Environmental Engineering Reference
In-Depth Information
sediment that was removed from the hole. Tamp the sediment repeatedly as you fill
the hole so the sediment is tightly consolidated. This will prevent any preferential
flow of water along the outside of the well casing during recharge events. If unused
segments of auger rod are used for this purpose, place duct tape over the end of the
rod to prevent damage of the threads.
If the sediment is sufficiently cohesive that the augered hole remains open below
the water table, inserting the completed well screen and casing is as simple as
placing the assembly into the auger hole. In this case, you will then need to pour
sand coarser than the well-screen slot size down the hole so that it surrounds the
entire screened interval. This backfill, often called a sand pack, will ensure that the
well screen does not become clogged with fine-grained sediment that otherwise
would be situated next to the well screen. Once sufficient sand is added to fill the
annular space to just above the screened interval, material removed from the auger
hole can be added to fill the remainder of the augered hole. As described before, this
sediment should be tamped to ensure that the density of the sediment filling the
annular space is not less than the undisturbed material. It is common to add soil to
create a small mound of soil at the base of the well that will direct rainfall away
from the well casing.
Now all that is left is to install a well cap, install well protection, and make
several measurements. A well cap can be as simple as a plastic slip cap that stays on
the casing via friction and gravity. You might instead wish to glue on an assembly
that has a threaded cap or that allows access to the well to be protected with a keyed
lock. In either case, make sure that the well cap can easily be removed from the
casing for measurements of depth to water. Shallow monitoring wells are not well
anchored to the soil because of the smaller contact area with the soil that surrounds
the well casing. Some wells can easily be moved, even in an attempt to remove a
firmly attached well cap, which may change the vertical positioning of the top of the
well and introduce error in determinations of hydraulic gradient. A small hole also
may be drilled through the well casing to facilitate equilibration of the pressure
inside of the well casing with changes in atmospheric pressure. If air cannot readily
enter the well casing, the position of the water table inside of the well may not
represent the water table.
In many areas, regulations require some form of protection that will minimize
the chance of the well casing being inadvertently broken by a falling tree or branch
or a wayward automobile or lawnmower. This may entail placing a steel casing of
larger diameter over the top of the well casing and into the ground (Fig.
3.41
), or
installation of three or four wooden or metal posts positioned so that wayward
objects will strike the posts rather than the well casing (Fig.
3.41
photo inset).
Lastly, make measurements of the stickup length and the distance to the bottom of
the well. Survey to the top of the well casing and determine the spatial coordinates
of the well with a global positioning system (GPS) or similar device.
Piezometer Installation
The piezometer will be installed in a location where the wetland bed is beneath the
water surface. In this situation, the piezometer will indicate the vertical hydraulic
