TIME SCALE OF SEAWATER CIRCULATION AND DOLOMITIZATION WITHIN THE LOWER FLORIDAN AQUIFER

In cooperation with the South Florida Water Management District, groundwater samples were collected from parts of the Upper and Lower Floridan aquifer system, south of Lake Okeechobee for analysis of dissolved noble gases, stable isotopes of water, chlorinitiy, radiocarbon, and strontium isotopes. In the upper Floridan aquifer freshwater derived from precipitation circulates. However, the groundwater becomes increasingly saline with depth, eventually reaching the salinity of the coastal ocean. The circulation time of the saline groundwater (seawater) in the lower Floridan aquifer is relatively fast. Noble gas recharge temperatures and the stable isotopes of water are identical to modern bottom water (<400 m) from the Florida Straits indicating that seawater recharged the aquifer after the end of the last glacial period when sea level returned to its present height. Hence, the time scale of seawater circulation is less than 10,000 years. As indicated by excess helium, noble gas recharge temperatures, and radiocarbon, the over lying freshwater in the upper aquifer recharged during the last glacial period and, hence, its circulation time is longer. Thus, the unusual situation of older groundwater lying above younger exists in the highly conductive layers of the Floridan aquifer system. Strontium isotope data and major ion chemistry from the lower Floridan aquifer samples were used to examine water rock interactions. The 87Sr/86Sr ratios range from the modern seawater value to the expected value of the aquifer material, which is an Eocene-age carbonate. On an 87Sr/86Sr ratio vs 1/Sr plot, the groundwater samples lie along a mixing line between these two end members. Additionally, the Mg/Ca ratio of groundwater correlates well with the 87Sr/86Sr ratio. These data indicate that modern seawater flows through the lower Floridan aquifer and reacts with the aquifer material, gaining the Sr isotope signature of the aquifer. Subsequent precipitation of dolomite occurs lowering the Mg/Ca ratio of the saline groundwater. A dolomitization rate is estimated using chemical mass balance calculations and geochemical dating techniques.