to the shallow groundwater table 

 forming freshwater lenses (Parker 

 et al. 1955, Klein 1970, Chesher 

 1974, and Hanson 1980). Figure 42 

 shows a vertical profile of the 

 freshwater lens on Big Pine Key dur- 

 ing March 1977 (Hanson 1980). The 

 size of freshwater lens is control- 

 led by at least six factors as out- 

 lined by Hanson (1980) and Klein 

 (1970): (1) freshwater recharge 



(rainfall); (2) freshwater discharge 

 (seepage, pumpage, runoff, andevap- 

 otranspiration) ; (3) response to 

 tidal fluctuations; (4) proximity to 

 saltwater bodies; (5) permeability 

 of the subsurface materials; and (6) 

 elevation of the island above sea 

 level. As expected, the dimensions 

 of the freshwater lenses increase 

 and decrease in response to precipi- 

 tation and therefore wet and dry 

 season conditons (Klein 1970, Hanson 

 1980). Parker et al. (1955) de- 

 scribed freshwater lenses on Key 

 West and Big Pine Keys as increasing 

 in thickness during the rainy sea- 

 son. During the dry season, even 

 without pumpage, the freshwater 

 tends to disappear quickly by seep- 

 age to the sea and by evapotranspi- 

 ration processes. As mentioned 



earlier, the Florida Keys ground- 

 water is intricately linked to the 

 sea; thus the water table of each 

 key undulates with the tides (Parker 

 et al. 1955, Klein 1970, Chesher 

 1974, Hanson 1980), accelerating the 

 lateral seepage of freshwater to the 

 sea. 



The development of canals for 

 housing (Chesher 1974), mosquito 

 control (Alexander and Dickson 1972, 

 Hanson 1980), and ports (Chesher 

 1974) has short-circuited the previ- 

 ous residence times of the island's 

 freshwater supplies. Alexander and 

 Dickson (1972) report that a ditch- 

 ing program on Big Pine Key for 

 mosquito control had within two 

 years (1964 and 1965) ditched 4,976 

 acres of the 6,000 acres available. 



MIAMI OOLITE 



♦ 320 Chloride Concentration; 

 Milligrams per liter ; 



Land Surface 



10,000 20,000 30,000 40,000 50,000 



CONDUCTANCEjmtcro mho per cm. at 25°CI 



Figure 42. Conductivity and chloride 

 concentration in relation 

 to geology on Big Pine 

 Key, Florida (adapted 

 from Hanson 1980). 



These ditches are approximately 

 4.9 m (16 ft) wide, vertical-sided, 

 and are "deep enough to allow tide- 

 water to flow in them from the near- 

 by open sea". Consequently such 

 ditches tend to accelerate runoff, 

 increase saltwater intrusion, and 

 thus decrease the freshwater lens on 

 the island. 



Evaporation/Transpiration 



Of the two processes of evapo- 

 ration and transpiration, the former 

 is far more influential in the Flor- 

 ida Keys hydrologic budget. Tran- 

 spiration tends to be localized and 

 confined to individual islands, 

 where it exerts some control of 

 freshwater lenses (Hanson 1980). 

 Evaporation, on the other hand, 

 exerts an important effect on the 



96 



