![]() As bypass flow emerges to be the dominant flow regime, the second objective is to image how water enters the subsurface and spreads out in the limestone via infilled dissolution holes. The first is to test which of three end-member conceptual models describing water flow (plug flow, lateral flow, bypass flow) within the Miami oolite are effective in draining large amounts of rainwater into the subsurface. This paper demonstrates how densely sampled and precisely repeated ground-penetrating radar (GPR) surveys can track fluid movement and characterize the subsurface flow regime at a field site in the Miami oolitic limestone. Even combined, these methods lack the resolution to describe how or where the water is moving on the centimeter to meter scales. Traditional investigations rely on interpolation between point measurements (rock samples and borehole tests), and/or low resolution two-dimensional (2-D) geophysical methods (e.g., surface or borehole resistivity and electromagnetic techniques). It is essentially impossible to characterize drainage using standard hydrogeological methods. Subsequent to the rapid infiltration, little is known regarding the nature of water flow paths and rates in this complex environment. Water from heavy summer storms that ponds on the surface to depths of several centimeters disappears within a fraction of an hour. For example, in large parts of urban Miami the vadose zone is characterized by oolitic carbonates, where laterally varying small-scale stratigraphy and crosscutting dissolution features control transport pathways. Three-dimensional connectivity creates preferential flow paths allowing large volumes of water, nutrients, and contaminants to rapidly bypass the less permeable bulk rock volume. Rainfall/water table response times, well drawdown tests, slug tests, pump tests, and tracer experiments exhibit surprisingly high hydraulic conductivities, orders of magnitude higher than those derived from measurements on rock plugs taken from both outcrops and cores. Subsurface fluid flow in carbonates is often controlled by a highly heterogeneous hydraulic conductivity field resulting from the interplay of small-scale sedimentary structure, diagenetic alteration, and fracturing. High-precision time-lapse GPR imaging can therefore be used to noninvasively characterize natural drainage inside the vadose zone ranging from transient loading to seasonal variation. Comparing 3-D surveys acquired after wet summer and dry winter conditions shows good GPR event correspondence, but also time shifts up to 20 ns caused by the change of overall water content within the vadose zone. On a seasonal time frame, redistribution involves the entire rock volume. ![]() Average lateral propagation measured over a 28-hour period was of the order of 0.1 m/h. At the same time, some of the water migrates laterally into the host rock guided by stratigraphic unit boundaries. Hourly repeated 3-D imaging of a dissolution sink in response to surface infiltration shows how the wetting front propagates at a rate of 0.6–1.2 m/h traversing the 5-m-thick vadose zone within hours. Two-dimensional (2-D) GPR time-lapse surveying at a 3-min interval before, during, and after rainfall shows how buried sand-filled dissolution sinks efficiently drain the bulk of the rainwater. ![]() A new rotary laser-positioned ground-penetrating radar (GPR) system enables centimeter-precise and rapid acquisition of time-lapse surveys in the field. ![]() Once the water enters the ground, little is known about the flow paths in the oolitic rock. The vadose zone of the Miami limestone is capable of draining several centimeters of rainfall within a fraction of an hour.
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