More than 80 shallow (5-20 m) water monitoring wells have been installed off the Florida Keys. Twenty are configured in a cluster that has been used for three tracer studies. The studies provide new data on the subsurface movement, flow rate and seepage of saline ground water in Florida Bay and the reef tract. Three dye tracer tests and one sulfur hexafloride (SF₆) tracer study show that 1) flow rates range from 1 to 3.5 m/day, and 2) flow is mainly eastward (perpendicular to the Keys) and away from Florida Bay. Reverse flow was measured during a period of strong easterly winds.

Measurement of pressure heads in monitoring wells indicates that tidal pumping, combined with higher average sea level in Florida Bay than oceanside, is the major cause of cross-Keys ground water movement and dispersal. Low tide east of the Keys (Atlantic side) results in a 1 m or more head on the bay side of Key Largo. Under these conditions, ground water head pressure under Florida Bay is negative (even though surface water head is positive), causing eastward "downhill" flow toward the Atlantic. Negative head pressure develops because the upper meter or two of the limestone under Florida Bay is relatively less permeable than the underlying rock. Thus, water in the permeable zone can flow "downhill" toward the Atlantic faster than water can leak through the relatively impermeable surface zone. High tide on the Atlantic side of the Keys produces the opposite situation. Under these conditions, subsurface flow is generally "downhill" toward Florida Bay. Because mean sea level in Florida Bay is 10 to 20 cm above mean Atlantic sea level, net flow is toward the Atlantic. The exception is during periods of sustained easterly winds combined with high spring Atlantic tides. Water is "blown" westward in Florida Bay, reducing water level by as much as 30 cm along the west side of the Florida Keys while water is piled 30 cm or more on the eastern side of the Keys. During these events, which can last several days to weeks, ground water flow is mainly westward into Florida Bay.

Because the upper 1 to 2 m of limestone are relatively impermeable compared to the underlying limestone, tidal springs occur wherever there are small sinkholes, fractures, or manmade breaks in the upper surface, such as our monitoring wells, canals, or dredged channels. These saline tidal springs are often reported as freshwater boils. Boils occur on both sides of the Florida Keys but only during the 6-hour period when tidal phase produces positive ground water heads. Slower seepage, not visible as boils, also occurs through smaller pores but only where an impermeable blanket of modern sediment is absent.

Tracer tests in well clusters also show upward movement associate d with lateral transport. Tracers placed in the deep 13.6 m (45 ft) center well invariably appear in the shallow 6 m (20 ft) peripheral wells faster than in the 13.6 m (45 ft) peripheral wells, regardless of flow direction. In addition, tracers injected into the shallow 6 m (20 ft) central wells first appear in shallow peripheral wells. These observations indicate that deeper ground water moves more slowly than near-surface ground water and ground water migrates both laterally and upward. Near-surface ground water eventually seeps into the overlying water column. These observations are compatible with systematic ground-water tidal head measurements. In all wells tested, tidal pumping heads have been found to decrease with depth.

New flexible seepage meters were devised and successfully tested because the 50 "hard" fiberglass meters previously installed were found to pump water from the ground. Though the results are preliminary (data based on only 4 m), seepage rates range from 5 to 13 l/m²/day, a range of flow sufficient to replace or recycle Florida Bay water one or more times/year. The pumping action of rigid seepage meters, including those constructed from the ends of oil drums, occurs only in the presence of waves.

Nutrient levels of saline ground water in the Keys is everywhere greater than that of surface waters. Total phosphorous (TP) and nitrite and nitrate (NO₂+NO₃) are generally in the range of surface waters, whereas ammonium (NH₄) is generally 20 to 40 times greater in the same anoxic H₂S-rich ground waters. The exception is fresh water from quartz sands underlying a thin limestone cap in the Shark River slough. TP in these fresh ground waters is elevated 6 or more times than that in surface waters. Seepage of nutrient-rich ground waters is most pronounced in areas close to the Keys because of tidal pumping. Tidal pumping is present but much reduced in central Florida Bay.

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