GROUND WATER IN NORTH-CENTRAL TENNESSEE

88 GROUND WATER IN NORTH-CENTRAL TENNESSEE
at a sink hole, well, or temporary test pit, and then to take samples
of water from all possible points of discharge in springs or wells and
examine them for traces of the dye.
The sodium salt of fluorescein is sold under the commercial name
"uranin." Its solution has a characteristic fluorescent green color
by reflected light, and it can be detected by the eye in a solution whose
concentration is as low as 1 part of the dye in 40,000,000 parts of clear
water, if proper technique is followed.60 When the concentration of
the solution is very low the characteristic color is seen best if the sam­
ple is placed in a test tube or a long glass tube and viewed in full
daylight before a white background by looking along the axis of the
tube. If the examination is made by a suitable fluoroscope the limit
of visibility has been placed at 1 part of pure uranin in 10,000,000,000
parts of clear water.
Uranin is altogether harmless in water used for domestic and other
common purposes. It is not decolorized by contact with leached sand
Or gravel and has been shown to persist as long as three years in such
material. 61 However, dilute solutions of uranin may be partly decol­
orized by calcareous soils or by waters such as those that generally
issue from limestone, which contain a large amount of calcium car­
bonate or other calcium salts, and may be completely decolorized by
peaty formations and by mineral acids other than carbon dioxide.
A solution of uranin is somewhat heavier than pure water, so that
it sometimes settles to the bottom of stagnant pools until the stream
is agitated, as by the sudden influx of a larger amount of water.
Consequently, the rate of movement of the dye may be slightly less
than that of the water, and one dose of the dye may produce distinct
color two or more times at a related point of discharge.
On the other hand, it was found in the experimental work of the
United States Public Health Service at Fort Caswell, N. C., that
uranin floated on the water table in unconsolidated sand and was even
drawn up into the capillary fringe and there trapped.62
The dye is most readily handled as a solution containing 2 or 3
ounces of uranin for each gallon of water, which is poured into the
selected well, sink hole, or other opening at the rate of about 1 ounce
of uranin an hour for each 500 gallons a minute of estimated under­
ground flow. Dosage at this rate should be continued for at least an
hour where the distance to the most remote point of observation is
less than 1 mile, or for a longer period where the distance is greater.
If the solution is poured into a dry sink or natural well, enough addi­
tional water should be poured in to assure the uranin being carried
down to the water table. Before the introduction of the uranin, a
80 Stiles, C. W., and others, Experimental bacterial and chemical pollution of wells via ground water and
the factors involved: U. S. Public Health Service Hyg. Lab. Bull. 147, pp. 84-85* 1927.
«Idem, pp. 86-86.
*» Idem, p. 79.