2006 Philadelphia Annual Meeting (2225 October 2006)
Paper No. 193-11
Presentation Time: 11:00 AM-11:15 AM

IMPLICATIONS OF DIURNAL RIVER FLUCTUATIONS ON MASS TRANSPORT IN A VALLEY-FILL AQUIFER

FLEMING, Brandon J., Geosciences, University of Massachusetts, Amherst, 611 N. Pleasant St, Amherst, MA 01003, bfleming@geo.umass.edu, BOUTT, David F., Department of Geosciences, University of Massachusetts, Amherst, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003, and MABEE, Stephen B., Office of the Massachusetts State Geologist, Dept. of Geosciences, University of Massachusetts-Amherst, Morrill Science Center, 611 North Pleasant Street, Amherst, MA 01003

Aquifers located in isolated stratified drift deposits in the northeastern portion of the US are extremely fragile and important groundwater resources. These aquifers, when restricted to bedrock valleys, are often strongly coupled to significant surface water drainage systems. In northwestern Massachusetts, surface water associated with the Deerfield River watershed is highly regulated by dams to protect against flooding and to generate hydroelectric power. Regular releases of water from these dams cause diurnal fluctuations in river stage. In a previous study performed by the USGS, measurements from two clusters of wells show a significant response to river stage fluctuations in the aquifer. Fluctuations in river stage and resulting changes in head levels in the aquifer cause a switch from a losing to a gaining stream. The flow reversals have implications for mass transport and nutrient cycling within the hyporheic zone. In this paper we investigate the physical hydrologic controls on mass transport in the shallow aquifer.

Using a coupled groundwater flow and transport code, we built a quasi three dimensional transient numerical model to approximate the head changes in the aquifer caused by the stage fluctuations in the river. Flow velocities and residence times were estimated in the aquifer for a variety of flow conditions. The mixing process driven by the aquifer head changes were quantified in the proximity of the hyporheic zone and shown to significantly influence both vertical and horizontal flow velocities in a region close to the stream-aquifer boundary. The diurnal river stage changes also appear to influence farfield hydrologic conditions and potentially hydrologically isolate the river and hyporheic zone. To further investigate these mixing processes we applied a mass transport code with conservative tracers to the aquifer. Fluctuation of the river stage combined with the heterogeneous nature of the aquifer creates a pumping mechanism that creates excess mixing within shallow portions of the aquifer. Aquifer dispersivity and molecular diffusion both contribute to the anomalous mixing modeled in the shallow aquifer. Mixing driven by stream stage changes has important implications for nutrient cycling as well as contaminant transport in the shallow aquifer.

2006 Philadelphia Annual Meeting (2225 October 2006)
General Information for this Meeting
Session No. 193
Stream-hyporheic Interactions: Hydrology, Geochemistry, and Biology
Pennsylvania Convention Center: 103 A
8:00 AM-12:00 PM, Wednesday, 25 October 2006

Geological Society of America Abstracts with Programs, Vol. 38, No. 7, p. 468

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