Near Shore Submerged

Oil Assessment

September 2010

In the context of the BP Deepwater Horizon (DWH) 

oil spill in the Gulf of Mexico, submerged oil refers to 

near shore oil which has picked up sediments and become 

heavier than water and sunk. Without the addition of 

these sediments, extensive laboratory study has shown 

that this particular oil will not sink. The oil can be visible 

to the naked eye, but is not always easy to locate as it 

may be covered by sand and sediment, or in deeper water. 

Although it is common to use the terms submerged and 

subsurface interchangeably, they are actually distinct and 

result from very different physical and chemical processes. 

In this spill, the oil was released more than 5,000 feet 

below the surface and some was either naturally or chemically dispersed in the process of degrading. Some of it rose 

to the surface and formed “slicks” and a smaller amount 

of it remained dispersed in the water column in the deep 

ocean as tiny droplets, most too small to be seen by the 

naked eye, to be consumed by naturally occurring bacteria. This remaining oil is referred to as subsurface oil. The 

rest of this fact sheet is about submerged oil and reflects 

sampling results to date. 

Submerged oil is mostly in very near shore areas and the 

sampling to date confirms that DWH oil has been submerged only after be mixed with sand or other types of 

sediment or organic matter heavier than seawater. This is 

usually a result of waves moving oil over sandy areas. 

Weathered oil (oil that has been exposed to the elements 

for days or weeks) can become sticky and is more susceptible to picking up small particles, adding enough weight 

to sink the oil. During the DWH spill response, as with 

most spills, there have been many reports of submerged 

oil, many of these end up being false, due to things like 

sargassum, algae and diatom blooms, being mistakenly 

thought to be submerged oil. Accurately identifying oil 

has always been a challenge during oil spills. 

DWH Programs to Detect Submerged Oil

To address ongoing concerns about submerged oil, the 

Unified Area Command has implemented sampling, in 

the nearshore (depths of less than 200 ft deep) and in the 

deep water. The sampling has been carried out by NOAA, 

EPA, BP, and academic and consultant scientists, directed 

by the Unified Area Command, using both research vessels and vessels of opportunity. 

Sampling teams are towing nets to detect and capture 

tar balls, taking water column and sediment samples at 

a variety of depths, placing sorbent material in the water 

column to detect any oil that might be carried toward 

land by currents and even deployed teams of skin divers 

to survey in the shallow water. Such measures are necessary because submerged oil can be very difficult to locate. 

However, because of the intensity and variety of sampling, 

we are becoming more confident that we understand the 

conditions under which this oil will submerge, the location we expect it to happen and extent it has happened. 

Things You Should Know

About Submerged Oil

1. Submerged oil is relatively uncommon: DWH oil is 

a light crude oil that floats. In some cases, weathered oil 

encounters sand or other types of sediment and organic 

debris that weigh it down and it sinks to the bottom in 

the very near shore and surf zone. 

2. Submerged oil can be difficult to find: it can be covered by sediments; it can be in deeper or cloudy water 

and there are many things that look like submerged oil 

which are not, for example, sea grasses or organic matter. 

3. Submerged oil can be difficult to recover: it requires 

significant skill to remove without risking response personnel and additional environmental impact. 

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The Unified Area Command has 

directed response scientists to:

• Define the extent of submerged 

oil by systematic surveys

• Aggressively sample those areas 

where submerged oil is reported

• Comprehensively sample water 

and sediments where submerged 

oil might be suspected

Submerged oil monitoring programs 

currently underway in the Gulf 

include: 

• EPA surveys to collect water and 

sediment samples from locations 

along the Louisiana coast and 

later expand to collect mid-water 

column samples. 

• Programs using nets to detect 

and capture tar balls and submerged oil. These programs are 

systematic and responsive, following a design and investigating 

individual reports (for example if 

someone reports having pulled up 

oil on an anchor).

• Deployment and regular checking 

of snare sentinels that indicate the 

presence or absence of submerged 

oil.

Forms of DWH Submerged Oil

At this time, DWH response scientists have seen the following forms of 

submerged oil: 

1) patches of weathered oil that landed 

on sandy beaches, picked up sand and 

then were moved off the beach into 

the nearshore shallow water by waves 

as mats or rollers; 

2) weathered oil that mixed with 

sand as a result of waves breaking on 

offshore sand bars, then sank to low 

areas between sandbars (observed off 

Alabama); and 

3) in sheltered inshore bays, small bits 

of oil that are on or below the surface 

stuck to sediments and organic matter, then accumulated on the muddy 

bottom. 

Submerged oil

Near shore sentinal snare

Oiled snare

A sentinal snare deployed

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