A MODFLOW-BASED APPROACH FOR MODELING FLOW IN KARST AQUIFERS

Karst aquifers with well-developed conduit systems present special challenges to groundwater modeling. In particular, the high degree of flow channeling and the complicated multiple time-scale responses of spring hydrographs to infiltration events are difficult to reproduce with conventional single-porosity models. Several approaches for incorporating explicit conduits in flow models are available. These approaches include conventional single-porosity models with highly heterogeneous distribution of hydraulic properties to represent conduits, finite-element models with conduits represented by line elements, and dual-conductivity models that couple explicit conduit networks with continuum representations for the diffuse flow component. In the latter approach, conduit and diffuse flow are modeled as separate but interacting flow systems, similar to the classical dual-continuum approach used to model flow in fractured rock. A new module (DCM) has been developed that implements the dual-conductivity approach for modeling karst aquifers within the MODFLOW (A. W. Harbaugh, E. R. Banta, M. C. Hill, and M. G. McDonald, USGS Open File Report 00-92, 2000) system. The DCM module includes options for laminar or turbulent flow within the conduits and accommodates confined or unconfined conditions. The new DCM module is particularly convenient for groundwater supply applications because it maintains compatibility with legacy data that are often in MODFLOW format. The authors are grateful to the Awwa Research Foundation, Edwards Aquifer Authority, and Southwest Florida Water Management District for supporting this work.