AQUIFER HETEROGENEITY RELATED TO DOLOMITIZATION, LOWER HAWTHORN AQUIFER (LATE OLIGOCENE - EARLY MIOCENE), COLLIER COUNTY, FLORIDA

An interdisciplinary study of a lower Hawthorn aquifer wellfield in Collier County, Florida, revealed the presence of large variation in artesian flow and specific capacity between wells that is related to diagenesis. Flow meter logs and borehole television surveys indicate that the wells produce primarily from thin intervals of high hydraulic conductivity dolomite that are locally fractured. These high hydraulic conductivity beds have limited lateral extent, as evidenced by a strong boundary effect detected in aquifer performance test data.

Lower Hawthorn aquifer limestones, including the precursors of the dolomite, have carbonate mud-bearing textures (fossil wackestones and packstones) and contain fossils of stenohaline organisms, suggesting deposition in a low-energy, marine environment. The dolomite has crystalline and micro-sucrosic textures and formed relatively late during diagenesis, postdating both skeletal aragonite dissolution and some mold-filling calcite cementation. Stable isotope data suggest that dolomitization occurred in either marine or brackish pore waters. Fracturing of the dolomite appears to be related to folding that occurred no earlier than the late Miocene.

The lower Hawthorn aquifer behaves as a large scale dual-porosity system in which the high hydraulic conductivity dolomite intervals increase the specific capacities of some wells, but have little effect on the total volume of water that can be produced from the aquifer, which is controlled by the surrounding lower hydraulic conductivity limestone. There is no obvious sedimentologic, structural, or hydrologic template that can predict the geometry of the high hydraulic conductivity dolomite units. The results of this investigation demonstrate the high degree of heterogeneity and unpredictablity that may occur in carbonate aquifers as the result of diagenesis. Because of uncertainties over the geometry of high hydraulic conductivity units, and thus fluid flow paths, heterogeneities in aquifer hydraulics related to diagenesis may greatly impact projects requiring the recovery of specific volumes of water, such as pump and treat remediation and aquifer storage and recovery systems.