REPORT OF INVESTIGATIONS No. 40


aquifer in southern Escambia and Santa Rosa counties ranges from 840
to 920F. The wells that tap this aquifer are from 900 to 1,500 feet deep.
 The temperature of ground water in this area usually increases about
1F. for each 52 to 85 feet of depth. For example, the geothermal gra-
dient 9 miles southwest of Pensacola, as shown by measurements made
in an oil test hole, is about l0. for each 81 feet of depth down to 12,500
feet. The temperature at the bottom of the hole was 2220F.

 SPECIFIC CAPACITY
 The specific capacity is used to indicate the amount of water, in
 gallons per minute, that can be obtained from a well for each foot of
 drawdown of water level in the well. The specific capacity is obtained
 by dividing the yield of the well in gallons per minute by the difference
 of the static water level and the pumping water level in feet. Factors
 that affect the yield of wells include: (1) the diameter of the well;
 (2) depth of aquifer penetrated; (3) transmissibility of the aquifer;
 (4) efficiency of the pump; (5) amount of well development; (6)
 amount and size of well screen (if any); and (7) the friction loss
 within the well.
 An example of the specific capacity of wells drilled into the sand-
 and-gravel aquifer can be shown by data from 8 wells at the Chemstrand
 Corporation nylon plant. The wells are constructed similarly with 24-
 inch casing at the surface, 16-inch casing in the middle, and 12-inch
 screen at the bottom. The amount of well screen is usually 110 feet and
 the wells are pumped at 1,500 gpm for 24 hours. The specific capacity
 ranged from 39.5 to 76.8 gpm per foot of drawdown and the average
 specific capacity was 53 gpm per foot of drawdown.
 Little information is known about the specific capacity of wells from
 the Floridan aquifer in the area of study. However, large-capacity wells,.
 10 to 16 inches in diameter, drilled into the Floridan aquifer in southern
 Okaloosa and Walton counties had a specific capacity ranging from 10 to
 100 gpm per foot of drawdown. The average specific capacity was 88
 gpm per foot of drawdown. Similar values could be expected from wells
 drilled into the Floridan aquifer in Escambia and Santa Rosa counties.

 QUANTITATIVE STUDIES
 The withdrawal of water from an aquifer creates a depression in the
 water table or artesian pressure surface around the point of withdrawal.
 This depression generally has the form of a cone with its apex down
 and is referred to as the cone of depression. The amount by which the