Date of Award

5-2013

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor(s)

Laura K. Lautz

Keywords

geochemistry, hyporheic zone, restoration, storms, stream-groundwater interactions

Subject Categories

Earth Sciences

Abstract

The body of literature on stream-groundwater interactions is rapidly growing, but little is known about the effects of either storm events or in-stream restoration on the interactions between surface water and groundwater. The chapters of this thesis explore two of these questions at the same locality of interest: (1) subsurface geochemical dynamics across a riffle bedform in an unrestored stream during a storm event and subsurface geochemical, and (2) hyporheic exchange dynamics at baseflow conditions pre- and one year post- stream restoration.

The study site was a 30 m stretch of stream that underwent stream restoration through installation of a cross-vane and engineered rock riffle over a natural pool-riffle-pool sequence. Pre-restoration, mini-piezometers and temperature profile rods were spatially located around the riffle. Fourteen mini-piezometers were installed at a 15 cm depth into the streambed and coupled with temperature profile rods that recorded temperature in the water column as well as at 5 cm intervals to a depth of 30 cm into the streambed. Sampling of pore water occurred during baseflow conditions as well as during and after Tropical Storm Irene. Principal component analysis was used to understand the controls on both spatial and temporal stream and pore water chemistry. Through the use of a MATLAB program that utilizes a one-dimensional heat transport model, vertical exchange rates in the streambed were calculated using the measured temperature fluctuations in the streambed during baseflow conditions. Similar to pre-restoration, 19 mini-piezometers and 10 temperature profile rods recorded pore water geochemistry and vertical exchange rates around the installed cross-vane and engineered rock riffle during baseflow conditions one year after restoration.

Pre-restoration, the majority of spatial variability in pore water geochemistry (62%) is driven by differential mixing of surface and ground water across the hyporheic zone. The second largest driver of pore water geochemistry (17%) was temporal dilution and re-enrichment of infiltrating surface water during Tropical Storm Irene. Hyporheic sites minimally affected by upwelling groundwater showed temporal fluctuations in pore water geochemistry across the reach influenced by both changes in infiltrating stream chemistry as well as hyporheic residence time and flowpath length. The streambed zone influenced by groundwater discharge increased in size during Tropical Storm Irene, indicating that the area of localized groundwater inputs grows in response to storm events.

After restoration, hyporheic exchange rates increased by an order of magnitude immediately adjacent to the cross-vane and engineered rock riffle, when compared to exchange rates around the riffle in pre-restoration conditions. Away from the restoration structure, exchange rates are similar between pre- and post- restoration. A Rhodamine WT injection suggested 100% of streambed pore water immediately adjacent to the structure originated from the stream, while the rest of the study site received roughly 20 percent stream water. Evidence of nitrate production and uptake were seen across the pre-restoration riffle, but the post-restoration cross-vane and rock riffle showed evidence of only nitrate uptake. Therefore, although restoration produces hot spots of hyporheic exchange, the high exchange rates reduce hyporheic flow path residence time such that nitrate production cannot change nitrate concentrations in the streambed. While zones of groundwater inputs are present pre- and post- restoration, the zone of groundwater upwelling increased post-restoration, suggesting cross-vane installation may have disturbed subsurface hydraulic conductivity.

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