Implications of anthropogenic river stage fluctuations on mass transport in a valley fill aquifer

Abstract

[1] 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.

Modeling the Impact of Stream Discharge Events on Riparian Solute Dynamics

“…Therefore, frequent, short‐term stream fluctuations will be more efficient than flow event of longer duration with lower peaks in mobilization and consequent export of solutes into the stream. Boutt and Fleming () also demonstrated that frequent stream fluctuations transport solute mass from the stream to the aquifer under zero net water flux by enhancing mixing process inside the aquifer. Gu et al () found that strong stream events significantly influence the chemistry of both surface water and groundwater by enhancing mixing and reaction efficiency in the near stream zone.…”

Investigation of Groundwater Contaminant Discharge into Tidally influenced Surface-water Bodies: Theoretical Analysis

“…Numerical simulations have been used to examine the effect of tidal fluctuation on the spreading of a solute plume (Zawadzki et al 2002;Elfeki et al 2007;Robinson et al 2007;Boutt and Fleming 2009). Yim and Mohsen (1992) showed mathematically that tidal fluctuations cause the concentration levels discharging to coastal waters to be lowered due to dilution.…”

Groundwater flow, nutrient, and stable isotope dynamics in the parafluvial-hyporheic zone of the regulated Lower Colorado River (Texas, USA) over the course of a small flood

“…More recent studies presented direct hydraulic observations of hyporheic flows in regulated river systems (Arntzen et al 2006;Boutt and Fleming 2009;Fritz and Arntzen 2007;Gerecht et al 2011;Sawyer et al 2009), and geophysical observations revealed the extent to which these exchanges may occur (Cardenas and Markowski 2011;Johnson et al 2012). Some studies have focused on processes occurring within the streambed (Arntzen et al 2006), while others emphasized processes within the banks (Boutt and Fleming 2009;Gerecht et al 2011).…”

“…This may promote additional denitrification in the bank sediment, though this increase is relatively small compared to denitrification in the riverbed (Gu et al 2012). Still, some modeling studies showed that because of dispersion, conservative solutes that enter the banks during repeated flood pulses may linger well beyond the flood duration (Boutt and Fleming 2009;McCallum et al 2010), potentially providing opportunities for a host of biogeochemical reactions to occur.…”