METER-SCALE TRACER TESTS TO ESTIMATE SEEPAGE VELOCITY AND HYDRAULIC CONDUCTIVITY IN SALMON SPAWNING GRAVELS OF THE AMERICAN RIVER, SACRAMENTO CA

Hydraulic conductivity is difficult to measure in gravel. Sieve tests that use relationships based on grain size distribution do not apply to coarser sediments, and slug tests or well tests rely on piezometer installations that have mechanical problems and limited lateral effect. Heterogeneity is also a significant factor in gravel bed streams, so hydraulic conductivity estimates that cover larger areas of the substrate are desirable.

To avoid some of these limitations, meter-scale tracer tests were conducted to estimate lateral and vertical seepage velocity through salmon spawning gravels. Seepage velocity was then used to estimate hydraulic conductivity by assuming an intergravel porosity of 20%. For each test, piezometers were spaced 50 cm apart in a longitudinal array that covered 1.5 or 2 m of stream gravel. Four liters of salt water were used as a conservative tracer, and injected to a depth of 30 cm in the upstream piezometer. Conductivity meters were used to monitor downstream travel of the saltwater plume, and first arrival was recorded through 50 cm, 100 cm, 150 cm and sometimes 200 cm of stream gravel. Riffles were emphasized, and results were compared to nearby glides and pools.

Measured seepage velocity ranged from 0.06 cm/s to 0.7 cm/sec in a cobble-sized riffle and glide sequence, resulting in hydraulic conductivity estimates of 0.07 cm/s to 0.81 cm/s. These estimates reflect hyporheic transport through coarse cobble-sized material with up to 10% infiltrated fines. Tracer tests have a broader zone of influence than slug tests and well tests, and estimated seepage velocities are averaged through a section of the hyporheic zone. Seepage tests may provide hydraulic conductivity estimates in coarse stream gravel where other methods are not appropriate.