Statistical analysis relating well yield to
construction practices and siting of wells in the Piedmont and Blue
Ridge Provinces of North Carolina
Water-Supply
Paper 2341-A
By C.C. Daniel, III
Abstract
A statistical analysis was made of data from more than 6,200
water wells drilled in the fractured crystalline rocks of the
Blue Ridge, Piedmont, and western edge of the Coastal Plain
where crystalline rocks underlie sediments at shallow depths.
The study area encompassed 65 counties in western North
Carolina, an area of 30,544 square miles, comprising nearly
two-thirds of the State. Additional water supplies will be
needed in western North Carolina as population and industrial
development continue to increase. Ground water is an attractive
alternative to surface water sources for moderate to large
supplies. The statistical analysis was made to identify the
geologic, topographic, and construction factors associated with
high-yield wells. It is generally held that the crystalline
rocks of Blue Ridge and Piedmont provinces yield only small
amounts of water to wells, that water is obtained from vertical
fractures that pinch out at a depth of about 300 feet because of
lithostatic pressure, and that the function of a larger diameter
well is primarily for storage. These concepts are reasonable
when based upon the average well drilled in these rocks: a
domestic well, 125 feet deep, 6 inches or less in diameter, and
located on a hill or ridge. However, statistical analysis shows
that wells in draws or valleys have average yields three times
those of wells on hills and ridges. Wells in the most productive
hydrogeologic units have average yields twice those of wells in
the least productive units. Wells in draws and valleys in the
most productive units average five times more yield than wells
on hills and ridges in the least productive units. Well diameter
can have significant influence on yield; for a given depth,
yield is directly proportional to well diameter. Maximum well
yields are obtained from much greater depths than previously
believed. For example, the average yield of 6-inch diameter
wells located in draws and valleys can be expected to reach a
maximum of about 45 gallons per minute at depths of 500 to 525
feet; for similarly located 12-inch diameter wells, the average
yield can be expected to reach a maximum of about 150 gallons
per min at depths of 700 to 800 feet.
Citation:
Daniel, C.C., III, Smith, D.G., and Eimers, J.L., 1989, Statistical analysis relating well yield to construction practices and siting of wells in the Piedmont and Blue Ridge provinces of North Carolina, in Ground-water resources of the Piedmont-Blue Ridge Provinces of North Carolina: U.S. Geological Survey Water-Supply Paper 2341-A, 27 p.
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