DELINIATION OF SHALLOW GROUNDWATER IN THE MANTLED KARST AQUIFERS OF BASIN TWO, SAVOY EXPERIMENTAL WATERSHED, NORTHWEST ARKANSAS

The Savoy Experimental Watershed (SEW) is a forest and pasture setting located 24 km west of Fayetteville, Arkansas. The SEW is designed as a state-of-the-science long-term field-scale laboratory for shallow karst environments. The site is being studied to determine the effects of livestock production and other farming practices on a watershed underlain by a mantled karst. The karstic Mississippian Boone and St. Joe Formations are overlain by cherty regolith, forming a mantled karst aquifer that is vulnerable to water-quality degradation from surface sources such as animal manures. The specific purpose of this project was to delineate the boundaries of subsurface water flow in Basin Two of the SEW and to characterize the dominant hydrologic processes. Understanding the extent of subsurface flow is critical to determining contaminant fate and transport in this mantled karst aquifer. A base map for the basin was prepared using ArcGIS 8.3x software in combination with GPS data of key sampling sites. These data, along with field reconnaissance, were used to determine optimum locations for dye injection and locations of discharge points, such as springs and hydraulic sinks or boundaries. Rhodomine WT, Fluorscein, Eosine, and Fluorwhite (optical brightener) dye were injected into sinkholes and losing streams. These dyes were chosen because they have proven to be effective and cost efficient in tracing studies elsewhere in the Ozarks. Positive dye detections and amounts were recorded by passive detectors and automatic samplers. Dye samplers were placed strategically in the anticipated down-flow direction of springs, seeps and waterways to trap any dye. To determine the presence and amount of the dye a spectrofluorometer was used for analysis because it can detect trace amounts of the dye. Based on the presence and amount of dye recovered from the basin, a map showing the extent of movement and time of travel throughout the watershed was generated. Preliminary results suggest that perching, epikarst, and vertical leakage at specific structural features concentrate groundwater flow. Flow determinations from dye tracing were further constrained by water-quality sampling, field parameter measurement, contiguous spring basin boundary determination, and assessment of basin area size using normalized base flow techniques.