Quarterly samples of Groundwater from 45 monitoring wells in Pleistocene limestone beneath Florida Bay, the reef tract and on the Keys, was sampled and analyzed quarterly. Well depths range from 5 to 20 m. Nutrients NO2, NO3 and NH4 in the offshore ground water were elevated about 10 times that of sea water and NH4 increased progressively up to 40 times that of seawater under coral reefs 8 to 10 km offshore. Salinity, except for shallow wells onshore, ranged from 36 to 42 ppt and the waters were generally anoxic. Onshore ground waters were equally saline except in shallow near-surface wells, where salinity measured 10 ppt or less. Fecal bacteria were identified in saline ground water from both onshore and offshore wells.

Subsurface hydrology is controlled by lithology, buried subaerial unconformities, and by Holocene carbonate mud overlying karstic Pleistocene grainstones and reef deposits. Tidal pumping, sufficient to raise water 20 cm above sea level, suggests leakage of nutrients and bacteria into surface marine waters, especially nearshore where there is no overlying Holocene sediment. Higher sea level in Florida Bay causes ground water to flow through the Keys and likely incorporates nutrients and bacteria from the 30,000 septic-tank drain fields and approximately 700 shallow sewage water injection wells. Because flow is toward the reef tract, both natural and anthropogenic nutrients may cause observed blooms of benthic algae and coral diseases.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1995)

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