Page 148 - Water and wastewater engineering

WELLS 4-3

quality standards, the strata that serve as aquifers may differ both in the constituents and the
concentration of the constituents.

Adverse Resource Impact
As noted in Chapter 3, one method of withdrawing water from a surface water body is by the
installation of a gallery or a Ranney well system. Individual wells may also cause water from a
surface water body to flow out of the water body into the aquifer. If the pumping rate is too high,
this may lower the water level in the surface water body enough to have a negative impact on its
aquatic life.

4-3 WELL PROTECTION

The elements employed to protect the well include:
• Isolation from sources of contamination.
• Sanitary considerations in well construction.

• Provision of structural protection.
Although the provisions of the Michigan Safe Drinking Water Act (MSDWA, 1976) that
are discussed in the following paragraphs serve as guidelines for good practice, individual state
requirements vary, and these should be consulted as a matter of normal engineering practice.

Isolation
For the purpose of this discussion, the Michigan Safe Drinking Water Act definitions of different
types of water supplies discussed in Chapter 2 are classified as follows:

• Type I: all community supplies.
• Type II: all noncommunity supplies.

• Type IIa: average daily water production for the maximum month equal to or greater
3
than 75 m /d.
3
• Type IIb: average daily water production for the maximum month less than 75 m /d.

• Type III: all public supplies that are not Type I or Type II.
This classification is used to set isolation distances based on the probable radius of influence
of the well and the risk to the population exposed. The recommended isolation distances are
shown in Table 4-1 on page 4-5.

Well Construction
Geologic conditions dictate two general types of well construction. A well that taps an aquifer
of water-bearing sand is cased through the overburden and screened in the water-bearing sand as
shown in Figure 4-1 a (Johnson, 1975). A well that taps an aquifer of consolidated rock consists