Yueqing Xie

When can inverted water tables occur beneath streams

Decline in regional water tables (RWT) can cause losing streams to disconnect from underlying aquifers. When this occurs, an inverted water table (IWT) will develop beneath the stream, and an unsaturated zone will be present between the IWT and the RWT. The IWT marks the base of the saturated zone beneath the stream. Although a few prior studies have suggested the likelihood of an IWT without a clogging layer, most of them have assumed that a low-permeability streambed is required to reduce infiltration from surface water to groundwater, and that the IWT only occurs at the bottom of the low-permeability layer. In this study, we use numerical simulations to show that the development of an IWT beneath an unclogged stream is theoretically possible under steady-state conditions. For a stream width of 1 m above a homogeneous and isotropic sand aquifer with a 47 m deep RWT (measured in an observation point 20 m away from the center of the stream), an IWT will occur provided that the stream depth is less than a critical value of 4.1 m. This critical stream depth is the maximum water depth in the stream to maintain the occurrence of an IWT. The critical stream depth decreases with stream width. For a stream width of 6 m, the critical stream depth is only 1 mm. Thus while theoretically possible, an IWT is unlikely to occur at steady state without a clogging layer, unless a stream is very narrow or shallow and the RWT is very deep.

On the limits of heat as a tracer to estimate reach‐scale river‐aquifer exchange flux

For the past few decades, heat has been used to estimate river-aquifer exchange flux at discrete locations by comparison of river and groundwater temperature. In recent years, heat has also been employed to estimate reach-scale river-aquifer exchange flux based only on river temperature. However, there are many more parameters that govern heat exchange and transport in surface water than in groundwater. In this study, we analyzed the sensitivities of surface water temperature to various parameters and assessed the accuracy of temperature-based estimates of exchange flux in two synthetic rivers and in a field setting. For the large synthetic river with a flow rate of 63 m3 s−1 (i.e., 5.44 × 106 m3 d−1), the upper and lower bounds of the groundwater inflow rate can be determined when the actual groundwater inflow is around 100 m2 d−1. For higher and lower fluxes, only minimum and maximum bounds respectively can be determined. For the small synthetic river with the flow rate of 0.63 m3 s−1 (i.e., 5.44 × 104 m3 d−1), the bounds of the groundwater inflow rate can only be estimated when the actual groundwater inflow rate is near 10 m2 d−1. In the field setting, results show that the inflow rate must be less than 100 m2 d−1, but a lower bound for groundwater inflow cannot be determined. The large ranges of estimated groundwater inflow rates in both theoretical and field settings indicate the need to reduce parameter errors and combine heat measurements with other isotopic and/or chemical methods. This article is protected by copyright. All rights reserved.

Limits of heat as a tracer to quantify transient lateral river‐aquifer exchanges

The application of heat as a tracer for assessing river-aquifer exchanges has been mainly limited to vertical flow through the riverbed. Lateral river-aquifer exchanges become more important than vertical riverbed exchanges if the river is deeply incised into an aquifer. Few studies have examined lateral river-aquifer exchanges and the ability of heat to constrain such exchanges. This study aims to perform a robust assessment of the limits of heat as a tracer to quantify lateral river-aquifer exchanges. The study is based on a section of the Meuse River in Belgium, a river predominantly gaining in the studied area and becoming intermittently losing in the winter time. A calibrated transect model shows that river temperature can affect groundwater temperature up to 9 m into the aquifer. An accompanying synthetic modelling investigation using Monte Carlo simulation shows that heat data for distances between 4 and 9 m from the river can reduce the uncertainty of river-aquifer exchanges for conditions similar to those of the transect model. The ability of heat to reduce the river-aquifer exchange uncertainty improves with distance from the river because of the reduction in the number of acceptable model realizations. The optimal distance is 8 m from the river where the groundwater temperature is no longer affected by the river temperature. The synthetic modelling also indicates that heat alone cannot constrain river-aquifer exchanges better than the commonly used hydraulic head. However, when combined with hydraulic head, heat can significantly reduce the uncertainty of river-aquifer lateral exchanges under gaining conditions.

Impact of kinetic mass transfer on free convection in a porous medium

We investigate kinetic mass transfer effects on unstable density-driven flow and transport processes by numerical simulations of a modified Elder problem. The first-order dual-domain mass transfer model coupled with a variable-density-flow model is employed to describe transport behavior in porous media. Results show that in comparison to the no-mass-transfer case, a higher degree of instability and more unstable system is developed in the mass transfer case due to the reduced effective porosity and correspondingly a larger Rayleigh number (assuming permeability is independent on the mobile porosity). Given a constant total porosity, the magnitude of capacity ratio (i.e., immobile porosity/mobile porosity) controls the macroscopic plume profile in the mobile domain, while the magnitude of mass transfer timescale (i.e., the reciprocal of the mass transfer rate coefficient) dominates its evolution rate. The magnitude of capacity ratio plays an important role on the mechanism driving the mass flux into the aquifer system. Specifically, for a small capacity ratio, solute loading is dominated by the density-driven transport, while with increasing capacity ratio local mass transfer dominated solute loading may occur at later times. At significantly large times, however, both mechanisms contribute comparably to solute loading. Sherwood Number could be a nonmonotonic function of mass transfer timescale due to complicated interactions of solute between source zone, mobile zone and immobile zone in the top boundary layer, resulting in accordingly a similar behavior of the total mass. The initial assessment provides important insights into unstable density-driven flow and transport in the presence of kinetic mass transfer.

The Effect of Porous Medium Storage on Unstable Density-Driven Solute Transport

Unstable density-driven groundwater flow and solute transport (i.e., free convection) leads to spatiotemporal variations in pressure. Specific storage (So ) indicates the capability of a confined aquifer to release or store groundwater associated with a pressure change. Although So is known to dampen pressure propagation, So has been implicitly assumed to have a negligible impact on the unstable free convective process in prior studies. This work explores the effect of So on both the classic onset criterion and the fingering process using numerical models. Results show that the classic onset criterion is applicable when So is smaller than 10(-1) m(-1) . Results also demonstrate that So does not play a significant role in the free convective fingering process unless it is greater than 10(-3) m(-1) . For most practical purposes in hydrogeology (large Rayleigh number and small So ), the implicit assumption of small or zero So is appropriate.

Efficient natural attenuation of acidic contaminants in a confined aquifer

As the demand for energy grows steadily and fossil fuels are depleted gradually, nuclear power will continue to play an important role in meeting energy needs in the future. Uranium, as the fuel for nuclear power generation, has experienced the increase in production in recent years. Acid in situ leach uranium mining is an effective and popular technique to extract uranium without exposure of workers to radioactive ore deposits. Despite this advantage, this technique causes extremely high concentrations of contaminants in confined aquifers after mining is completed. These contaminants will undergo natural attenuation while migrating downgradient with regional groundwater. Mounting concerns have been raised regarding widespread groundwater contamination due to concentrations of contaminants and lack of published studies for comparison. In this study, the fate of various contaminants was examined to infer the extent of natural attenuation in a confined aquifer in northwest China. Results indicate the efficiency of natural attenuation in the confined aquifer. In addition, the contaminant plume migrated much slower than regional groundwater. The dual-domain theory has been invoked to aid in the interpretation of the decoupling of the plume migration with the regional groundwater flow. This study also implies that widespread contamination in confined aquifers is more likely to be caused by external factors (e.g., mechanical failure, human errors) than post-mining spreading of contaminants. To be always safe, strict monitoring schemes must still be established and operated.