HYPORHEIC EXCHANGE FLOWS AND THEIR RESIDENCE-TIME DISTRIBUTIONS IN MOUNTAIN STREAMS

The hyporheic zone can be an important location for biologically-mediated chemical reactions that modify the chemical nature of stream water. Exchange flows bring stream water into close contact with biofilms on sediment surfaces where these reactions tend to occur. However, the extent to which transformations occur is dependent, in part, on the contact time between water and biofilms. We investigated the residence time of water in the hyporheic zones of mountain streams, and how different morphologic features of the stream channel control both the amount of water and the frequency distribution of residence times of this water in the subsurface.

We used both groundwater flow models and solute transport models to examine water residence times in the hyporheic zone. We also made direct measurements from well networks to follow the movement of tracer-labeled water through the hyporheic zone and to estimate nominal travel time. We simulated subsurface flows through our study sites using MODFLOW, MODPATH, and MT3D to simulate hyporheic exchange flows, movement of tracer-labeled water through the hyporheic zone, and to estimate the residence time distribution of exchange flows.

Results showed that residence times tended to be short in small, steep headwater streams and much longer in larger, lower gradient streams. Valley floor width had relatively little effect on residence times in small streams, but strongly influenced residence times in the larger streams. Pool-step and pool-riffle sequences accounted for the greatest amount of hyporheic exchange flow and were dominated by relatively short residence time flow paths. Channel sinuosity and secondary channels accounted for less total exchange flow, but these had much longer residence times.