Title

Hyporheic interaction and nitrate uptake in a semi-arid stream

Date of Award

2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Donald I. Siegel

Keywords

Semiarid, Groundwater, Hyporheic, Nitrate, Stream water, Debris dams

Subject Categories

Earth Sciences | Geology | Physical Sciences and Mathematics

Abstract

The exchange of surface and ground water back and forth across streambeds, in the hyporheic zone, strongly influences stream water chemistry by increasing solute residence times and exposing solutes to biogeochemically active sediments. The purpose of my dissertation research was to complete an intensive study of the geomorphic features that drive hyporheic interactions and how those interactions influence nitrate uptake.

Conservative and nitrate addition tracer tests were used to quantify the degree of hyporheic interaction and nitrate uptake along various segments of Red Canyon Creek, a semi-arid stream in Wyoming. Hyporheic exchange rates observed during snowmelt recession equal or exceed the exchange rates others have observed during baseflow at other streams. Along gaining reaches, debris dams and severe meanders generate comparable rates of hyporheic exchange. Along non-gaining reaches, hyporheic interactions have less impact on solute residence time because stream water is diverted away from the stream and captured by long near-stream flow paths.

Nitrate uptake along the most downstream reach of Red Canyon Creek is rapid compared to results from other studies, but other sites within the watershed show little nitrate uptake. Nitrate uptake does not appear to be related to hyporheic interaction along Red Canyon Creek. Statistical analyses of several other nitrate addition tracer tests show that almost half the variability in nitrate uptake can be explained by specific discharge and transient storage area. However, predictive models are most effective for sites with very different stream characteristics and cannot be effectively used to predict nitrate uptake variability along Red Canyon Creek.

To understand better where hyporheic interactions are occurring along Red Canyon Creek and what geomorphic features are driving those interactions, I built a groundwater flow and solute transport model. I found debris dams are a key driver of stream water into the subsurface, causing the largest flux of water across the streambed and creating hyporheic zones with up to twice the cross-sectional area of other hyporheic zones. My modeling approach, which simulates both advective transport and mixing of solutes, is more comparable to field-based observations of hyporheic exchange than standard particle tracking simulations.

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