REPORT OF INVESTIGATIONS No. 40


714-4, 500 feet west of the Escambia River, increased from about 6 to
more than 1,100 ppm.
 During 1957 and 1958, the chloride content of the Escambia River at
the Chemstrand nylon plant cooling water intake was above 25 ppm for
about 25 percent of the time. Thus, the salt-water front had advanced
at least 7 miles above the mouth of the river.
 The flow of the Escambia River during the summer of 1955 was
sufficient to keep the salt-water front downstream from the Chemstrand
nylon plant. In September 1955, the flow of the Escambia River de-
creased and salt water probably occurred at the plant. As the ground-
water level was below river level, salt-water encroachment began. The
time required for the river water to move to Chemstrand well (035-
714-4) 500 feet from the river was calculated. Assume an average draw-
down of 20 feet (which would also be about 20 feet below sea level); an
aquifer thickness, m, of 300 feet; and a porosity, p, of 30 percent. The
coefficient of transmissibility, T, is 150,000 gpd per foot. The hydraulic
gradient, g, would be 20 feet in 500 feet or 0.04. These values and the
following calculation would give the minimum time required for river
water to move to the well. No allowance is made for the time required
for the water to move through the clay beds in the sand-and-gravel
aquifer.
 V- Tg
 7.48 mp
 (150,000) (0.04)
 V (150,000) (0.04) 8.9 feet per day
 S(7.48) (300) (0.30)
 500 feet
 = 56 days
 8.9 feet per day -
 The time required for the salty water from the river to move to the
 well field would be about two months and the salt water in the well
 could have been expected sometime in November 1955. The salt water did
 not show up in the well until almost one year later. This lag can be ex-
 plained by the time required for the water to pass through several thin
 clay beds.
 Figure 45 shows the lowering of ground-water levels and their rela-
 tion to the level of the Escambia River. The figure is a cross section from
 Cantonment eastward to the Escambia River. Early data imply that the
 ground-water level close to the river was 25 to 30 feet above sea level
 in 1940. The water table was 12 to 15 feet above sea level near the river
 in 1951 and about 1.5 mgd per mile was moving toward the Escambia