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 100 m-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. Introduction Wind Cave is located within Wind Cave National Park in the southern Black Hills in Custer County, South Dakota. The cave is an extensive three-dimensional labyrinth that as of this writing consists of more than 230 kilometers of surveyed passages contained within an area of roughly 2.5 square kilometers (Ohms, 2015, personal communication). It is contained within the Madison formation, which consists of beds of limestone and dolomite of Mississippian age roughly 100 meters in thickness in the Wind Cave area (Palmer and Palmer, 2008). Overlying the Madison formation in the vicinity of the cave is the Minnelusa formation, consisting of sandstone, limestone, dolomite, and shale (Strobel and others, 1999). The vast majority of Wind Cave is capped by the lower 30 to 100 meters of the Minnelusa formation, but a small number of cave passages are located beneath areas where the overlying Minnelusa formation has been largely or entirely eroded away. The only known completely-natural entrance to the cave is located in such an area, in a drainage known as Wind Cave Canyon (Figure 1). The contact between the Minnelusa and Madison formations is complex. The Madison was exposed soon after deposition, and a karst topography formed, with many caves, sinkholes, and other typical karst features. As a result, the base of the Minnelusa was deposited atop an irregular surface, and in the process it filled many former solutional features up to tens of meters below the highest remaining reaches of the Madison (Palmer and Palmer, 2008). The area above Wind Cave is semi-arid with a meanannual precipitation of 45.8 cm/year (Ohms, 2012). Streambeds above the cave area have highly intermittent flow, and there are few locations within them that would appear to capture what flow they occasionally have in any obvious way. Wind Cave National Park was established in 1903, making it one of the oldest national parks in the United States. For the convenience of the visiting public, the park headquarters and its associated buildings, utilities, and parking lot were established near the cave’s enJames Nepstad Effigy Mounds National Monument, 151 HWY 76, Harpers Ferry, Iowa, USA, jim_nepstad@nps.gov Figure 1. An aerial image of surface features and developments located in the vicinity of the entrance to Wind Cave.