A COMPARISON OF SAMPLE COLLECTION DEVICES FOR ISOLATING SUB-SURFACE FLOW PATHS IN A GAINING MOUNTAIN STREAM

Developing a means of quantifying aquifer characteristics such as storage volume and recharge rate by sampling flow-weighted flow paths in areas of groundwater discharge is important for global water resource assessments. To do so in a gaining mountain stream, the groundwater discharge must be isolated from the stream and hyporheic flow paths, then both concentrations and flow rates measured. We compared three sample collection devices, all capable of measuring flow rates, at 12 sites in Red Butte Creek, Salt Lake City, Utah, including: (1) a conventional bucket seepage meter, (2) a flexible rubber mat seepage meter, and (3) a Shelby tube-like device that is driven to depth in the stream bottom. To determine the extent of flow path isolation in each collection device, a bromide injection tracer test was conducted and mixing models developed using the general chemistry of samples from the stream, from nests of shallow piezometers installed in the stream at the study sites, and from groundwater-monitoring wells in the drainage basin. Results show that all three devices are successfully isolating samples from the stream; additionally, we have shown that specific flow paths can be targeted with these devices in settings with hyporheic flow. The delay in arrival time (hours to days) and the lower bromide concentration peak relative to the stream during the tracer test along with higher specific conductance values indicates that both of the seepage meter devices are sampling a flow-weighted average that largely targets shallow hyporheic flow paths. The early arrival (on the order of days) in the Shelby tube-like device of bromide concentrations an order of magnitude lower than stream concentration but present two months after the tracer test indicates a sampling of short hyporheic flow diluted by deeper hyporheic and groundwater flow paths. Using data from nests of piezometers, it appears that shallow flow paths can be successively eliminated from the sample in this device by driving the tube to depths up to 50 cm; results of the mixing models concur. Additional advantages of the Shelby tube-like collection device are a large sampling reservoir with optional continuous purging and the ability to amplify flow rate measurements.