The vertical positions and lengths of monitoring well intakes should be based on the

physical/chemical characteristics of the hazardous waste or hazardous waste constituents that

control the movement and distribution of the hazardous waste or hazardous waste constituents

in the subsurface. These characteristics include, but are not limited to: solubility, Henry's

Law constant, partition coefficients, specific gravity, contaminant reaction or degradation

products, and the potential for contaminants to degrade confining layers. Considering both

contaminant characteristics and hydrogeologic properties is important when choosing the

vertical position and length of the well intake. Some contaminants may migrate within very

narrow zones.

Different transport processes control contaminant migration depending on whether the

contaminant dissolves or is immiscible in water. Immiscible contaminants may occur as

LNAPLs, which are lighter than water, and DNAPLs, which are denser than water. Most

LNAPLs are hydrocarbon oils and fuels. Most DNAPLs are highly chlorinated hydrocarbons

(e.g., carbon tetrachloride, tetrachloroethylene, and PCBs). Identifying whether or not a

compound may exist as an DNAPL or an LNAPL is complicated by the substance in which it

is dissolved. For example, free phase PCBs may be denser than water (DNAPL), but PCBs

in oil can be transported as an LNAPL. Additional information on NAPL migration is

provided by USEPA (1989) and USEPA (1991).

LNAPLs migrate in the capillary zone just above the water table. Wells installed to

monitor LNAPLs should be screened at the water table/capillary zone interface, and the

screened interval should intercept the water table at its minimum and maximum elevation.

LNAPLs may become trapped in residual form in the vadose zone and become periodically

remobilized and contribute further to aquifer contamination, either as free phase or dissolved

phase contaminants, as the water table fluctuates and precipitation infiltrates the subsurface.

The migration of free phase DNAPLs may be primarily influenced by the geology,

rather than the hydrogeology, of the site. That is, DNAPLs migrate downward through the

saturated zone due to density, and then migrate by gravity along less permeable geologic units

(e.g., the slope of confining units, the slope of clay lenses in more permeable strata, bedrock

troughs), even in aquifers with primarily horizontal groundwater flow. Consequently, if

wastes disposed at the site are anticipated to exist in the subsurface as a DNAPL, the

potential DNAPL should be monitored:

At the base of the aquifer (immediately above the confining layer);

In structural depressions (e.g., bedrock troughs) in lower hydraulic conductivity

geologic units that act as confining layers;

Along lower hydraulic conductivity lenses and units within units of higher

hydraulic conductivity; and/or

November 1992

5 6