"Down the dip" of lower hydraulic conductivity units that act as confining

layers, both upgradient and downgradient of the waste management area.

Because of the nature of DNAPL migration (i.e., along structural, rather than hydraulic,

gradients), wells installed to monitor DNAPLs may need to be installed both upgradient and

downgradient of the waste management area. It may be useful to construct a structure

contour map of lower permeability strata and identify lower permeability lenses upgradient

and downgradient of the unit along which DNAPLs may migrate; then locate the wells

accordingly.

The lengths of well screens used in ground water monitoring wells can significantly

affect their ability to intercept releases of contaminants. The complexity of the hydrogeology

of a site is an important consideration when selecting the lengths of well screens. Most

hydrogeologic settings are complex (heterogeneous and anisotropic) to a certain degree.

Highly heterogeneous formations require shorter well screens to allow sampling of discrete

portions of the formation that can serve as contaminant migration pathways. Well screens

that span more than a single saturated zone or a single contaminant migration pathway may

cause cross contamination of transmissive units, thereby increasing the extent of

contamination. Well intakes should be installed in a single saturated zone. Well intakes

(e.g., screens) and filter pack materials should not interconnect, or promote the

interconnecting of, zones that are separated by a confining layer.

Even in hydrologically simple formations, or within a single potential pathway for

contaminant migration, the use of shorter well screens may be necessary to detect

contaminants concentrated at particular depths. A contaminant may be concentrated at a

particular depth because of its physical/chemical properties and/or because of hydrogeologic

properties. In homogeneous formations, a long well screen can permit excessive amounts of

uncontaminated formation water to dilute the contaminated ground water entering the well.

At best, dilution can make contaminant detection difficult; at worst, contaminant detection is

impossible if the concentrations of contaminants are diluted to levels below the detection

limits for the prescribed analytical methods. The use of shorter well screens allows for

contaminant detection by reducing excessive dilution and, when placed at depths of predicted

preferential flow, shorter well screens are effective in monitoring the aquifer or the portion of

the aquifer of concern.

Generally, screen lengths should not exceed 10 feet. However, certain hydrogeologic

settings may warrant or necessitate the use of longer well screens for adequate detection

monitoring. Unconfined aquifers with widely fluctuating water tables may require longer

screens to intercept the water table surface at both its maximum and minimum elevations and

to provide monitoring for the presence of contaminants that are less dense than water.

Saturated zones that are slightly greater in thickness than the appropriate screen length (e.g.,

12 feet thick) may warrant monitoring with longer screen lengths. Extremely thick

homogeneous aquifers (e.g., greater than 300 feet) may be monitored with a longer screen

November 1992

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