MULTI-VELOCITY TRANSPORT OF WATER THROUGH MANTLED KARST AQUIFERS: THE POTENTIAL EFFECT ON AQUIFERS USED FOR WATER SUPPLY

Karst aquifers are an important source of water supply but have features such as sinkholes, caves, and conduits that make that source vulnerable to contamination. In south-central Pennsylvania, karst aquifers commonly are covered by a mantle of unconsolidated materials that can obscure the surface expression of karst features, thus making it difficult to locate where a contaminant can be directly introduced into the aquifer. The combination of unconsolidated materials, fractures, and conduits results in a multi-velocity flow field with components ranging from slow to rapid. Recent studies of two springs in a carbonate aquifer in south-central Pennsylvania used different methods to identify slow and rapid components in spring discharge. Traditional methods showed that both springs had a deep, slow-moving, diffuse source of water; however, other methods showed that these springs also had a component of discharge that moved rapidly through the aquifer. For example, at one spring, continuous monitoring of temperature and specific conductance during storms provided the evidence of rapid flow through the aquifer. At the other spring, a dye-tracing study linked the spring to a sinkhole 5 miles away with the initial breakthrough in 5 days, indicating flow rates several orders of magnitude faster than previously published values. The flow direction was parallel to strike, which differs about 90 degrees from the flow direction determined by other methods. These results provide an invaluable insight to the understanding of the mantled karst aquifer. In some cases, the unconsolidated mantle and fracture system overlies the conduit system, providing storage, attenuating traveltime, and feeding the conduit system below. In other parts of the same system, the conduit system receives water directly from the surface and transmits it rapidly to the spring. Because the contribution of the conduits that bypass the slower part of the system is a small percentage of the total flow and is also transient, this part of the system could be easily missed; however, when it is active it dominates the potential for contaminant transport. Traditional hydrology would need to be supplemented with karst studies to provide the understanding of where the rapid transport component completely bypasses the other parts of the system.