The lateral placement of monitoring wells also 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. For example,

contaminants with low solubilities and high specific gravities that occur as DNAPLs may

migrate in the subsurface in directions different from the direction of ground water flow.

Therefore, in situations where the release of DNAPLs is a concern, the lateral placement of

compliance point ground water monitoring wells should not necessarily only be along the

downgradient edge of the hazardous waste management unit. Considering both contaminant

characteristics and hydrogeologic properties is important when determining the lateral

placement of monitoring wells.

5.1.2.3

Vertical Placement and Screen Lengths

Proper selection of the vertical sampling interval is necessary to ensure that the

monitoring system is capable of detecting a release from the hazardous waste management

area. The vertical position and lengths of well intakes are functions of: (1) hydrogeologic

factors that determine the distribution of, and fluid/vapor phase transport within, potential

pathways of contaminant migration to and within the uppermost aquifer, and (2) the chemical

and physical characteristics of contaminants that control their transport and distribution in the

subsurface. Well intake length also is determined by the need to obtain vertically discrete

ground water samples. Owners and operators should determine the probable location, size,

and geometry of potential contaminant plumes when selecting well intake positions and

lengths.

Site specific hydrogeologic data obtained during the site characterization should be

used to select the length and vertical position of monitoring well intakes. The vertical

positions and lengths of monitoring well intakes should be based on the number and spatial

distribution of potential contaminant migration pathways and on the depths and thicknesses of

stratigraphic horizons that can serve as contaminant migration pathways.

The depth to, and thickness of a potential contaminant migration pathway can be

determined from soil, unconsolidated material, and rock samples collected during the boring

program, and from samples collected while drilling the monitoring well. Direct physical data

can be supplemented by geophysical data, available regional/local hydrogeological data, and

other data that provide the vertical distribution of hydraulic conductivity. The vertical

sampling interval is not necessarily synonymous with aquifer thickness. Monitoring wells are

often screened at intervals that represent a portion of the thickness of the aquifer. When

monitoring an unconfined aquifer, the well screen typically should be positioned so that a

portion of the well screen is in the saturated zone and a portion of the well screen is in the

unsaturated zone (i.e., the well screen straddles the water table).

November 1992

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