DETECTION OF GROUND-WATER DISCHARGE TO MOUNTAIN STREAMS THROUGH TRACER-INJECTION AND SYNOPTIC SAMPLING STUDIES

Ground-water pathways to streams can be very complex. Tracer-injection studies with synoptic sampling are used to help understand these pathways. Dilution of an ionic-salt tracer indicates the total inflow of water to a stream for a specific stream segment and offers enhanced precision relative to flow-meter measurements, especially in cobble-bottomed, high-gradient streams. This approach has been used recently in two upland Rocky Mountain watersheds. Red Butte Creek, Utah, is in a highly fractured sedimentary sequence of rocks. Handcart Gulch, Colorado, is in fractured crystalline rock. The tracer accounts for both surface and hyporheic down-valley flow. In the absence of visible inflows, ground-water discharge is evident in the dilution of the tracer. Chemical mass-balance comparisons for solutes like sulfate or chloride also provide evidence of ground-water discharge. If the sampled inflows do not adequately account for load increases measured in the stream, the discrepancy may indicate ground-water discharge of water that has solute concentrations that differ from sampled inflows. These tracer methods do not explicitly define the pathways of such subsurface inflows but do provide quantitative evidence of their significance. In combination with age-date techniques, these methods may define specific pathways of flow to the stream with potentially useful information for many applications.