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).