Hyporheic exchange in streams supports many important ecosystem services but can also contribute to legacy pollution, by trapping less reactive contaminants in streambed sediments that are then slowly released back to the stream over time. In this study we evaluated three different analytical representations of the backward transit time distribution (bTTD) of water leaving the hyporheic zone, corresponding to different mechanisms for how water and solutes in hyporheic zone storage are sampled for outflow: (1) uniform sampling (exponential bTTD), plug-flow sampling (Dirac delta bTTD), and preferential sampling of either young or old water (Gamma bTTD). Using the Method of Moments, these three bTTDs were tested against data from 47 previously published hyporheic exchange experiments conducted in laboratory flumes over a range of flow conditions, sediment grain sizes, and bedform sizes and types. Based on measures of model fit and parsimony (AICc), in all 47 experiments hyporheic exchange was best represented by either the Gamma or exponential distributions. Further, values for key process variables, including hyporheic exchange flux and the Gamma distribution's shape parameter are correlated with readily measured field variables, including mean grain diameter of the streambed, streambed roughness, and mean stream discharge and velocity. This work advances understanding of hyporheic exchange processes and their representation in models of pollutant fate and transport in streams. / Master of Science / The sediments beneath a stream, where surface water and groundwater interact, play an important role in supporting aquatic ecosystems and nutrient cycling. In natural streams, conservative pollutants enter into the permeable sediments and slowly return to the stream over time, known as legacy pollution. This study looks at different models of how conservative pollutants circulate through the in-stream sediments, and tests how well each model represents published laboratory experiments. Model parameters were also correlated to physical parameters such as flow velocity, mean grain size diameter, and porosity. This study helps clarify the role that sediments play in pollutant transport in streams and articulates implications for legacy pollution in stream bed sediments.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/117303 |
| Date | 04 January 2024 |
| Creators | Werber, Nelson Norris |
| Contributors | Civil and Environmental Engineering, Grant, Stanley, Monofy, Ahmed Mostafa Ibrahem Mostafa, Hotchkiss, Erin R. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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