Data obtained from multiple piezometers or wells placed in a single borehole

may be erroneous. Placement of vertically nested piezometers or wells in

closely spaced, separate, boreholes, or single multi level devices in single

boreholes, is preferred.

The vertical component of hydraulic gradient should be calculated, and the

vertical direction of ground water flow should be determined, for a minimum

of two vertical profiles at the site. These profiles should be aligned roughly

parallel to the horizontal direction of ground water flow as indicated by the

potentiometric surface or water table map.

All other procedures for water level measurement described in this Manual

should be met.

When reviewing data obtained from multiple placement of piezometers or wells in

single boreholes, the construction details of the well should be carefully evaluated. Not only

is it extremely difficult to adequately seal several piezometers/wells at discrete depths within

a single borehole, but sealant materials may migrate from the seal of one piezometer/well to

the screened interval of another piezometer/well. Therefore, the design of a piezometer/well

nest should be carefully considered. Placement of piezometers/wells in closely spaced

boreholes, where piezometers/wells have been screened at different, discrete depth intervals, is

likely to produce more accurate information. The primary concerns with the installation of

piezometers/wells in closely spaced, separate boreholes are: 1) the disturbance of geologic

and soil materials that occurs when one piezometer is installed may be reflected in the data

obtained from another piezometer located nearby, and 2) the analysis of water levels

measured in piezometers that are closely spaced, but separated horizontally, may produce

imprecise information regarding the vertical component of ground water flow. The

limitations of installing multiple piezometers either in single or separate boreholes may be

overcome by the installation of single multi level monitoring wells or sampling devices in

single boreholes. The advantages and disadvantages of these types of devices are discussed

by Aller et al. (1989).

The owner or operator should determine the vertical direction(s) of ground water flow

using the water levels measured in multi level wells or piezometer/well nests to construct

flow nets. Flow nets should depict piezometer/well depth and length of the screened interval.

It is important to accurately portray the screened interval on the flow net to ensure that the

piezometer/well is actually monitoring the desired water bearing unit. A flow net such as that

presented in Figure 5 should be developed from information obtained from piezometer/well

clusters or nests screened at different, discrete depths. Detailed guidance for the construction

and evaluation of flow nets in cross section (vertical flow nets) is provided by USEPA

(1989c). Further information can be obtained from Freeze and Cherry (1979).

November 1992

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