UMass Sesquicentennial

Implications of diurnal river fluctuations on mass transport in a valley-fill aquifer

TitleImplications of diurnal river fluctuations on mass transport in a valley-fill aquifer
Publication TypeConference Proceedings
Year of Publication2006
AuthorsFleming, BJ, Boutt, DF, Mabee, SB
Conference NameAbstracts with Programs - Geological Society of America
Pagination468 - 468
Date Published2006/10/01/
PublisherGeological Society of America (GSA) : Boulder, CO, United States
Conference LocationUnited States
ISBN Number00167592
Keywords#StaffPubs, aquifers, BEDROCK, clastic sediments, controls, diffusion, diurnal variations, drainage, drift, Eastern U.S., Environmental geology 22, floods, fluctuations, geochemical cycle, geologic hazards, ground water, measurement, mixing, models, Northeastern U.S., numerical models, nutrients, pollution, processes, pumping, quantitative analysis, residence time, sediments, shallow aquifers, surface water, three-dimensional models, tracers, transport, United States, valleys, water pollution, water resources, water wells

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.

Short TitleAbstracts with Programs - Geological Society of America
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