GROUND WATER IN NORTH-CENTRAL TENNESSEE

180 GROUND WATEK IN NORTH-CENTRAL TENNESSEE
No. 435, which, is about 200 feet southeast of the boiler room, derives
most of its water from a cavernous zone in the Murfreesboro limestone
between 50 and 65 feet below the surface, although several water­
bearing beds of small capacity were penetrated between this cavernous
zone and the bottom of the well, at a depth of 217 feet. The well is
pumped by air lift. It is reported by the driller that when first
tested in May, 1927, the well yielded about 400 gallons a minute for 24
hours, the drawdown indicated by the difference between starting and
running pressures in the air line being approximately 70 feet. Hence
the specific capacity is rather small about 6 gallons a minute for each
foot of drawdown. During this first test at least 1 cubic yard of fine
silt and corroded fragments of limestone as large as 2 inches in diame­
ter were discharged from the well, indicating that the solution chan­
nels that transmit the water are partly clogged with such debris.
During the period July to September, 1927, the well was pumped
almost continuously, and its yield ranged approximately between 75
and 150 gallons a minute. A part of this variation was perhaps due
to changes in the efficiency of the air4ift pump as the water level
declined, but most of the variation was presumably due to seasonal
fluctuations in the ground-water supply. This presumption is
strengthened by the fact that the yield of the well increases about 24
hours after a heavy local rain. It is noteworthy that this well is
approximately on the projected trace of prominent joints that cut the
Murfreesboro limestone at the orifice of the Murfreesboro city spring
(No. 439, p. 187), about a mile to the southeast. Another well (No.
434), about 150 feet northwest of No. 435, passed through the same
water-bearing zones; its ultimate capacity is only about 5 gallons a
minute. A third well (No. 433), which is 650 feet deep, penetrates a
small water-bearing channel in the Murfreesboro limestone at a depth
of 80 feet; a small amount of water may also be derived from a depth
of about 350 feet. The rocks at the projected horizon of the St.
Peter (?) sandstone were probably penetrated, although little if any
water was found in them. The total capacity of this well, when
pumped continuously, is about 12 gallons a minute, with a drawdown
of about 180 feet. A fourth well (No. 436) comprises a sump 18 feet in
diameter and 23 feet deep and an 8-inch drilled hole extending 63 feet
below the bottom of the sump. Water-bearing zones in the limestone
were penetrated 46 and 55 feet below the surface. Even after the
well had been shot with dynamite at the bottom, its total capacity
was only about 10 or 15 gallons a minute, of which about 5 gallons
seeped from fractures in the walls of the sump and presumably
originated in a near-by perennial creek. In general, the greatest
likelihood of wells of large capacity exists where the course of a cavern­
ous zone in the limestone is indicated at the surface by a linear
arrangement of sink holes or along a line parallel to master joints