Environmental Engineering Reference
In-Depth Information
formation from entering the well. The filter pack contains granular sand or gravel of uni-
form size and is placed between the aquifer formation and the well screen. The filter pack
aids in screening out the fine particles within the aquifer mobilized by well pumping, so
this device is important in aquifers composed of unconsolidated sediments that tend to be
poorly sorted. Removing solid particles before they enter the well keeps the groundwater
sample free from analytes of concern that may be sorbed onto sediment particles.
Many states require that the filter pack and the slot size of the well-screen be “engi-
neered,” based on the textural characteristics of the aquifer material. This is accomplished
by performing a sediment size analysis of the aquifer material and creating a grain-size
distribution curve. The desirable percentage of open slots in a well screen should be at
least equal to the porosity of filter pack. Although desirable, this is not always achievable,
particularly if the aquifer material consists of fine sand. Moreover, the cost of performing a
grain size analysis of aquifer material to optimize the filter pack and the well screen is fre-
quently too great, especially considering the cost of having the drill rig and crew standing
by while this analysis is being performed. As a result, we cannot overstate the importance
of contracting an experienced drilling company that has already successfully installed
monitoring wells in the area where you are working.
An annular or bentonite seal composed of bentonite clay is placed above the filter pack
and above the well screen surrounding the casing. This placement of the seal helps to
minimize potential infiltration from surface water and other fluids via the outside casing
of the well.
Most monitoring well screens are 2 or 3.5 m (5-10 ft) long and 5 cm (2 in.) in diame-
ter. Many local regulatory agencies require well screens to be no more than 2 m (5 ft)
in length. Monitoring wells installed in an unconfined aquifer generally straddle the
water table, and these usually have a 3.5 m (10 ft) long screen (if allowed by the regulatory
agency) to allow for the seasonal fluctuation of the water table. Half the well screen (2 m)
is installed below the water table, with the other half above the water table. Monitoring
wells installed entirely below the water table (termed fully submerged) have well screens
2 m in length.
Installing more than one monitoring well per location may be required in cases of sig-
nificant aquifer thickness of over 6-8 m (20-25 ft). Multiple monitoring wells at a single
location are installed to evaluate the groundwater quality vertically through the length
of the aquifer. Figure 4.24 shows a cross section of an aquifer composed of sand with a
saturated thickness of approximately 25 m (≈75 ft). In addition to the monitoring wells
installed straddling the water table, three monitoring wells were installed as a set at
three depths within the aquifer. One set straddles the water table, one set was placed
at the approximate mid-point of the aquifer, and the last set was located at the bottom
of the aquifer. However, in some cases, even installing three monitoring wells at one
location may not be sufficient for gathering enough data to adequately understand the
inner dynamics of a contaminant plume. An example of this is where multiple plumes are
present. In this scenario, a technique termed High Resolution Aquifer Profiling is used
and involves collecting numerous groundwater samples vertically at several locations
to develop a detailed three-dimensional depiction of a groundwater plume or plumes
(Welty et al. 2010).
Requirements for the design and installation of monitoring wells exist and vary state
by state and often county to county. The differences occur for items such as obtaining a
permit to install, location, depth, seals, certification, length of well screen, diameter of well
screen, placement of screen, construction materials, and installation procedures. Before
installing a monitoring well, state and local requirements should be examined.
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