Temporal and Spatial Analysis of Stream and Groundwater Interactions

Water chemistry and ecology of streams are impacted by the amount of water that exchanges between the surface water system and the adjacent saturated area, called the hyporheic zone, a dynamic area of stream channel sediments, which undergoes down-welling or up-welling of stream water. The rate and volume of water exchange between the surface water and the hyporheic zone are primary controls on stream ecology, but are challenging to assess. A common approach is to model the exchange rate with a one-dimensional advection-dispersion equation that includes solute exchange with transient storage zones, which is referred to as a transient storage model. OTIS, a computerized transient storage model, utilizes four hydraulic parameters that represent the stream and the hyporheic zone, which gives a simple measure of the size of the hyporheic zone and its exchange rate with the surface water. This study investigates the influences on hyporheic exchange across temporal and spatial scales to better understand the parameter variability associated with the transient storage model. Thirteen conservative tracer experiments were conducted, which involved multiple tracer injections in the same reach of a small, plane-bed stream in order to determine how seasonal changes affect hyporheic parameters. Another nine alluvial streams were used to conduct more conservative tracer experiments involving two reaches per stream with varying stream channel types but with similar alluvium geology to determine how spatial variations affect the hyporheic parameters. The tracer experiments repeated in the same stream were done across various discharges ranging from 500 to 8 L/s and across different seasons. The other nine alluvial streams tracer experiments were conducted within the same month with various discharges ranging from 100 to 29.8 L/s. Relationships between the hydraulic parameters and transient storage metrics were compared to discharge, velocity, stream unit power, and Darcy-Weisbach friction factor, which are simple channel descriptors to compare different streams and reaches at different discharge rates. Results show that at a specific reach location, discharge relates linearly to As and α, showing that discharge is a major contributor to the hyporheic processes. When comparing the spatially different but similar sites, the correlation of discharge to the hydraulic parameters weakens to a slight correlation, showing that other site-specific processes are occurring at each reach that are influencing the variability of hyporheic processes. Preliminary results show that α may not spatially change across streams that are similar in characterization, although further research needs to be conducted to see if spatially different, but similar streams, result in the same temporal trend of the exchange coefficient, α