TY - Generic T1 - Implications of diurnal river fluctuations on mass transport in a valley-fill aquifer T2 - Abstracts with Programs - Geological Society of America Y1 - 2006 A1 - Brandon J Fleming A1 - David F Boutt A1 - Stephen B Mabee KW - #StaffPubs KW - aquifers KW - BEDROCK KW - clastic sediments KW - controls KW - diffusion KW - diurnal variations KW - drainage KW - drift KW - Eastern U.S. KW - Environmental geology 22 KW - floods KW - fluctuations KW - geochemical cycle KW - geologic hazards KW - ground water KW - measurement KW - mixing KW - models KW - Northeastern U.S. KW - numerical models KW - nutrients KW - pollution KW - processes KW - pumping KW - quantitative analysis KW - residence time KW - sediments KW - shallow aquifers KW - surface water KW - three-dimensional models KW - tracers KW - transport KW - United States KW - valleys KW - water pollution KW - water resources KW - water wells AB - 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. JF - Abstracts with Programs - Geological Society of America PB - Geological Society of America (GSA) : Boulder, CO, United States CY - United States VL - 38 SN - 00167592 UR - https://gsa.confex.com/gsa/2006AM/finalprogram/abstract_115285.htm IS - 77 N1 - Accession Number: 2010-061334; Conference Name: Geological Society of America, 2006 annual meeting; Philadelphia, PA, United States; Conference Date: 20061022; Language: English; Coden: GAAPBC; Collation: 1; Collation: 468; Publication Types: Abstract Only; Serial; Conference document; Updated Code: 201034; Monograph Title: Geological Society of America, 2006 annual meeting; Monograph Author(s): Anonymous; Reviewed Item: Analytic JO - Abstracts with Programs - Geological Society of America ER -