USE OF DYE TRACING TO VERIFY DELINEATION OF SOURCE-PROTECTION AREAS FOR A KARST SPRING AND WELLS IN THE BEAR RIVER RANGE, NORTHERN UTAH

Drinking-water source-protection areas for wells and springs in fractured-rock terranes are often delineated by using analytical models and time-of-travel methods based on Darcian concepts. These methods, however, generally assume that the aquifers in these terranes behave as porous media at the scale of the study area. Fractured-rock aquifers that do not behave as porous media, such as those in carbonate (karst) terranes, generally cannot be studied with the same methods used to characterize porous-media aquifers. In these settings, delineation of source-protection areas or zones of contribution for wells and particularly springs, often can be done only by integration of hydrogeologic mapping techniques such as water-level measurements, discharge balancing, and dye-tracer tests. Dye tracing has proven to be one of the most effective methods for determining zones of contribution, general directions of ground-water flow, and residence times for water in karst aquifers where nonporous-media (conduit) flow predominates.

Source-protection areas and time-of-travel zones delineated for a karst spring and two wells discharging from carbonate rocks in the Bear River Range of northern Utah were compared to results of dye tracing. Results of five tracer tests indicate that time of travel based on porous-media concepts can be substantially overestimated in terranes where conduit flow paths are present. Maximum ground-water travel times ranging from 8 to almost 31 days were determined for losing stream reaches 3.0 to 7.2 miles upgradient from Dewitt Spring and within the delineated source-protection areas. These travel times indicate that average ground-water velocities ranged from about 530 to almost 2,000 feet per day. Because passive dye-tracing methods were used, however, ground-water travel times are probably substantially shorter and velocities are greater. Results of dye tracing also show that a large part of the source-protection area delineated for the wells probably lies within the recharge area for the spring. In addition, source-protection areas in these terranes may be under or overestimated when surface-water divides are assumed to be ground-water divides. As a result, dye tracing can be used to help delineate areas that do not contribute to springs or wells, and thus, minimize the area that is necessary for source-protection management.