESV. The lower the ESV, the more harmful the chemical. From her cursory inspection, said Subra, there are some compounds that are “definitely way over the ecological impact [thresholds].” And, she noted, some of the compounds in question, a class of toxic chemicals called PAHs, are worrisome exactly because they persist in the environment and build up in the bodies of larger organisms.
“This is in the sediment, so it’ll start with the benthic organisms that are in the sediment, which get consumed by organisms in the water column and build up the food chain. And it’s very persistent in the environment, so it’ll be a source for bioaccumulation.”
Benefield said that bioaccumulation was a legitimate concern, but emphasized that the cleanup of the cove should remove most of the toxins from the environment before they had a chance to accumulate.
“Generally, and it depends on the chemical, bioaccumulation is a longer-term concern, not something you see immediately. So we’re going to ensure that Exxon remediates the site so that we don’t later have levels of long-term bioaccumulation that are of concern in the fish.”