EXPERIENCE WITH A DUAL-CONDUCTIVITY FLOW MODEL APPLIED TO KARST AQUIFERS

Mature karst aquifers typically exhibit significant flow channeling and complicated, multiple time-scale responses to recharge events. These dynamics are difficult to reproduce with conventional aquifer models. One alternative approach is to use separate but interacting flow systems to represent conduit and diffuse flow, similar to dual-conductivity approaches that have been used successfully in fractured rock modeling applications. A new package that implements the dual-conductivity model (DCM) within the MODFLOW-2000 (A.W. Harbaugh et al., USGS Open File Report 00-92, 2000) system was recently developed and is currently being tested in two karst modeling applications. Unlike most previous tools for modeling karst aquifers, the DCM package includes storage in the conduits and allows conduit transmissivity to vary with water level within the conduit. Numerical tests demonstrate that these capabilities are needed to adequately model karst aquifers that are unconfined or partially confined. The DCM module also represents laminar and turbulent flow within the conduits. The DCM module is being applied to the Barton Springs segment of the Edwards Aquifer and the Santa Fe sink/rise system in the Upper Floridan Aquifer. Both aquifers have well-developed conduit systems, but they have strongly contrasting physical and hydrological characteristics. Preliminary experience with DCM applied to these aquifers suggests that the DCM package is a promising tool for capturing the multiple time-scale dynamics of karst aquifers. However, the approach tends to amplify existing numerical instabilities associated with drying cells in MODFLOW if water levels drop below the base elevation of a conduit. Possible approaches to improving numerical performance in the presence of drying cells will be described. Acknowledgment: The authors are grateful to the AwwaRF, Edwards Aquifer Authority, and Southwest Florida Water Management District for supporting this work.