Effects of Turbulent Groundwater Flow on Hydraulic Heads and Parameter Sensitivities In Preferential Groundwater Flow Layers Dominated by Biogenic Macroporosity In the Biscayne Aquifer, Southeastern Florida

The U.S. Geological Survey created a Conduit Flow Process (CFP) for the Modular Finite-Difference Ground-Water Flow Model, MODFLOW-2005. An application of the CFP on the Biscayne aquifer in southern Florida is described that examines (1) the potential for turbulent groundwater flow, and (2) the effects of turbulent flow on hydraulic heads and parameter sensitivities. Turbulent flow was spatially extensive in preferential groundwater flow layers that are characterized by biogenetic macroporosity with mean void diameters equal to about 3.5 centimeters, groundwater temperature equal to about 25 degrees Celsius, and critical Reynolds numbers less than about 400. In the Biscayne aquifer, biogenic macroporosity largely manifests as: (1) ichnogenic macroporosity primarily related to post-depositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems (Ophiomorpha); and (2) biomoldic macroporosity originating by dissolution of fossil hard parts, principally mollusk shells. Inter- and intra-burrow macroporosity dominates the porosity in two of the flow zones and fossil-moldic macroporosity dominates the third. Turbulence either increased or decreased simulated heads from laminar altitudes. Specifically, head differences from laminar altitudes ranged from about -18 to +27 centimeters, and were explained by the magnitude of net flow to the finite-difference model cell. Turbulence also influenced the sensitivities of model parameters. Specifically, the composite-scaled sensitivities of horizontal hydraulic conductivities decrease by as much as 70% when turbulence is removed. Resultant hydraulic head and sensitivity differences due to turbulent groundwater flow highlight potential errors in models which assume laminar flow in an equivalent porous-media having uniformly distributed void spaces.