Environmental Engineering Reference
In-Depth Information
6
Physical methods: saturated zone
6.1 Introduction
levels (
Table 2.1
). Groundwater levels can be
measured manually by using a graduated meas-
urement tape to determine the depth to water
in a well from a reference point at the top of
the well casing. Historically, groundwater levels
in some observation wells were automatically
recorded by using a float that was attached by a
steel tape or wire to a wheel sensor; a strip-chart
or paper-punch device was used to record move-
ment of the wheel (Rasmussen and Andreasen,
1959
). Submersible pressure transducers have
come into widespread use for monitoring
groundwater levels since the 1990s (Freeman
et al
.,
2004
). These electronic devices can auto-
matically sense and record groundwater levels
at user-selected frequencies. The depth (relative
to the reference point on the well casing) at
which a transducer is placed in a well must be
carefully measured. If groundwater elevation
is desired, the elevation of the reference point
needs to be determined.
Total head within an aquifer varies by loca-
tion and depth. A measured groundwater level
represents a head value that is averaged across
the depth interval over which an observation
well is screened. So the length and depth of the
screened interval affects measured water levels
(Taylor and Alley,
2001
). A water level measured
in a well screened across the entire thickness of
an aquifer represents a depth-averaged head for
the aquifer at the well location. Multiple wells
installed at different depths and with narrow
screened intervals are required to determine
vertical head gradients within an aquifer.
Among the most widely used techniques for
estimating recharge are those based on meas-
urement of groundwater levels over time and
space. The abundance of available ground-
water-level data and the simplicity of these
methods facilitate straightforward applica-
tion. The water-table fluctuation method uses
fluctuations in groundwater levels over time
to estimate recharge for unconfined aqui-
fers; it is the focus of most of this chapter.
Included in the discussion of the method are
an analysis of mechanisms that can cause
water-table fluctuations and a review of meth-
ods for estimating specific yield. Other meth-
ods addressed in this chapter are based on
the Darcy equation and include an approach
developed by Theis (
1937
), the Hantush (
1956
)
method for estimating interaquifer flow, and
the application of flow nets. The chapter also
includes a discussion of approaches based on
time-series analyses of measured groundwater
levels. The content of this chapter draws from
and expands upon the material presented in
Healy and Cook (
2002
).
6.1.1 Groundwater-level data
Many local, state, and federal agencies maintain
databases of measured groundwater levels in
individual countries. Within the United States,
the US Geological Survey maintains the largest
database on real-time and historic groundwater
