FLOW VELOCITIES IN KARST AQUIFERS

Tracer studies have documented several different patterns of rapid flow in the multiple porosity karst aquifers of the Upper Mississippi Valley Paleozoic carbonates and sandstones. The first, "classical" pattern is observed in shallow flow systems with rapid flow from sinking streams and sinkholes via conventional dendritic conduit systems to springs. Tracer studies in such systems typically yield fast flow velocities in the km/day range with well-defined, smooth, narrow tracer pulses at the springs. Single tracer injections have yielded two overlapping pulses in individual springs under high flow conditions. Tracers injected near the boundaries of springsheds can yield pulses in two or more subsurface basins. A second pattern apparently results from flow though anatomising, horizontal, high transmissive zones. These zones can sometimes be correlated at the regional scale. The leading edges of tracer pulses in these systems move as rapidly as in conventional conduits but the tracer pulses are days to weeks long and can have abundant structure at the minutes to hours time-scale. A third pattern was observed when tracers were flushed into a normally dry crevice in the bottom of a shallow limestone quarry. This study recorded an initial rapid movement of tracer that decayed on the week timescale. The next major recharge event produced a second, smaller tracer pulse that decayed away at a rate similar to the first. This pattern of recharge driven pulses repeated itself for several years. A range of intermediate flow systems exists, which display apparent flow velocities from km/day through km/yr to m/yr. Much of the storage in these aquifers occurs in the parts of the systems with slow flow velocities. Several of different flow systems are typically present in each volume of aquifer.

Such diverse karst systems a priori directly violate many of the explicit and implicit assumptions built into resource managers' tools and governing regulations. Effective and economical management of karst aquifers requires that those tools and regulations be based on detailed understanding of flow in karst aquifers.