Geoinsight, Inc -Assessing and Remediating No. 6 Fuel Oil in Lakes and Ponds

GeoInsighter Summer 2004 Newsletter

Assessing and Remediating No, 6 Fuel Oil in Lakes and Ponds

Number 6 (No. 6) fuel oil, also known as bunker oil, presents certain challenges when it is released to a surface water body because of its chemical and physical properties. No. 6 fuel oil is a viscous, black, tar-like liquid that becomes semi-solid in cooler temperatures. Having a specific gravity just less than or equal to that of water, when released to a water body it may float, sink, or remain suspended in the water column. Ocean releases have been extensively studied and assessment and remedial technologies to address them are generally established. However, releases to lower energy fresh water bodies, such as lakes and ponds, are not as well studied and assessment and remediation in these environments presents different sets of issues to be considered.

A No. 6 fuel oil release to a small pond in Maine provides an example of how releases to fresh water environments can be assessed and remediated. In this case, the release occurred when a tanker truck crashed and turned over on a narrow beach adjacent to a pond where it released approximately 6,000 gallons of No. 6 fuel oil, which quickly covered more than half of the pond’s 100-acre surface area. Immediately following the incident, State and private emergency response teams recovered most of the released oil and performed cleanup operations to remove gross oil impacts from the shoreline. Residual oil re-impacted portions of the shoreline during two consecutive summers following the spill.

Re-oiling occurred during the two summers in July when water temperature increases extended to approximately 15 feet deep in the pond and changed the specific gravity of the oil on the cove bottom, causing it to float. In addition, oil globules appeared to primarily originate from the cove where the release occurred and drifted under the influence of prevailing winds to impact the downwind shoreline of the pond. These impacts were mitigated once booms were deployed across the mouth of the cove.

A cove-bottom survey was developed that accounted for the pond’s low visibility to evaluate if oil remained on the cove bottom and to identify oil hot spots for potential remediation. The survey was conducted by first placing a rope grid on the cove bottom. The grid consisted of 29 transect lines aligned roughly perpendicular to a boom deployed across the mouth of the cove and spaced approximately 15 feet apart. Each grid node was pre-marked with an identification label at 15-foot intervals. A specialized Hazmat dive team was then deployed with sample jars pre-marked to match each grid node label. The divers then collected a pond bottom sample at each grid node. Samples were delivered to the surface where they were characterized and ranked based upon visually observed oil content. The ranking for each node sample was then placed on a map and contoured. The resulting map was used to identify “hot” and “clean” zones and target areas for remediation.

Several remedial approaches, ranging from draining the pond, or a portion of the pond using a coffer dam, to physical disturbance of cove bottom sediments to re-suspend “tar balls” were considered. Based upon costs and the bottom survey results, which indicated relatively well-defined hot spots and a relatively limited quantity of oil remaining on the cove bottom, an approach incorporating physical disturbance (i.e., raking) of the pond bottom to resuspend and collect oil at the surface was selected as the primary remedial approach. Prior to commencing remedial activities, which would disturb cove-bottom sediments, a silt barrier was constructed across the mouth of the cove using a standard, 15-foot wide geotextile material. In addition, sorbent booms were placed along the cove shoreline.

Remedial activities were performed from a motorized work platform (barge). Various low-tech agitation techniques were implemented during the remedial activities. The most effective of which included the use of rigid 3-inch pronged rakes attached to extension poles operated manually from the work platform to agitate bottom sediments, suspend oil in the water column, and float it to the surface. The rakes were covered with polyethylene bags to increase the surface area and provide an adhesion surface. Liberated oil globules and sheen within the cove were collected from the pond surface using polyethylene sheeting attached to poles. The sheeting provided an adhesive surface for the oil that proved more effective than sorbent booms or polyethylene snares.

Pond bottom surveys completed during and near the end of remedial efforts indicated a significant reduction in oil on the cove bottom. The project demonstrated that techniques not applicable to releases in higher energy ocean environments may be utilized in more quiescent pond and lake environments.

Michael F. Dacey, P.G., L.S.P.
mfdacey@geoinc.com

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