Comparison of transient storage in vegetated and unvegetated reaches of a small agricultural stream in Sweden:: seasonal variation and anthropogenic manipulation

In this work, we compare solute transport and hyporheic exchange in vegetated and unvegetated reaches of Sava Brook, an agricultural stream in Sweden subject to extreme variations in channel vegetation and morphology due to both natural seasonal effects and anthropogenic manipulation. A solute injection experiment was conducted in September, 2001 (late summer), at which time there was extensive in-channel vegetation in upstream reaches but none in downstream reaches due to channel excavation by farmers. Experimental results are interpreted using both the advective storage path model and the transient storage model. Results from the vegetated and excavated reaches of the stream are compared both with each other and with the results of a previous experiment conducted in April, 1998 (early spring), when the stream was not excavated but there was only minimal vegetation present in the area due to natural seasonal effects. Results from the two injection experiments are compared by using scaled parameters that appropriately include the effects of stream velocity and depth on hyporheic exchange. This analysis indicates that the variation of solute storage time in all non-excavated agricultural reaches is attributable to differences in stream flow depth, velocity, and the hydraulic conductivity of the streambed sediments. Mixing in vegetated reaches is characterized by rapid exchange and considerable lag of the mean solute peak relative to the mean channel velocity. In addition, excavation altered the stream channel geometry so as to increase the storage time of solutes and reduce the effective exchange rate. This work indicates the need to consider the effect of specific processes when analyzing hyporheic exchange using tracer-injection methods, and supports the use of model frameworks with the potential to explicitly include different formulations for various hyporheic and dead zone transport processes. (C) 2003 Elsevier Ltd. All rights reserved.