Solute Transport

Streams and rivers, whether in urban or rural settings, contain stagnant zones due to recirculating eddies (from meanders) and obstructions (cobbles, woody debris, etc.). The fraction of stream water that enters these stagnant (or dead) zones is thus temporarily retained before being released to the main channel. Similarly, a fraction of stream water enters the permeable subsurface zone surrounding the stream (the hyporheic zone) and moves downstream at a slower velocity than that of the main channel. It eventually exits the hyporheic zone back to the stream. Due to this transverse exchange, the dead zones and the hyporheic zone behave as "storage zones" that attenuate the peak concentration of an instantaneous input of solute (such as may occur due to an accidental contaminant spill). The storage zones also provide slow release of the solute back to the stream after the pulse has passed resulting in a longer time period of elevated instream concentration. This slow release is manifested by a long tail in breakthrough curves.

(Transient Storage Mechanisms (Runkel, 1999), (a) Storage zone caused by small pockets of slow-moving water. (b) The hyporheic zone.)

The major question being addressed by our current research is: what impact does the heterogeneity of the streambed hydraulic conductivity have on solute transport and storage? We hypothesize that stream bed heterogeneity will engender hyporheic exchange and its effect will be mechanistically similar to hyporheic exchange engendered by topography. To test this hypothesis we are conducting multiple conservative solute tracer tests and measuring the streambed hydraulic conductivity in situ using a portable permeameter. Solute tracer tests are conducted by slowly adding a conservative tracer (i.e., a chemical that does not decay or absorb to the soil) to the stream and measuring the concentration in the stream at several downstream locations over the course of the experiment.