REPORT OF INVESTIGATIONS NO. 42


 EFFECTS OF WATER IMPOUNDMENT IN
GREEN SWAMP RESERVOIR ON GROUND-WATER LEVELS

 Comparison of figures 31 and 54 show that the proposed Green
Swamp Reservoir is underlain at shallow depth by the Floridan
aquifer. A reconnaissance of the area indicated that limestone of
the aquifer crops out for about 6 miles along the north-south
portion of the proposed levee. The head created by the reservoir
will cause additional amounts of water to move through the aquifer
beneath the levee.
 The principal factors controlling the underseepage are the
 horizontal and vertical permeabilities of the materials beneath the
 reservoir and the percentage of the reservoir area immediately
 underlain by the Floridan aquifer.
 Aquifer tests in adjacent areas indicate that the coefficient of
 transmissibility varies considerably from point to point. The
 coefficient of transmissibility in the reservoir area is estimated to
 range from 125,000 gpd/ft to 500,000 gpd/ft. Significant amounts
 of underseepage will probably occur along about 14 miles of the
 north-south portion of the proposed levee because the predominant
 direction of water movement is from the east to the west.
 Underseepage will be greatest in the channel of the Withlacoochee
 River because the channel is well incised into the aquifer.
 The hydraulic gradients scaled from the piezometric maps
 (figs. 35 and 36) were used to compute the existing wet and dry
 period flow of ground water beneath the area of the proposed levee.
 During the wet period (1959), the average elevation of the
 piezometric surface beneath the 14-mile length of proposed levee
 was about 95 feet, the average hydraulic gradient across the
 proposed levee was about 2 feet per mile, and the estimated mean
 daily underflow ranged from about 5 to 20 cfs. During the dry
 period (1962), the average elevation of the piezometric surface
 beneath the 14-mile length of proposed levee was about 89 feet,
 the average hydraulic gradient across the proposed levee was about
 2.6 feet per mile, and the estimated mean daily underflow ranged
 from about 10 to 30 cfs.
 In order to estimate future underflow it was assumed that the
 amount of water that seeps into the limestone will raise the
 piezometric surface to the same level as that of the conservation
 pool (100 ft). If the impoundment occurred during a wet period,
 then the piezometric surface would rise from 95 to 100 feet, the
 gradient would increase from 2 to 7 feet per mile, and the estimated


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