Mixing and dispersion in coastal aquifers are strongly influenced by periodic temporal flow fluctuations on multiple time-scales ranging from days (tides), seasons (pumping and recharge) to glacial cycles (regression and transgressions). Transient forcing effects lead to a complex space- ant time-dependent flow response which induces enhanced spreading and mixing of dissolved substances. We study effective mixing and solute transport in temporally fluctuating one-dimensional flow for a stable stratification of two fluids of different density using detailed numerical simulation as well as accurate column experiments. We quantify the observed transport behaviors and interface evolution by a time-averaged model that is obtained from a two-scale expansion of the full transport problem, and derive explicit expressions for the center of mass and width of the mixing zone between the two fluids. We find that the magnitude of transient-driven mixing is mainly controlled by the hydraulic diffusivity, the period and the initial interface location. At an initial time regime, mixing can be characterized by an effective dispersion coefficient and both the interface position and width evolve linearly in time. At larger times, the spatial variability of the flow velocity leads to a deceleration of the interface and a compression of its width, which is manifested by a subdiffusive evolution of its width as t1/2. This article is protected by copyright. All rights reserved.

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Transient forcing effects on mixing of two fluids for a stable stratification

A 2 D cross-shore transect model with variable density and saturation was established in FEFLOW to investigate the groundwater dynamics and solute transport under the influences of three tidal cond...

Simulated groundwater dynamics and solute transport in a coastal phreatic aquifer subjected to different tides

.............................................................................................................................................................. 5 Résumé ................................................................................................................................................................ 7 Knowledge dissemination ................................................................................................................................ 9 Acknowledgements ......................................................................................................................................... 15 Table of content............................................................................................................................................... 17 List of symbols and acronyms ....................................................................................................................... 19 List of figures ................................................................................................................................................... 21 List of tables ..................................................................................................................................................... 27 Introduction ..................................................................................................................................................... 29 The current challenge of mass flux measurement in contaminated aquifers .................................... 29 Research objectives ..................................................................................................................................... 31 Thesis outline ............................................................................................................................................... 31 1. Mass flux as fundamental metric for contaminated aquifers management ................................. 33 1.1. The environmental challenge of groundwater resources protection and management ....... 34 1.2. Contaminated sites in Europe and Wallonia ............................................................................... 35 1.3. Concepts of mass flux and mass discharge.................................................................................. 37 1.4. Methods for contaminant mass discharge calculation ............................................................... 42 1.5. Techniques and methods for contaminant mass flux measurement ....................................... 46 1.6. Techniques and methods for groundwater fluxes measurement ............................................. 50 1.7. Time variability of groundwater flow and consequences on mass flux measurements ........ 64 1.8. Conclusion ........................................................................................................................................ 65 2. The Finite Volume Point Dilution Method: theoretical concepts and developments .............. 67 2.1. Origin and fundamental principles of the FVPDM. .................................................................. 68 2.2. Mathematical basis for the FVPDM ............................................................................................. 68 2.3. Analytical solution for FVPDM interpretation in steady state groundwater flow ................ 71 2.4. Generalization of the FVPDM to transient state groundwater flow ....................................... 73 2.5. General experimental setup ............................................................................................................ 76 2.6. Dimensioning flow chart for FVPDM experiments .................................................................. 77 18 3. Study of the accuracy, of the precision and of the resolution of the FVPDM .......................... 81 3.1. Laboratory validation of the FVPDM for measuring horizontal flows .................................. 82 3.2. Evaluation of the precision of the FVPDM versus PDM in a fractured aquifer .................. 92 3.3. Field monitoring of a controlled transient groundwater flux using the FVPDM ............... 110 4. Field scale applications of the FVPDM .......................................................................................... 125 4.1. Groundwater flux measurement in aquifers of the discontinuous permafrost zone .......... 126 4.2. Field scale mass discharge measurement from a controlled solute plume ........................... 146 4.3. Innovative contaminant mass flux monitoring in an aquifer influenced by tidal effects. .. 160 General conclusion and perspectives ......................................................................................................... 177 Synthesis of the main research outcomes ............................................................................................. 177 Conclusions ................................................................................................................................................ 178 Perspectives ................................................................................................................................................ 179 References ....................................................................................................................................................... 181

Groundwater and contaminant mass fluxes monitoring in heterogeneous aquifers

Data from an one-dimensional homogeneous sand column, which is utilized to investigate the effect of tides on the concentration of groundwater contaminants discharging to a surface-water body, demonstrate that the tidal fluctuations in water level elevation create concentration oscillations upgradient of the groundwater discharge locations and there is a resulting decrease in average contaminant concentration at the point of groundwater discharge to a surface-water body. The further upgradient an observation point is located, the smaller the amplitude of the tidally induced concentration oscillations. In addition, an excessive upstream migration of concentration oscillations is observed although there is a net downgradient flow. As the classical groundwater flow and transport model could not reproduce this phenomena, a multi-mobility model is proposed with one highly mobile liquid phase, one less mobile liquid phase and a solid phase. Averaging theory is applied in a first step to develop the macroscopic mass conservation equation from its microscale counterpart and then, in a second step, averaging is again used to reduce dimensionality to one-dimensional governing equations defined along the axis of the column. The simulation confirms the existence of an enhanced tidally induced mixing process and the suitability of our mathematical-physical representation of it.

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

Wind waves, which frequently occur on large surface water bodies such as lakes, may temporarily alter flow patterns in a subsurface zone and the corresponding water and nutrient interactions between surface waters and shallow groundwaters. To better understand these processes, soil flume experiments were carried out to investigate wind wave-driven water and chloride interactions across the lake–groundwater interface, and the Hydrus-2D model was used to analyze and evaluate the observed experimental results. Two interaction cases between the lake and groundwater systems were considered: groundwater discharging into a lake (the GDL case), and lake water recharging groundwater (the LRG case). For comparison, no-wave conditions for both the GDL and LRG cases were also analyzed. The results revealed that, similarly to no-wave conditions, water and chloride exchange fluxes between the lake and groundwater systems under wave conditions occurred mainly within narrow bands near the intersection of the water level in the lake and the interface in both the GDL and LRG cases, and then exponentially decreased along the interface. Most water and chloride that infiltrated into the subsurface zone through the upper part of the interface during a wave crest returned to the lake through the lower part during a wave trough in both the GDL and LRG cases, creating local recirculation zones in the subsurface near the interface. Such recirculation produced a more frequent exchange of water and solute across the interface compared with those under no-wave conditions. During a one-day period after wind waves started, the total exchange fluxes of water and chloride to the lake decreased by 36.2% and 71.9%, respectively, compared to the no-wave conditions in the GDL case. In the LRG case, the total exchange water fluxes to the subsurface increased by 89.7%, while the total exchange chloride fluxes increased only slightly (4.5%) compared to the no-wave conditions due to the difference in chloride concentrations between the upper and lower parts of the interface. The sensitivity analysis revealed that the hydraulic conductivity of the lakeshore zone and the characteristics of the waves were important factors influencing water and chloride exchange between the lake and groundwater systems. The simulated results helped us to better understand water and solute interactions in the lake–groundwater system during windy periods.

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Simulating the Effects of Lake Wind Waves on Water and Solute Exchange across the Lakeshore Using Hydrus-2D

An investigation of the effect of tidally influenced water elevations on the concentration of groundwater contaminants discharging to a surface-water body is studied using a one-dimensional homogeneous sand column. A constant water level is imposed upstream, and the downstream water level is controlled by a wave generator that controls the hydraulic head to mimic a 12-h tidal fluctuation. The experimental results demonstrate that the tidal fluctuations in the downstream reservoir result in a decrease in average contaminant concentration at the point of discharge to the tidally influenced surface-water body. The further upstream an observation well is located, the smaller the amplitude of the concentration oscillation. Fourier analysis suggests that the dominant frequency of the pressure at different locations along the length of the column is identically two cycles per day and that the concentration data have a dominant frequency of two cycles per day, but also exhibit harmonics.

Hua Chen

Papers

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

Investigation of Groundwater Contaminant Discharge into Tidally Influenced Surface-Water Bodies: Experimental Results