Florida Geological Survey


21,000 mg/L for the period from 1979 to 1991. Interestingly, chloride data for the lower Floridan monitor zone do show a pattern. From 1979 to 1987 values fluctuate between 2,000 and 4,000 mg/L, but then concentrations increase steadily to over 12,000 mg/L by mid-1991, indicating displaced formation water. Again, TDS and TKN data were not collected until 1990, and no patterns are observed. Data for the middle and upper Floridan monitor zones also show no patterns.

 DISCUSSION AND CONCLUSIONS

 The geologic framework of the lower Floridan aquifer system in Brevard County embodies a shallow water carbonate platform sequence, the character of which has been determined by a diversity of factors including depositional environment, diagenesis, and geologic structure. Variations in these components can result in considerable differences in local lithofacies, porosity, permeability, and hydrogeologic character of the aquifer.

 Ground-water chemistry trends for several injection wells indicate that injected waste liquids are migrating upward through the "confining" rocks immediately above the injection zones. Since apparently low permeability dolostones are common in the "confining" sequence,


injected waste waters are probably moving vertically along fractures and possibly along fault planes where present. Fractures commonly observed in borehole cores and videos justify this supposition. Faults, however subtle and small scale, can enhance fracture-related permeability locally and serve as conduits for vertical fluid migration. If injected waste fluids migrate preferentially upward along dissolutionally enlarged fault planes, conceptually, the fault could effectively mask contamination detection in monitor wells depending on the location of the monitor zone relative to the fault.

 A more satisfactory understanding of the lower Floridan aquifer system in Brevard County can only be achieved by further study accompanied by the acquisition of additional data. Thorough coring of strata overlying injection zones is highly desirable so that lithofacies and hydrologic characteristics can be adequately detailed. A seismic survey program should be considered in order to identify and map the extent of possible faulting in proximity to current and proposed injection well sites. Because of their value to subsurface geological evaluations, borehole videos and complete geophysical log suites (including gamma-ray, sonic, and neutron-density) should be run over the entire borehole of future injection and monitor wells.