Characterizing the Hydraulic Conductivity of the Biscayne Aquifer of South Florida
Abstract:
The karstic Biscayne aquifer is an eastward-thickening wedge of limestone that serves as the sole source of water for 5.5 million people. The karst is eogenetic (formed during early burial) and in many cases characterized by thick and laterally extensive zones of touching-vug porosity related to the trace fossil Ophiomorpha (Figure 1). The vugs are commonly about 2 cm in diameter and porosity of 50% or more can be attributed to vugs. These connected vugs can impart an exceptionally high hydraulic conductivity (K), commonly exceeding 0.1 m s-1, to the Biscayne aquifer. Despite the likelihood that this is the best-studied aquifer of such high K, reliable values of its K have been challenging to establish due to the inapplicability or inconclusiveness of various test methods for extreme K.Multiple methods over a broad range of scales were used to estimate K values for the Biscayne aquifer, including:
● detailed Lattice Boltzmann Modeling (LBM) at pore scale;
● LBM and laboratory measurements at core scale;
● high-resolution borehole scale geostatistical and flow modeling based on borehole images;
● borehole scale slug testing; and
● aquifer test meta-analysis.
Results indicate that maximum K values from these methods vary over 5 orders of magnitude from 10-4 to 101 m s-1. Some variations in K are due to disparities in the physical samples and aquifer reconstructions tested, but most are intrinsic to the methods. For example, limitations of standard laboratory K measurements of core samples truncate the distribution of Biscayne aquifer core K values at about 10-4 m s-1. Slug tests in appropriately-constructed wells in the Biscayne aquifer are generally underdamped and appear to underestimate K (returning maximum values of 10-2 m s-1 comparable to a sand aquifer), possibly due to the Darcian flow assumption that underlies analysis methods for such tests. Aquifer tests are difficult to conduct in the Biscayne aquifer and are commonly inconclusive for a variety of reasons.
LBM applied at numerous scales yields K values that tend to converge and agree with pipe flow expectations and specialized lab measurements on a 0.1 m diameter core model. The K values from LBM at core scale are consistent with LBM K values from 2.72 m3-volume scale explicit pore/solid aquifer models based on novel geostatistical extrapolation of borehole optical images.