water are left behind in small, 

 closed depressions. Before the water 

 can seep into the ground, partial 

 evaporation takes place and salinity 

 increases. Eventually, the hyper- 

 saline water migrates downward, en- 

 ters the water table, and becomes 

 part of the ground water system. 

 This process has been going on since 

 the emergence of the area from the 

 sea, so that the salinity of the 

 ground water in the shallow aquifer 

 in this area is greater than normal. 



A second possible factor leading 

 to hypersalinity is evapotrans- 

 piration. The water table is very 

 close to the land surface virtually 

 all of the time, so that evapo- 

 transpirative losses are high. 

 Annual potential evapotranspiration 

 is more than 53.1 inches (1,350 mm). 

 The actual is estimated to be 39.4 

 inches (1,000 mm). While average 

 annual precipitation at Fort Myers is 

 about the same as the potential 

 evapotranspiration, studies conducted 

 on Marco Island (Weinstein et al. 

 1977) indicate that the coastal area 

 has even less rainfall. Precipi- 

 tation data for the years 1973 

 through 1977 from AES laboratory and 

 Rookery Bay sites, which bracket the 

 study area, indicate that the average 

 rainfall is 8 inches (203 mm) less 

 than at Fort Myers. Also, rainfall 

 is not evenly distributed throughout 

 the area so that there are periods 

 when ground water recharge does not 

 occur. Most of the precipitation 

 occurs from May through October. For 

 the remainder of the year very little 

 rain falls. During these dry peri- 

 ods, evaporation from the water table 

 undoubtedly occurs. Thus, a combina- 

 tion of the high potential evapo- 

 transpiration and the evaporative 

 losses that undoubtedly occur during 

 the dry months is believed to be one 

 of the causes of hypersaline ground 

 water. 



Water from the deep and shallow 

 zones at Well 7, located on the south 

 side of Lake Marco Shores, had chlor- 

 ides of 17,200 and 616 mg/1, respect- 

 ively, on March 6, 1980, and 16.215 

 and 353 mg/1 on May 1, 1980. Water 

 from the deeper zone is less saline 

 than sea water. The comparatively 

 low chloride concentration present 

 in water from the shallow well is a 

 reflection of the presence of a lens 

 of fresh ground water resulting from 

 lake and golf course construction. 

 Previously the area was a coastal 

 marsh with conditions similar to 

 those presently existing immediately 

 to the north of the lake where hyper- 

 saline ground water exists. Conse- 

 quently, fresh ground water could not 

 have been present in such an environ- 

 ment prior to creation of the land. 



When the golf course and airport 

 were built, a mound or island of 

 sandy material was created. Rainfall 

 and golf course irrigation water per- 

 colated downward, eventually creating 

 a lens of fresh ground water float- 

 ing on more dense, saline ground 

 water. Although potential evapo- 

 transpiration and rainfall are nearly 

 equal in the area, there are times 

 when rainfall exceeds the require- 

 ments of evapotranspiration and the 

 surplus enters the ground, particu- 

 larly where the soil is sandy. It 

 is estimated that on a long term 

 basis six inches of the normal yearly 

 rainfall serves as recharge to the 

 water table aquifer on the golf 

 course, in addition to that which 

 occurs as a result of irrigation. 



Thus, over the years, a lens of 

 comparatively fresh ground water has 

 developed in the golf course area. 

 The lens is dynamic; it is (on the 

 average) constantly being replenished 

 by rainfall while, at the same time, 

 fresh ground water is being dis- 

 charged to shallow portions of the 



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