To determine ground water flow directions and hydraulic gradient, owners and
operators should develop and implement a water level monitoring program. The water level
monitoring program should be structured to provide precise water level measurements in a
sufficient number of piezometers or wells at a sufficient frequency to gauge both seasonal
average flow directions and temporal fluctuations in ground water flow directions
( 264.97(f)). Ground water flow direction(s) should be determined from water levels
measured in wells screened in the same hydrostratigraphic position. In heterogeneous
geologic settings (i.e., settings in which the hydraulic conductivities of the subsurface
materials vary with location in the subsurface), long well screens can intercept stratigraphic
horizons with different (e.g., contrasting) ground water flow directions and different heads. In
this situation, the resulting water levels will not provide the depth discrete head measurements
required for accurate determination of the ground water flow direction.
In addition to evaluating the component of ground water flow in the horizontal
direction, a program should be undertaken to accurately and directly assess the vertical
component of ground water flow. Vertical ground water flow information should be based at
least in part on field data from wells and piezometers such as multi level wells, piezometer
clusters, or multi level sampling devices, where appropriate. The following sections provide
acceptable methods for assessing the vertical and horizontal components of flow at a site.
Ground Water Level Measurements
To determine ground water flow directions and ground water flow rates, accurate
water level measurements (measured to the nearest 0.01 foot) should be obtained.
Procedures for obtaining water level measurements are presented in Section 7.2.2. At
facilities where it is known or plausible that immiscible contaminants (i.e., light non aqueous
phase liquids (LNAPLs) or DNAPLs) occur (or are determined to potentially occur after
considering the waste types managed at the facility) in the subsurface at the facility, both the
depth(s) to the immiscible layer(s) and the thickness(es) of the immiscible layer(s) in the well
should be recorded. Section 7.2.3 provides procedures for measuring the thickness of
immiscible layers in wells.
If accurate documentation cannot be produced to show that the procedures for well
surveying contained in Section 6.6, water level elevation measurements contained in Section
7.2.2, and detection of immiscible layers contained in Section 7.2.3 were met during the
collection of water level measurements, the information generated may be judged inadequate.
For the purpose of measuring total head, piezometers and wells should have as short a
screened interval as possible. Specifically, EPA recommends that the screens in piezometers
or wells that are used to measure head be less than 10 feet long. In circumstances including,
but not limited to the following, well screens longer than 10 feet may be warranted: