GROUND WATER IN NORTH-CENTRAL TENNESSEE

HUMPHREYS COUNTY
157
prove inadequate as a source of municipal supply throughout the year. A well
300 feet deep drilled some years ago near the pressure tank on the hilltop south of
Waverly failed to find a water-bearing bed adequate for the demand of the town
and has been abandoned. No deeply buried water-bearing stratum is known to
underlie the vicinity. The chief advantages of drilled wells in the limestone
would be that the capacity of the source would presumably not show large seasonal
variations and that the wells could be located in or near the town, with a consequent
saving in capital expenditure for pipe lines. The chief disadvantages would be
that the uncertainty of developing wells of adequate yield renders a comprehensive
and costly program of exploratory drilling advisable before permanent construc­
tion and that a supply adequate for emergencies and for future expansion of the
town may not be obtainable.
At Waverly, Trace Creek flows over a rock bed cut on limestone, but farther
east its valley embraces a flat alluvial plain underlain by stream gravel. This
material supplies several household wells from 11 to 25 feet deep, such as Nos. 168
and 170 (pp. 159-160), although none is known to have a capacity approaching the
requirements of a municipal supply. The thickness of gravel that lies below the
water table is unknown; moreover, much of the gravel is poorly sorted as to size and
contains considerable sand and clay. Hence the permeability of the material is
presumably relatively small and the size of the ground-water reservoir is unknown,
although the total volume of water in the gravel may be considerable and a
thorough test of this potential source is warranted. Test pits or wells should
first be dug or drilled systematically over the chosen site to ascertain the depth to
bedrock and the thickness of water-bearing gravel below the water table. Second
if the gravel is found to be reasonably thick, its water-yielding capacity should be
determined from a test well or wells at the point where the greatest thickness of
water-bearing gravel exists. This test should be made when the water table is at a
a low stage.
If the results of preliminary tests are satisfactory, one or more permanent wells
should be excavated to bedrock by digging while using temporary lagging to sup­
port the walls of the hole or by sinking a temporary casing by well-drilling
methods. It is essential that the well be sunk to bedrock in order that the largest
possible yield may be obtained. A permanent casing should then be set axially
in the well and extending from the surface to the bottom of the well, with that
part of the casing which penetrates the water-bearing beds thoroughly perforated.
This casing should be 12 inches or more in diameter; preferably it should be per­
forated in the shop before being placed in the well. The space between the
permanent inner casing and the temporary outer casing should then be filled with
well-rounded gravel that has been screened so that all the particles are more than
a quarter of an inch but less than half an inch in diameter. As the gravel is
inserted the outer temporary casing should be raised and the well pumped vig­
orously, so as to draw the fine sand from the surrounding stream gravel as com­
pletely as possible. The rate of pumping should be increased gradually, and
pumping should be continued until the well attains its maximum yield and no
more fine material can be drawn into the well. More screened gravel may be
added in the space outside the perforated casing until a condition of stability is
attained. By properly developing the well in this way an envelope of highly
permeable clean gravel is created about the perforated casing, so that for a given
draft the water enters the well with the least possible velocity and consequently
with the minimum burden of entrained sand and silt. An air-lift pump is
especially well suited to developing a well, inasmuch as its yield can be changed
easily to suit the water-yielding capacity of the gravel. From the yield of the
well determined in this way when the water table was at its lowest stage the
number of wells necessary can be determined.