Using Sodium-Chloride Tracers and Grain Size Analysis to Determine Hyporheic Permeability in Salmonid Spawning Habitat

Embryonic mortality rates of salmonids are greatly affected by gravel permeability and grain size distributions within the host gravel. Typical permeability testing methods use a single standpipe to measure the permeability. For studies on the American and Feather Rivers in northern California, tracer tests were used to measure seepage velocity using a main "injection" well and several downstream monitoring wells. Bulk samples and pebble counts were used to measure grain size. Measurements were recorded at approximately 30cm depth in the gravel, where salmonid species typically lay their eggs. Sites were examined before and after stream restoration to compare subsurface habitat conditions. During each tracer test, a super-saturated NaCl solution was introduced into an "injection" standpipe with a short well screen located 30cm deep in the gravel. Identical downstream standpipes contained conductivity meters that sensed the NaCl as it passed through the gravel, causing a spike in specific conductance. Plotting the peak conductance against the arrival time allowed a seepage velocity to be measured in cm/second. Seepage velocity ranged from 0.2 - 0.7 cm/sec in restored gravel, and was less than 1.6 x 10-4 cm/sec in some un-restored areas. Grain size analysis showed that un-restored areas had an armored surface with d50 values ranging from 4-10 cm, while the subsurface showed excessive fine material supporting large grains smaller than those in the surface sample with d50 values ranging from 1.3 - 3.0 cm. Restored areas were found to contain a well sorted composition containing little to no fine material and a subsurface which closely matched the surface showing d50 values from 1.6 - 3.8 cm. Comparing results of tracer tests with grain size distributions in both restored and un-restored spawning gravels gives an indication of the relative health of a particular portion of a hyporheic river system, and the relative success of some restoration projects.