WATER RESOURCES OF ORANGE COUNTY 95
some locations, very large (4,000 gpm or more) yields can be obtained from the limestone, but most high yield wells also' penetrate the underlying dolomitic limestone. However, many domestic wells and small public supply wells draw all their water from the limestone section of the upper zone. The municipal supply wells for the Cities of Orlando and Winter Park are developed in the lower (1,100-1,500 feet) producing zone. These wells generally yield 3,000 to 5,000 gpm with 10 to 25 feet of drawdown.
The relatively impermeable zone (600 to 1,100 feet below the land surface) separating the two major producing zones is composed of layers of relatively soft, mealy limestone and dolomitic limestone. It contains some water-bearing layers, but generally this separating zone yields much less water than the zones above and below it. In many parts of the country the separating zone would be considered a good aquifer; but because much larger supplies can be obtained above and below this zone in Orange County, very few wells are developed in it.
The occurrence of reported cavities is shown in figure 40. The number of cavities shown for different depths actually does not represent the true distribution of the cavities because many more wells penetrate the upper part of the aquifer than penetrate the lower part. However, the illustration does show that although cavities have widespread vertical distribution, they are more prevalent in some zones than in others.
Interrelation of Zones
The interrelation of the upper and lower producing zones is of vital importance to the people of Orlando and Winter Park because excess surface water is disposed of in the upper zone while most of the municipal water supplies are developed from the lower zone. Contaminated water can enter the upper producing zone through the numerous drainage wells (See section on drainage wells, page 83) and it is important to know if this contaminated water can move into municipal supply wells.
It has been postulated that some dense dolomitic beds between 400 and 600 feet in the upper zone might be continuous and act as an impervious layer to protect the lower zone. To test this idea, a well was drilled at Lake Adair in Orlando into the upper zone of the aquifer adjacent to an existing well in the lower zone. The shallow well is cased to the top of the aquifer (105 feet) and bottomed at the top of the hard dolomitic zone at 400 feet. The