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by Earl A. Greene^sup a^
Abstract
Stable isotopes of hydrogen and oxygen were used to trace the sources of recharge from sinking streams to wells and springs several kilometers downgradient in the karst Madison aquifer near Rapid City, South Dakota. Temporal sampling of streamflow above the swallets identified a distinct isotopic signature that was used to define the spatial dimensions of recharge to the aquifer. When more than one sinking stream was determined to be recharging a well or spring, the proportions were approximated using a two-component mixing model. From the isotopic analysis, it is possible to link sinking stream recharge to individual wells or springs in the Rapid City area and illustrate there is significant lateral movement of ground water across surface drainage basins. These results emphasize that well-head protection strategies developed for carbonate aquifers that provide industrial and municipal water supplies need to consider lateral movement of ground-water flow from adjacent surface drainage basins.
Introduction
Carbonate aquifers occur in many parts of the world, including the United States, and in some areas these aquifers supply most of the domestic, industrial, and agricultural water use of a region. Approximately 20% of the U.S. and about 40% of its eastern land area are underlain by limestone or dolomite (Quinlan, 1989). The hydrology of carbonate rocks is unique because through dissolution of bedrock they give rise to karst terrain.
One of the unique geomorphic features of a karst aquifer is the relation of stream recharge from swallets (sinkholes) to water derived from wells and springs. This phenomenon has been the subject of much research and study in karst hydrology because of the potential for contamination of the aquifer from surfacewater recharge. Commonly, this recharge-discharge relation is studied by using conventional dye-tracing methods to determine the direction of ground-water flow and delineate recharge areas of a well or spring (Rahn, 1971; Atkinson et al., 1973; Thrailkill, 1985; Mull et al., 1988; and Smart, 1988). However, conventional dye-tracing methods in karst terrain usually are not able to quantify the percentage or contribution of recharge water to wells and springs over the spatial dimensions of kilometers. Therefore, continued research into field tracing methods that provide this type of information to water managers is needed. recharge source, the...