| Summary: | Surface transient storage (STS) in stream ecosystems serve an important function in retaining nutrients and refugia for aquatic communities. Unfortunately, they can retain contaminants as well. Therefore, it is of importance to determine the residence time distribution (RTD). A RTD of a particular STS zone encompasses the time it takes for the first pulse of water to leave the STS zone, and for the mean residence time of water in that zone, among other things. The RTD of STS is also useful to subtract from the RTD of the total transient storage in streams in order to determine the hyporheic transient storage (HTS) of streams, which is difficult to measure.
Currently, there is no definitive method of determining the RTD of STS. They have been determined with tracer injection alone, though this is time consuming and subject to interference from HTS. A relationship between STS physical characteristics and a RTD would be desirable, as this would characterize the time of entrainment of STS based upon a few easily measured physical parameters. This exists for groyne fields and flumes, which both have artificial STS. However, direct application of these equations to natural STS leads to errors due to simplistic geometries.
The focus of this study determines RTDs in lateral STS, which is adjacent to the main channel of a stream and a significant proportion of STS, and its relationship to physically measurable parameters of lateral STS. Twenty sites throughout Oregon were each injected with NaCl to determine four residence timescales: Langmuir time (��[subscript L]), negative inverse slope of the normalized concentration curve of the primary gyre (��[subscript 1]), negative inverse slope of the normalized concentration curve of the entire STS zone (��[subscript 2]), and the mean residence time (��[subscript STS]). The RTDs of these sites were then compared to the length, width, and depth of each lateral STS zone, as well as the velocity of the adjacent main channel. This data also was used to calculate dimensionless parameters submergence, a measure of bed roughness, and k, a measure of exchange that relates ��STS to lateral STS and associated parameters.
��[subscript 1] was found to be identical to ��[subscript STS], and ��[subscript 2] could not be defined. ��[subscript STS] was found to be approximately 1.35 times ��[subscript L], the ratio of which (��[subscript L]/��[subscript STS]) is positively correlated with lateral STS submergence. ��[subscript L] and ��[subscript STS] are positively correlated with lateral STS parameters, and inversely correlated with main channel velocity. The value of k from this study was comparable to the value of k from other studies in flumes, and so there is a relationship between RTDs and lateral STS parameters. === Graduation date: 2013
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