PROCESSES AFFECTING SOLUTE TRANSPORT IN STREAMS

The transport and transformation of solutes in a stream depends on open-channel and ground-water flow. Solute transport in most streams is described as advection-dispersion in the open channel and transient storage in both the channel and adjacent porous streambed. Transient storage in the porous streambed, or hyporheic zone, is usually conceptualized as a first-order mass transfer process. For relatively small channels complete exchange of hyporheic porewater with stream water generally occurs within a timeframe that is 2 or 3 times greater than the advective transport time. Results from a sodium-bromide tracer experiment conducted in Red Butte Creek, a 2nd order stream located in north-central Utah, indicate that the time required for complete hyporheic exchange is 30 to 40 times greater than the advection transport time.

The hyporheic exchange time for Red Butte Creek was estimated using data collected during a 69-hour tracer injection experiment. The estimated exchange time was derived from a comparison of time-series bromide concentrations at set locations along the stream and physical stream-flow measurements. Three approaches were used to examine the observed and estimated exchange times. Lateral hyporheic-zone exchange and transient storage were examined by optimizing hyporheic cross-sectional areas and exchange coefficients using OTIS-P. Downstream transport in the hyporheic zone was examined for likely channel-deposit geometries using a 1-dimensional decoupled open-channel/porous media technique that summed the effects of open-channel and ground-water flow velocities. Those same geometries were also examined using a 2-dimensional MT3D modeling approach.