THE KARST PERMEABILITY SCALE EFFECT OF SETE LAGOAS, MG, BRAZIL

Collecting and interpreting permeability data in karst systems is considered complicated due to three distinct properties of these systems. First, the permeability distribution of high permeability features may be as one-dimensional features that are difficult to detect with wells, or may be so high in the wells that the upper measurement limit is encountered during aquifer testing. Secondly, turbulent flow may make the application of continuum hydraulic principles difficult. Finally, permeability commonly has a scale effect in which permeability in these systems increases with the scale of measurement. The aquifer for Sete Lagoas, Brazil, is used to evaluate a permeability combination evaluation methodology to test the permeability structure across a range of spatial scales in order to develop a quantitative model of permeable features that is consistent across all scales of measurement, from matrix properties to regional-scale flow. The aquifer in this study has some wells that do not have measurable drawdown during pumping due to high permeability. Permeability data indicated an increase in permeability from the small- to the well-scale and a decrease from the well- to regional-scale due to the localized development of karst bedding plane dissolution in one structurally controlled region of the aquifer, which the matrix permeability is very low and the secondary porosity is mostly filled by secondary precipitation of calcite. The results show that, based on measurement technique, the permeability data vary over many orders of magnitude, while the physical size of permeable features of the aquifer are consistent across the scales of data collection and their geometry provides a quantitative understanding of the scale effects of permeability measurements.