Dye Tracing Through the Vadose Zone Above Wind Cave, Custer County, South Dakota

During the 1990s, in an attempt to better understand threats posed by surface developments overlying the cave, National Park Service staff at Wind Cave National Park in Custer County, South Dakota carried out a series of dye traces through portions of the vadose zone overlying the cave. Wind Cave is located within the 100m-thick Madison formation (limestone and dolomite), which in most locations is capped by varying thicknesses of the basal units of the Minnelusa formation (intermingled beds of sandstone, limestone, and shale). A variety of cave locations with dripping or pooled water were monitored for up to five years following dye injection. Transit times to the cave varied from less than six hours to as much as 4.8 years. Despite a variety of positive results, there appears to be little correlation between transit time and lateral or vertical distance from the injection site. Data analysis produced traditional-shaped dye recovery curves in some locations, albeit stretched out over hundreds and possibly even thousands of days beyond dye injection. The results strongly suggest that chemical or sewage spills in the vicinity of the dye injection sites would quickly enter multiple sites in the cave system, and could persist for years.