Exxon Mobil has proposed that it reduce oil sheening in Lake Conway’s Dawson Cove by excavating up to 1 foot of affected soil and sediment from a channel of water leading into the cove and by capping sheens in the open-water area with a mixture of sand and an absorbent clay.

Arcadis U.S. Inc., a Colorado-based company Exxon Mobil hired for sampling after a major oil spill last spring, submitted the recommendation and four alternatives in a report it prepared for the oil giant in response to requests from the Arkansas Department of Environmental Quality.

Last month, the department gave Exxon Mobil until last Friday to submit remediation recommendations for the cove and three drainage ditches leading to it after the Pegasus pipeline ruptured in a Mayflower neighborhood and spilled an estimated 210,000 gallons of heavy crude oil into the area March 29, contaminating a subdivision and those waterways.

Exxon Mobil and state authorities have said there is no evidence that oil reached the main portion of the popular fishing lake.

The channel where excavation would take place runs between Interstate 40 and the cove’s open-water area.

Arkansas 89 separates the cove from the main portion of the lake. Water flows through two culverts beneath the highway. Emergency workers barricaded those culverts shortly after the spill, but workers have since removed the barriers.

Exxon Mobil submitted the 121-page report from Arcadis just before the Environmental Quality office closed for the week Friday.

Arcadis said in the report that it settled on the excavation depth because samples had “indicated no sheen-bearing material present below 1 foot.” The company said it would conduct a study “to refine the horizontal and vertical boundaries of removal and to evaluate whether a thinner removal thickness would be adequate.”

The report offered alternative corrective measures to the one recommended for sheening and also summarized samples taken of water, soil and sediment since last spring in the ditches, the cove and Lake Conway.

In a Dec. 13 letter to Jeff Bunce, project developer for Exxon Mobil Environmental Services Co., Tammie Hynum, chief of the state department’s Hazardous Waste Division, wrote that samples taken in the cove and other drainage sites had found that some chemicals were higher than established ecological screening values for “benthic receptors.”

Such receptors are organisms such as blood worms and insect larvae that live on or in the water’s bottom and which are a major food source for smaller fish.

But in its report, Arcadis wrote, “The results from the screening data evaluation and the subsequent refined risk evaluation indicate that there are no unacceptable ecological risks in the drainage ways, Dawson Cove, and Lake Conway. Therefore, no action is necessary to mitigate constituent levels in the soil and sediment” in those areas.

Environmental Quality spokesman Katherine Benenati said Monday that the department has not had a chance to review the report but should know more by the end of the week on how the review is progressing. The report is subject to the department’s approval. Exxon Mobil also would need some permits to proceed.

On the suggested course of sheen remediation, Arcadis wrote that activities would include removing vegetation and debris from targeted areas as needed to allow for excavation. “Large-diameter trees would be left in place,” it added.

Excavated materials eventually would be taken to licensed, off-site disposal facility.

Workers would fill in excavated areas with clean material “if needed for restoration”and would restore those areas “by re-grading and re-planting with native species,” Arcadis wrote.

Capping in the cove’s open-water area also would involve removal of vegetation and debris from targeted areas as needed, again with large trees left alone. The cover, called a “reactive cap,” would consist of sand and organoclay.

This type of clay is used to absorb oil and sheens. “After the cap is placed, staging areas would be re-graded and restored by planting native vegetation,” Arcadis wrote.

The company said the goal is “to mitigate surface water sheens resulting from the crude oil from the Pegasus Pipeline, to the extent practicable.”

Arcadis said it does not expect any such remedial work to be needed in the area’s drainage ways “because sheens observed in the areas from October 21, 2013 through January 5, 2014 were not likely related to the crude oil from the Pegasus Pipeline, as demonstrated by sheen sampling and forensics analysis.”

“However, there were four sheens observed with oil spots at three locations in the drainage ways. The potential presence of sheen-bearing materials in these three areas of the drainage ways … will be evaluated,” it added.

Arcadis also has been monitoring surface-water samples and said data collected from March 29 through Dec. 31 also indicate no further action is warranted in that regard. The company said, though, that monitoring for polycyclic aromatic hydrocarbons should continue weekly at five specified sites in the cove and near the cove’s outlet “until a remedial action is implemented.”

Polycyclic aromatic hydrocarbons, or PAHs, are “a group of chemicals that are formed during the incomplete burning of coal, oil, gas, wood, garbage, or other organic substances, such as tobacco and charbroiled meat. There are more than 100 different PAHs,” according to the federal Agency for Toxic Substances and Disease Registry’s website.

Exxon Mobil has estimated that 5,000 barrels of oil - at 42 gallons per barrel - were released during the spill and that 2,000 barrels have been recovered.

Exxon Mobil spokesman Aaron Stryk said Monday, however, that the estimated amount recovered “is a preliminary estimate that includes only barrels of recovered oil currently measured in liquid tanks.”

“Oil recovered in the form of oiled soil, vegetation and debris, which includes sorbent booms, pads and wipes,cannot be accounted for as easily and, therefore, was not included in this estimate. This preliminary estimate will be revised after the line is refilled,” Stryk added in an email.

Front Section, Pages 1 on 01/21/2014