A Response Guide for Sunken Oil Mats (SOMs): Formation, Behavior, Detection, and Recovery

Abstract

Based on past spills, the conditions under which floating oil mixes with enough sand to form sunken oil mats (SOMs) are identified. SOMs form mostly during spills of heavy crudes or heavy fuel oils, but also highly weathered and viscous crude oils. They usually form when oil and sand are both suspended in the water column by breaking waves or by the erosion of heavily oiled sand from the beach. The oil–sand mixture needs an area in the nearshore where it can accumulate into mats, such as in troughs or inside lagoons, where wave energy is reduced. SOMs can be oily (>40% oil) or sandy (<<40% oil), with oily SOMs posing risks of the oil separating from the sand and refloating. Methods are described for SOMs detection and recovery based on their location, in either the surf zone or the open-water environment seaward of the surf zone. A matrix provides guidance on methods based on effectiveness and environmental impacts for different site conditions.