Groundwater最新文献

In simulations of groundwater flow through dipping aquifers, layers of model cells are often “deformed” to follow the top and bottom elevations of the aquifers. When this approach is used in MODFLOW, adjacent cells within the same model layer are vertically offset from one another, and the standard conductance-based (two-point) formulation for flow between cells does not rigorously account for these offsets. The XT3D multi-point flow formulation in MODFLOW 6 is designed to account for geometric irregularities in the grid, including vertical offsets, and to provide accurate results for both isotropic and anisotropic groundwater flow. A recent study evaluated the performance of the standard formulation and XT3D using a simple, synthetic benchmark model of a steeply dipping aquifer. Although XT3D generally improved the accuracy of flow simulations relative to the standard formulation as expected, neither formulation produced accurate flows in cases that involved large vertical offsets. In this paper, we explain that the inability of XT3D to produce accurate flows in the steeply dipping aquifer benchmark was not due to an inherent limitation of the flow formulation, but rather to the limited cell connectivity inherent in the most commonly used discretization packages in MODFLOW 6. Furthermore, we demonstrate that XT3D is able to produce the expected accuracy when adequate cell connectivity is introduced using MODFLOW's unstructured grid type and the aquifer is discretized vertically using at least two model layers.

Continental glaciations during the Pleistocene Epoch created complex systems of aquifers and aquitards across many northern regions of the Earth. The low hydraulic conductivities of glacial till aquitards suggest that limited recharge will reach the underlying aquifers, potentially preserving old groundwaters. Here, we characterize the recharge history in intertill and buried valley aquifers in Saskatchewan, Canada using ¹⁴C, ³H, ⁴He δ²H, δ¹⁸O, and major ions. Intertill aquifers with depths of <30 m had corrected ¹⁴C ages ranging from 0 to 15.5 ka. These aquifers also contained ³H and/or elevated NO₃ in some locations, indicating that a component of modern recharge had mixed with older water. A single sample from the Judith River bedrock aquifer in the region had a corrected ¹⁴C age of 10.2 ka and elevated NO₃. Samples from buried valley aquifers with depths of 89 to 123 m contained older waters with ages >38 ka in some locations, indicating that recharge occurred before the last glacial advance over the region. While measuring tracers that cover a wide range of ages is necessary to understand these flow systems, δ²H and δ¹⁸O were less diagnostic because values of modern winter precipitation overlapped with groundwaters with a wide range of ages. The range of ages present in the intertill aquifers of the region indicates that these systems are currently being recharged, which indicates some development of groundwater resources is possible but also points to a need for groundwater protection measures.

Quantifying lacustrine groundwater discharge (LGD) is important for understanding the dynamics of lake ecosystems and their expansion. This study focuses on Lake Qinghai, employing radium isotope models to evaluate the contributions of both shallow and deep groundwater. The data indicate that the activity of ²²³Ra and ²²⁴Ra demonstrates a pronounced gradient, decreasing from the shoreline to the center of Lake Qinghai. Additionally, vertical stratification characteristics were observed. The spatial distribution of radium isotope activity suggests that there is discharge of both shallow and deep groundwater into the lake. Deep groundwater migrates slowly and its apparent age reflects the time elapsed since the water became enriched in Ra and was isolated from the source, in the study system this age is estimated to be 10.1 d. In contrast, shallow groundwater displayed varied apparent ages in different regions: 7.9 d in the north, 13.1 d in the south, and 7.4 d in the southeastern area of the lake. The LGDs of shallow groundwater discharge in the north, south, and southeast areas of Lake Qinghai were estimated by ²²⁴Ra as 1.89 × 10⁶ to 2.69 × 10⁶ m³/d, 3.25 × 10⁶ to 3.99 × 10⁶ m³/d, and 4.51 × 10⁶ to 6.33 × 10⁶ m³/d, respectively. For deep groundwater, the LGD was 0.16 × 10⁶ to 0.29 × 10⁶ m³/d. Annually, the total LGD fluxes of shallow and deep groundwater are 27.86 × 10⁸ to 37.59 × 10⁸ m³/year and 0.58 × 10⁸ to 1.06 × 10⁸ m³/year, respectively. This study is the first to evaluate shallow and deep groundwater discharge around the lake. Understanding these discharge dynamics is essential for developing effective management strategies to preserve lake environments.

Sea water intrusion (SWI) simulators are essential tools to assist the sustainable management of coastal aquifers. These simulators require the solution of coupled variable-density partial differential equations (PDEs), which reproduce the processes of groundwater flow and dissolved salt transport. The solution of these PDEs is typically addressed numerically with the use of density-dependent flow simulators, which are computationally intensive in most practical applications. To this end, model surrogates are generally developed as substitutes for full-scale aquifer models to trade off accuracy in exchange for computational efficiency. Surrogates represent an attractive option to support groundwater management situations in which fast simulators are required to evaluate large sets of alternative pumping strategies. Reduced-order models, a sub-category of surrogate models, are based on the original model equations and may provide quite accurate results at a small fraction of computational cost. In this study, a variable-density flow reduced-order model based on proper orthogonal decomposition (POD) and utilizing a fully coupled flow and solute-transport model is implemented with a finite-difference (FD) approach for simulating SWI in coastal aquifers. The accuracy and computational efficiency of the FD-POD approach for both homogeneous and—more realistic—heterogeneous systems are investigated using test cases based on the classic Henry's problem (Henry 1964). The findings demonstrate that the combined FD-POD approach is effective in terms of both accuracy and computational gain and can accommodate the output of the most popular variable-density flow models, such as those from USGS's MODFLOW family.

Due to increasing global demand for fresh water, it is increasingly necessary to understand how aquifer pumping affects groundwater chemistry. However, comprehensive predictive relationships between pumping and groundwater quality have yet to be developed, as the available data, which are often collected over inconsistent time intervals, are poorly suited for long-term historical correlation studies. For example, we needed an adequate statistical method to better understand relationships between pumping rate and water quality in the City of Norman (OK, USA). Here we used the interval-scaled change in mean pumping rate combined with the Quadrant method to examine correlations between pumping rates and changes in trace metal concentrations. We found that correlations vary across the study area and are likely dependent on a variety of factors specific to each well. Comparing the Quadrant method to the commonly used Kendall's tau correlation, which requires different assumptions about aquifer behavior, the methods produced similar correlations when sample sizes were large and the time interval between samples was relatively short. Sample sizes were then artificially restricted to determine correlation reproducibility. Despite being less reproducible overall, the Quadrant method was more reproducible when there were large time intervals between samples and very small sample sizes (n ~ 4), but not as reproducible as significant (p ≤ 0.1) Kendall's tau correlations. Therefore, the Quadrant method may be useful for further investigating the effects of pumping in cases where Kendall's tau does not produce significant correlations.

Our recent steady-state mass-balance modeling suggests that most global carbonic-acid weathering of silicate rocks occurs in the vadose zone of aquifer systems not on the surface by atmospheric CO₂. That is, the weathering solute flux is nearly equal to the total global continental riverine carbon flux, signifying little atmospheric weathering by carbonic acid. This finding challenges previous carbon models that utilize silicate weathering as a control of atmospheric CO₂ levels. A robust analysis utilizing global estimates of groundwater carbon concentration generated by a geospatial machine learning algorithm was coupled with recharge flux in a geographic information system environment to yield a total global groundwater carbon flux of between 0.87 and 0.96 Pg C/year to the surface environment. On discharging to the surface, the carbon is speciated between 0.01 and 0.11 Pg C/year as CaCO₃; 0.35 and 0.38 Pg C/year as CO₂ gas; and 0.49 and 0.51 Pg C/year as dissolved HCO₃⁻. This total weathering carbon flux was calculated for direct ocean discharge (0.030 Pg C/year); endorheic basins (0.046 Pg C/year); cold-wet exorheic basins (0.058 Pg C/year); warm-dry exorheic basins (0.072 Pg C/year); cold-dry exorheic basins (0.115 Pg C/year); and warm-wet exorheic basins (0.448 Pg C/year).

Drilling wells in unconsolidated formations is commonly undertaken to extract drinking water and other applications, such as aquifer thermal energy storage (ATES). To increase the efficiency of an ATES system, the drilling campaigns are targeting greater depths and enlarging the wellbore diameter in the production section to enhance the flow rates. In these cases, wells are more susceptible to collapse. Drilling fluids for shallow formations often have little strengthening properties and, due to single-string well design, come into contact with both the aquifer and the overburden. Drilling fluids and additives are experimentally investigated to be used to improve wellbore stability in conditions simulating field conditions in unconsolidated aquifers with a hydraulic conductivity of around 10 m/d. The impact on wellbore stability is evaluated using a new experimental setup in which the filtration rate is measured, followed by the use of a fall cone penetrometer augmented with an accelerometer to directly test the wellbore strengthening, and imaging with a scanning electron microscope (SEM) to investigate the (micro)structure of the filter cakes produced. Twelve drilling fluids are investigated with different concentrations of bentonite, polyanionic cellulose (PAC), Xanthan Gum, calcium carbonate (CaCO₃), and aluminum chloride hexahydrate ([Al(H₂O)₆]Cl₃). The filtration results indicate that calcium carbonate, average d_p <20 μm, provides pore throat bridging and filter cake formation after approximately 2 min, compared to almost instantaneous discharge when using conventional drilling fluids. The drilling fluid containing 2% [Al(H₂O)₆]Cl₃ forms a thick (4 mm) yet permeable filter cake, resulting in high filtration losses. The fall cone results show a decrease of cone penetration depth up to 20.78%, and a 40.27% increase in deceleration time while penetrating the sample with CaCO₃ compared with conventional drilling fluid containing bentonite and PAC, indicating a significant strengthening effect. The drilling fluids that contain CaCO₃, therefore, show high promise for field implementation.

The potential performance of a hypothetical colloidal-activated carbon (CAC) in situ remedy for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in groundwater in coastal zones was evaluated using estimated hydrogeologic and geochemical parameters for a coastal site in the United States. With these parameters, a reactive transport model (ISR-MT3DMS) was used to assess the effects of tidal fluctuations and near-shore geochemistry on CAC performance. The average near-shore ionic strength of 84 mM at the site was conservatively estimated to result in an increase in the adsorption of PFOA to CAC by about 50% relative to non-coastal sites with ionic strength <10 mM. The modeling also confirmed the hypothesis that tidally induced groundwater flow reversals near the shore would result in the accumulation of PFOA at the downgradient edge of the CAC zone. Slow desorption of PFOA from this downgradient CAC boundary may sustain downgradient plume concentrations above a strict cleanup criterion (e.g., USEPA MCL of 0.004 μg/L), for decades; however, there was still a large PFOA mass flux reduction (>99.9%) achieved after several decades at the shore. CAC longevity was substantially greater for PFOS with a similar source concentration; however, the higher PFOS distribution coefficient (K_d) in soil downgradient from the CAC zone resulted in substantially longer flushing times. It is recommended that short-term remedial action objectives for CAC remedies at coastal sites be based on mass flux reduction targets over a period of several decades, given the demonstrated challenges in trying to achieve very low cleanup criteria downgradient of a CAC zone in the short term.