REPORT OF INVESTIGATIONS NO. 38 25
Other investigators (Jacob, 1943) have shown that in humid areas where the water levels are not affected by pumping, the water table in general fluctuates with the accumulated departures from average rainfall. A graph of the accumulated departures from average rainfall at Venice is shown in figure 8. The graph shows that at Venice the accumulated departures from average rainfall in July 1962 were about average for the period 1955-62. Accordingly, an elevation of 7.6 feet above sea level for the water table is probably about average.
The water table probably fluctuates 6 or 7 feet over a period of several years. The water table under nonpumping conditions, therefore, probably drops to as low as 4 or 5 feet above sea level and rises to as high as 10 or 12 feet above sea level. The piezometric surface of the first artesian aquifer under non-pumping conditions, being higher than the water table, probably will not drop below about 5 feet above sea level except for periods of a few days or weeks. The elevation of the design piezometric surface is, therefore, considered to be 5 feet above sea level.
Computing the drawdown: The drawdown in the vicinity of a pumping well after an infinite period of pumping may be computed from the following formula developed by Hantush and Jacob (1955):
sin (Q/2T) K(, (r/B)
where Q is the discharge of the well; T is the coefficient of transmissibility; Ko is the modified Bessel function of the second kind and of zero order; r is the distance from the center of the well to any point in the field; and B = (Tm'/P') where P'/m' is the coefficient of leakage. The drawdown at any point near a group of pumping wells is equal to the sum of the drawdowns of the individual wells at that point.
The formula assumes that the water table will not be lowered by the leakage from the water-table aquifer into the first artesian aquifer. However, the leakage will result in some lowering of the water table, especially near the pumping wells. This lowering will result in reduced leakage to the first artesian aquifer, and consequently the drawdown will be greater than that computed from the formula.
For the purpose of computing the drawdown in the vicinity of the waterway, the supply wells are assumed to draw water from both the first and the second artesian aquifers. The coefficient of transmissibility (T) was assigned a value of 5,500 gpd per ft, and the coefficient of leakage (P'/m') was assigned a value of 1.3 x 10-3