Science of Water : Concepts and Applications

36 The Science of Water: Concepts and Applications
3. Vulnerability assessment
a. Wellhead area = 1000-ft radius from the well
b. What is the general land use of the area (residential, industrial, livestock, crops, undeveloped,
other)?
c. What are the geologic conditions (sinkholes, surface, subsurface)?
TYPE OF WELLS
Water supply wells may be characterized as shallow or deep. In addition, wells are classifi ed as
follows:
1. Class I—cased and grouted to 100 ft
2. Class II A—cased to a minimum of 100 ft and grouted to 20 ft
3. Class II B—cased and grouted to 50 ft
Note: During the well-development process, mud/silt forced into the aquifer during the drilling
process is removed, allowing the well to produce the best-quality water at the highest rate from the
aquifer.
SHALLOW WELLS
Shallow wells are those that are less than 100 ft deep. Such wells are not particularly desirable for
municipal supplies since the aquifers they tap are likely to fl uctuate considerably in depth, making
the yield somewhat uncertain. Municipal wells in such aquifers cause a reduction in the water table
(or phreatic surface) that affects nearby private wells, which are more likely to utilize shallow strata.
Such interference with private wells may result in damage suits against the community. Shallow
wells may be dug, bored, or driven.
Dug Wells
Dug wells are the oldest type of well and date back many centuries; they are dug by hand or by a
variety of unspecialized equipment. They range in size from approximately 4–15 ft in diameter and
are usually about 20–40 ft deep. Such wells are usually lined or cased with concrete or brick. Dug
wells are prone to failure from drought or heavy pumpage. They are vulnerable to contamination
and are not acceptable as a public water supply in many locations.
Driven Wells
Driven wells consist of a pipe casing terminating at a point slightly greater in diameter than the
casing. The pointed well screen and the lengths of pipe attached to it are pounded down or driven in
the same manner as a pile, usually with a drop hammer, to the water-bearing strata. Driven wells are
usually 2–3 in. in diameter and are used only with unconsolidated materials. This type of shallow
well is not acceptable as a public water supply.
Bored Wells
Bored wells range from 1 to 36 in. in diameter and are constructed with unconsolidated materials.
The boring is accomplished with augers (either hand or machine driven) that fi ll with soil and then
are drawn to the surface to be emptied. The casing may be placed after the well is completed (in
relatively cohesive materials), but must advance with the well into the noncohesive strata. Bored
wells are not acceptable as a public water supply.