GRAND CANYON DYE TRACING: CHALLENGES, CONCERNS, AND RESULTS

Grand Canyon National Park is home to over 150 perennial karst springs, including the sole source of drinking water for residence and visitors to the park. These springs are located in the semi-confined Redwall- Muav karst aquifer (R-aquifer) which is overlain by 1000 – 2000m of relatively flat-lying sedimentary rocks. These overlying rocks contain a variety of lithologies, including sandstones, shales, and a perched karst aquifer which drains to the underlying R-aquifer. This heterogeneity along with the spatial extent of the recharge and discharge areas has long prevented/precluded the use of artificial tracers in studying the aquifers.

Tracing has been on-going in the region for the last 3 years. These traces have presented a series of logistical challenges. The depth of the unsaturated zone and lack of significant and reliable infiltration from sinking streams and rain events, dye injections have focused on using snowmelt to push dye through the aquifer. Additionally, because of the spatial extent of the aquifer system and the lack of pre-existing quantitative information on aquifer flowpaths, dye receptors have been placed at springs and streams along a 240 km stretch of the Colorado River. Each receptor exchange requires 30 days of fieldwork. Therefore, efforts have been focused on finding the balance between these fieldwork demands and potential interference in dye detection. When receptors remain in the field for extended periods, fluorescence from biological activity can cause interference and noise in the analysis of the receptors. Receptor exchanges have been occurring on a 1 -3 month interval, with modifications to this interval tied to known peaks in discharge associated with spring snowmelt. While efforts seem to be pushing the limits of these techniques, tracer testing is proving successful and shedding new light onto this complex hydrologic system. Currently the dye has been recovered from sinkholes 1920m above and 41 km straight line distance in springs below the rim.