NUMERICAL MODELING OF MIXED-FLOW KARST SYSTEMS: APPROACH AND INPUT DATA

Modeling groundwater flow in a karst environment is both numerically challenging and highly uncertain because of potentially complex flowpaths and a lack of site-specific information. This paper presents an approach to numerical modeling in which drain cells in a finite-difference model are used as analogs for preferential flowpaths or conduits in karst environments. In this study, conduits in mixed-flow systems are simulated by assigning sequences of adjacent drain cells from the locations of tracer releases, sinkholes, or other karst features to outlet springs along inferred flowpaths. These paths are determined by the locations of losing stream segments, ephemeral stream beds, fracture lineaments, other surficial characteristics, or results of geophysical surveys, combined with the results of dye traces. The elevations of the drains at the discharge ends of the inferred flowpaths are set to the elevations of discharge springs; the elevations at the beginning of the inferred flowpaths are estimated from field data and are adjusted as necessary during model calibration. To simulate a free-flowing conduit, a high conductance is assigned to each drain to promote the removal of water by modeled conduits and to eliminate the need for drain-specific information that would be very difficult to obtain. Calculations were performed for two sites: one near Hohenfels, Germany, and one near St. Louis, Missouri. The potentiometric surfaces produced by these simulations agreed well with field data. The head contours in the vicinity of the karst features behaved in a manner consistent with a flow system having both diffuse and conduit components, and the sum of the volumetric flow out of the drain cells agreed closely with spring discharges and stream flows. Because of the success of this approach, it is recommended for testing of conceptual flow models and for regional studies in which little site-specific information is available, and general flow characteristics are desired.