RESULTS OF 25 GROUNDWATER DYE TRACER TESTS IN THE EAST SNAKE PLAIN AQUIFER OF SOUTHERN IDAHO NEAR SPRINGS AND FROM AQUIFER RECHARGE SITES SINCE YEAR 2008

Since 2008, 25 groundwater dye tracer tests have been conducted on western portion of the East Snake Plain Aquifer (ESPA) near spring areas and from aquifer recharge sites. The ESPA is a Basalt dominated aquifer type and covers approximately 10,000 square miles in Southern Idaho. A tracer test program was started with a cooperative effort between the Idaho Dept. of Water Resources and Idaho Power to further the understanding of water movement in the aquifer. The tracer tests document groundwater flow routing, velocities and spatial distribution of the dye cloud. Fluorescein dye is the tracer of choice due to its long-studied safety record and as a conservative tracer in basalt rock aquifers. Tracing started with the shortest trace from a well to a spring at just 1,100 feet distance; then step back further along a trajectory to the next distant well. A second type of tracing is from spreading basin aquifer recharge sites where dye was released into the recharge pools and tracked for up to 32 miles in the basalt aquifer over a 5-year period. Dye flow paths were also confirmed using general water chemistry and water table potentiometric mapping.

Results document groundwater velocities ranging from a few hundred feet per day to several thousand feet per day. Velocities increased as gradient increased near spring areas. Geologic controls on many of the spring areas have been shown to be from ancient canyon filling lavas (Malde, 1982) and a greater understanding of the subsurface geometry was gained from a recently developed 3D model of these canyons. Important information from dye tracing include: quantity of dye required for measurable responses, sampling methodology and instruments, relationships between groundwater gradient, velocities, distance, spatial distribution of dye clouds, breakthrough curve characteristics, Reynolds Numbers and hydraulic conductivity values. The traces have also helped validate studies of ancient buried canyons now filled with basalt by Harold Malde and others. Data at recharge sites was used to confirm the appropriate placement of water quality monitor wells. The dye tracing program has produced some unprecedented scale data about fractured volcanic aquifers with some unexpected results that enlighten how this aquifer behaves.