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