Groundwater最新文献

PEST++IES (White 2018; White et al. 2020) is widely used in the groundwater modeling community for its ability to perform computationally efficient history matching and uncertainty analysis in a highly parameterized context. One primary advantage of using an iterative ensemble smoother is that the number of model runs required per iteration depends on the number of realizations in an ensemble, not the number of parameters in each realization. However, this raises the question: what is the optimal number of realizations and iterations to use for any one model before the point of diminishing returns? Using a modified version of the Freyberg model (Freyberg 1988; Hunt et al. 2020), different parameter and observation scenarios were evaluated for four iterations and ensembles of 10, 25, 50, 100, 250, 500, 1000, and 2000 realizations. To match observations, PEST++IES altered hydraulic conductivity (k), both globally across the model and locally at three different pilot point densities, as well as global recharge (via a single multiplier), global river conductance, and individual well flow rates. Risk-based well capture zone results (Fienen et al. 2022a) and estimated hydraulic conductivity fields from each scenario were quantitatively and qualitatively compared against the “truth” model and its outputs. Across the cases examined, ensemble sizes of 100 to 250 realizations and two PEST++IES iterations were generally sufficient to achieve good results.

Cross-sectional (2D) groundwater models are commonly applied to simulate complex processes that are challenging to capture using the coarse grids of 3D regional-scale models. 2D models are often extracted from 3D models for this purpose. However, translating groundwater properties from 3D to 2D models so that regional flow patterns are preserved poses several challenges. A methodology is presented here to maximize agreement between the heads of 2D and 3D groundwater models, considering MODFLOW models with rectilinear grids. This includes careful averaging of hydraulic properties and stresses from the 3D model to create commensurate properties and stresses in cross section. The approach was evaluated by examining the statistical match of transient heads within 10 cross sections extracted from a 3D model of the Limestone Coast (Australia). Concordance between 2D and 3D models was generally poor but was improved by incorporating lateral flow as inflows/outflows in 2D models. Lateral flows required inputs from the 3D model, which limits the application of 2D models as independent predictive tools. Pumping in the 3D model was redistributed to neighboring cells to reduce errors in the 2D model that arise from the limited capability to simulate pumping effects. Although pumping redistribution led to minimal improvement for the case study model, simpler modeling scenarios with more intense, localized pumping showed substantially better head matches between 2D and 3D models when pumping redistribution was applied. The methodology for creating cross-sectional models offered in this article provides relatively simple steps for creating 2D models that are consistent with 3D parent models, although further work is needed to develop a methodology for 2D models that are oblique to 3D model grids.

This study investigates the spatial and temporal sensitivity of aquitard hydraulic conductivity and specific storage on drawdowns in pumping tests. The objective is to understand which area of the aquitard is represented by drawdowns in different observation wells. A three-layered MODFLOW 6 model was used to simulate pumping tests on a circular Voronoi grid for three transmissivity scenarios and both confined and semiconfined top boundary conditions. A local sensitivity analysis was performed using PEST++ to determine how perturbations in hydraulic conductivity and specific storage of the aquitard affect head changes at observation wells in the pumped and overlying aquifer. Results indicate that for observation wells in the pumped aquifer, sensitivity forms an elliptical shape that is symmetrical around the observation well and the pumping well for all scenarios. The sensitivity map for the observation well in the overlying aquifer depends on the transmissivity ratio between both aquifers. It favors the area surrounding the pumping well if the transmissivity of the pumped aquifer is lower than that of the overlying aquifer. Conversely, with higher transmissivity in the pumped aquifer, sensitivity primarily lies around the observation well. Sensitivity patterns evolve over time, expanding the area of influence and shifting the sensitivity toward the observation well for a semiconfined top boundary. These findings are relevant for understanding the information regarding aquitard heterogeneity that is present in pumping test drawdowns and optimizing pumping test design.

Climate change induced aridity and Euro-American settlement have altered the historical disturbance and flow regimes of large portions of the ponderosa pine forests of northern Arizona. The increased occurrence of high-severity wildfires due to these changes has led to the establishment of various forest restoration programs to protect the region's forests and their watersheds. In 2014, a paired-watershed monitoring project was implemented to compare the impacts of differing levels of forest thinning to watershed hydrology in seven experimental watersheds nested within the Upper Lake Mary (ULM) watershed in Arizona. This study expands the calibration phase of the ULM paired-watershed by synthesizing historic precipitation, surface runoff, groundwater recharge, soil moisture data, and evapotranspiration (ET) data to perform regression analyses and create a holistic water balance for each watershed. The magnitude and timing of seasonal groundwater recharge events were quantified for the first time in this region using a water table fluctuation method. The results showed that recharge did not occur every year and was heavily dependent (P < 0.05) on total winter season precipitation and snowpack duration. On average, recharge composed 9% of the total water budget when present. The results of this study lay the foundation for a greater understanding of how forest restoration alters northern Arizona's forest hydrology and will provide crucial information that should be used in water policy and water resource decision-making as the region plans for future water availability.

Reduction potentials of redox couples are fundamental for understanding subsurface geochemistry and guiding water resource exploration and management. Reduction potentials are routinely calculated with the Nernst equation, which requires detailed chemical composition data and complex speciation modeling—factors that limit its application in large-scale or data-limited field settings. To address these limitations, we developed a data-driven simplified Nernst equation that estimates the reduction potentials of individual redox couples using only pH and temperature. By integrating geochemical modeling with a global groundwater chemistry dataset, we demonstrate that pH is the dominant control on redox potential, while temperature and redox species activity play secondary roles. The resulting formulation reduces computational demands while maintaining high-predictive accuracy across diverse groundwater environments. This approach enables rapid and scalable estimation of reduction potentials, supporting applications in geochemical modeling, contaminant transport prediction, and groundwater quality assessments. Furthermore, it offers a thermodynamically grounded yet practical framework for interpreting electron transfer dynamics in natural groundwater systems.

Karst aquifers have evolved secondary porosity features that facilitate heterogeneous recharge and groundwater flow dynamics. These dynamics affect the natural spatial and temporal variability of water quality in the aquifer. However, when recharge occurs near urban and agricultural land use that can introduce contamination, the contamination can conflate natural water quality variability, generating convoluted signals in time and space. Most water quality investigations in karst aquifers rely on groundwater sampling at discrete locations such as wells or springs, which do not always capture the magnitude of water quality heterogeneity. Cave diving in phreatic caves can be used to explore this variability by using water quality sensors and discrete water chemistry samples to explore spatial and temporal water quality changes for improved and targeted water resource management. Our study uses cave diving to document the spatial and temporal variation in water quality within a phreatic cave system in the Floridan Aquifer System (FAS), a karst aquifer in northern Florida. We collect continuous 15-s measurements of dissolved oxygen (DO), temperature, pH, and specific conductance along a 1.1 km transect, which intersects multiple cave passages that drain into the primary cave passage. We also collect discrete water chemistry samples in three separate cave passages within the phreatic cave, as well as at the spring vent, to document spatial and seasonal variability in nutrients, organic matter, and major groundwater ions. Our results show that specific conductance, DO, temperature, and pH vary together spatially in consistent ways, which we use to identify cave passages that receive more direct recharge. Spatial and temporal variability across the cave system was most pronounced for NO_x-N (nitrate + nitrite), DO, and dissolved organic carbon, while major ions showed minimal spatial variability but greater temporal variability. Relationships derived between specific conductance and NO_x-N show a positive correlation, while relationships between ions associated with carbonate mineralogy and specific conductance are negatively correlated, which likely reflects the impact of recharge from agricultural land use surrounding the cave system. Our results highlight water quality complexity in phreatic caves and have implications for local water quality restoration efforts, interpreting water quality data collected at a discrete location, and provide guidance for future water quality studies in phreatic cave systems.