@proceedings {302, title = {Implications of diurnal river fluctuations on mass transport in a valley-fill aquifer}, volume = {38}, year = {2006}, note = {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}, month = {2006/10/01/}, pages = {468 - 468}, publisher = {Geological Society of America (GSA) : Boulder, CO, United States}, address = {United States}, abstract = {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.}, 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}, isbn = {00167592}, url = {https://gsa.confex.com/gsa/2006AM/finalprogram/abstract_115285.htm}, author = {Brandon J Fleming and David F Boutt and Stephen B Mabee} } @article {315, title = {Implications of anthropogenic river stage fluctuations on mass transport in a valley fill aquifer}, journal = {Water Resources Research}, volume = {45}, year = {2009}, month = {2009/01/01/}, pages = {@CitationW04427 - @CitationW04427}, publisher = {American Geophysical Union : Washington, DC, United States}, abstract = {In humid regions a strong coupling between surface water features and groundwater systems may exist. In these environments the exchange of water and solute depends primarily on the hydraulic gradient between the reservoirs. We hypothesize that daily changes in river stage associated with anthropogenic water releases (such as those from a hydroelectric dam) cause anomalous mixing in the near-stream environment by creating large hydraulic head gradients between the stream and adjacent aquifer. We present field observations of hydraulic gradient reversals in a shallow aquifer. Important physical processes observed in the field are explicitly reproduced in a physically based two-dimensional numerical model of groundwater flow coupled to a simplistic surface water boundary condition. Mass transport simulations of a conservative solute introduced into the surface water are performed and examined relative to a stream condition without stage fluctuations. Simulations of 20 d for both fluctuating river stage and fixed high river stage show that more mass is introduced into the aquifer from the stream in the oscillating case even though the net water flux is zero. Enhanced transport by mechanical dispersion leads to mass being driven away from the hydraulic zone of influence of the river. The modification of local hydraulic gradients is likely to be important for understanding dissolved mass transport in near-stream aquifer environments and can influence exchange zone processes under conditions of high-frequency stream stage changes.}, keywords = {$\#$Hydro, $\#$WaterResources, aquifers, boundary conditions, Charlemont, Deerfield River basin, fluctuations, fluvial features, Franklin County Massachusetts, ground water, human activity, Hydrogeology 21, hydrology, massachusetts, numerical models, preferential flow, rivers, shallow aquifers, streams, surface water, transport, two-dimensional models, United States, valleys}, isbn = {0043139719447973}, url = {http://onlinelibrary.wiley.com/doi/10.1029/2007WR006526/full}, author = {David F Boutt and Brandon J Fleming} }