Environmental Engineering Reference
In-Depth Information
are present. Doing so could deform the threads and make it impossible to attach a
coupling or additional sections of casing that would otherwise allow the well screen
to be driven deeper into the sediment. A drive cap or coupling should be screwed
onto the threads at the top of the well casing before striking the top of the casing to
drive it farther into the sediment. The drive cap or coupling should be tightened
occasionally as the casing is driven into the sediment; not doing so also may result
in damaged threads. It is prudent to periodically stop driving the well and swab the
well to remove sediment that may have clogged the well screen. It may be necessary
to pour water into the top of the well casing so the swab pushes and pulls water, and
not air, through the well screen. If additional sections of pipe are required, Teflon
tape or pipe dope should be used liberally, and the fittings tightened using pipe
wrenches, to ensure that no leaks occur at the junctions between pipe segments.
Once the well is driven to depth, it should be thoroughly developed by repeatedly
swabbing the well and screen, including periodic removal of water and suspended
sediment from the well with a pump or bailer, until the water level inside the well
casing recovers readily to the static water level. Once this occurs, the well is
considered developed and is functioning as a piezometer.
After well installation and development you will want to measure and record:
1. Distance from the top of casing to the well bottom,
2. Distance from top of casing to the wetland bed,
3. Screened interval, sump interval (if present), and
4. Distance from the water surface to the wetland bed.
With these values determined, the distance from the sediment-water interface to
the mid-point of the screened interval can be calculated. Commonly referred to as
l
in the Darcy equation (or sometimes
l
v
to indicate that the gradient is distributed on
a vertical axis), this is the distance that the head difference is divided by to
determine the vertical hydraulic gradient. The head difference can easily be deter-
mined by measuring the distance from the top of casing to the wetland water surface
and subtracting the distance from the top of casing to the water surface inside of the
well. For a small-diameter well completed in low-permeability sediments,
measurements of depth to water can be corrupted if a portion of the measuring
device needs to be immersed in the water to make a measurement. The volume of
the sensor device immersed in the water will cause the water level to rise inside of
the well. Low-permeability sediments will not permit the water level inside the well
to return to static equilibrium in a sufficiently short time, resulting in a false depth-
to-water measurement. Care should be taken to prevent this possibility by using a
measurement method that does not require immersion of a large sensor relative to
the well-casing diameter during a water-level measurement. The cut-off end of a
chalked-steel tape is a particularly good device for this purpose because the volume
of the steel tape immersed to make a measurement is very small.
Once the piezometer is installed, GPS coordinates and well-top elevations are
determined, and measurements are made to determine the hydraulic gradient.
Sensors also can be installed to continuously monitor hydraulic head, and tempera-
ture at one or more depths, inside of the piezometer (Fig.
3.41
).
