7.2.3

Detection and Sampling of Immiscible Layers

The QAPjP should include procedures for detecting and measuring the thicknesses of

immiscible contaminants (i.e., LNAPLs and DNAPLs) each time water level is measured, if

immiscible contaminants are known to occur (or are determined to potentially occur after

considering the waste types managed at the facility) in the subsurface at the facility.

LNAPLs, also known as "floaters," are relatively insoluble organic liquids that are less dense

than water and that spread across the water table. DNAPLs, also known as "sinkers," are

relatively insoluble organic liquids that are more dense than water and tend to migrate

vertically downward in aquifers toward underlying confining layers. The detection of

immiscible contaminants requires specialized equipment, and should be performed before a

well is purged for conventional sampling.

The QAPjP should specify the device(s) that will be used to detect LNAPLs and

DNAPLs. The procedures for detecting LNAPLs and DNAPLs should include procedures for

measuring depth to both the non aqueous phase liquid (NAPL) layer and to the water surface.

When opening wells that may contain LNAPLs or DNAPLs, the air above the well head

should be monitored to determine the potential for fire, explosion, and safety hazards, or

adverse health effects to workers. Air monitoring also serves as a first indication of the

presence of LNAPLs. The presence of LNAPLs precludes the exclusive use of sounders or

manometers to make a determination of static water level. A manometer or acoustical

sounder (for very shallow wells) may provide an accurate reading of the depth to the surface

of the liquid in the well, but neither is capable of differentiating between the water table and

the surface of an immiscible layer. Often an interface gauging probe or a weighted tape

coated with commercially available reactive indicator paste will be suitable for this purpose.

The interface probe serves two related purposes. First, as it is lowered into the well,

the probe registers when it is exposed to an organic liquid and thus identifies the presence of

LNAPLs. Careful recording of the depths of the air/LNAPL and LNAPL/water interfaces

establishes a measurement of the thickness of the LNAPL in the well casing. Secondly, after

passing through the LNAPL layer, the probe indicates the depth to the water level. Interface

probes are available that can be used to measure the thickness of DNAPLs. The Regional

Administrator should be notified when LNAPLs or DNAPLs have been detected in a well.

The QAPjP also should include the procedures that will be used to sample LNAPLs or

DNAPLs. A sample of the LNAPL or DNAPL should be collected prior to well purging.

The Agency understands that bailers typically must be used to collect LNAPLs and DNAPLs

because immiscible phases do not often occur in thicknesses that can be satisfactorily sampled

using recommended submersible pumps. The key to minimizing sample bias is controlled,

slow lowering (and raising) of the bailer within the well.

The approach to sampling LNAPLs depends on the depth to the floating layer surface

and the thickness of the layer. If the thickness of the LNAPL in the well casing is great

November 1992

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