Recent Developments in Karst Groundwater Flow Measurement in Southeastern Florida,USA

Groundwater seepage was first characterized in the early 1800's, when Henry Darcy determined that the flow of groundwater could be estimated from the head difference and the distance between two points. Since then, hydrogeologists have been struggling with ways to continuously measure groundwater flow in situ, and more recently have sought data in near-real time. Groundwater flow within aquifers that have relatively large head differences (several meters) are porous in nature and have low hydraulic conductivities, is linear in nature, and can be generally characterized by Darcy's solution. Prior to the research presented herein, it was assumed that aquifers within Miami-Dade County could also be characterized by Darcy's solution (with Reynolds numbers less than 10 or 20). The L-31N Canal lies on the eastern flank of Everglades National Park (ENP). In addition to conveying water to Florida Bay and Biscayne Bay, the canal's levees are intended to reduce surface-water sheet flow from ENP to eastern urban areas. In an effort to reduce groundwater seepage coming from ENP, the South Florida Water Management District (SFWMD) and the United States Army Corp of Engineers (USACE) have been tasked with evaluating the hydrogeology and the groundwater/surface-water interaction on the L-31N canal. This involved process of installation includes monitoring wells, recording automated water-level measurements, characterizing water-chemistry types and ages, and installation of instruments capable of measuring horizontal groundwater velocities and directions coming from ENP. The SFWMD initiated a cooperative agreement with the United States Geological Survey (USGS) for the geological and hydrogeological investigation and concurrently contracted the installation of borehole flowmeters in eight wells (two clusters). The USGS provided detailed core and sediment analysis, geophysical logging, in situ borehole flowmeter logging, and digital optical borehole imaging. In addition, the USGS produced a hydrogeologic cross-section using the new borehole data. The USGS delineated high-frequency cycles (HFCs) within the study area. The high-frequency cycles form the fundamental building blocks of the rocks composing the Biscayne aquifer. Vertical lithofacies successions, which have stacking patterns that reoccur, fit within the high-frequency cycles. An important observation is that a predictable vertical pattern of macroporosity and permeability commonly exists within the high-frequency cycles, thus preferential flow passageways can be constrained by the lower and upper cycle boundaries. In southeastern Florida, specific HFCs can contain relatively high hydraulic conductivities and vertical head gradients within centimeters of each other. This combination of high hydraulic conductivities (estimated at 1500 to 3000 m/d) and nearly flat water table gradients combine to convey large amounts of groundwater from ENP to the eastern urban areas, with the water ultimately discharging into Biscayne Bay. Horizontal groundwater flow velocity was measured with horizontal heat-pulse flowmeters installed in eight monitoring wells located on the western levee of the L-31N canal. Results show that flow velocity in the shallow wells (5.1 m in depth) is coupled to the surface water as measured by well water levels. A groundwater rise of about 0.5 m during the wet season of September-December 2007 led to a six-fold increase in horizontal groundwater flow rates.