Movement of soil moisture associated with tree root‐water uptake is ecologically important but technically challenging to measure. Here, the self‐potential (SP) method, a passive electrical ...geophysical method, is used to characterize water flow in situ. Unlike tensiometers, which use a measurement of state (i.e., matric pressure) at two locations to infer fluid flow, the SP method directly measures signals generated by water movement. We collected SP measurements in a two‐dimensional array at the base of a Douglas‐fir tree (Pseudotsuga menziesii) in the H.J. Andrews Experimental Forest in western Oregon over 5 months to provide insight on the propagation of transpiration signals into the subsurface under variable soil moisture. During dry conditions, SP data appear to show downward unsaturated flow, whereas nearby tensiometer data appear to suggest upward flow during this period. After the trees enter dormancy in the fall, precipitation‐induced vertical flow dominates in the SP and tensiometer data. Diel variations in SP data correspond to periods of tree transpiration. Changes in volumetric water content occurring from soil moisture movement during transpiration are not large enough to appear in volumetric water content data. Fluid flow and electrokinetic coupling (i.e., electrical potential distribution) were simulated using COMSOL Multiphysics to explore the system controls on field data. The coupled model, which included a root‐water uptake term, reproduced components of both the long‐term and diel variations in SP measurements, thus indicating that SP has potential to provide spatially and temporally dense measurements of transpiration‐induced changes in water flow. This manuscript presents the first SP measurements focusing on the movement of soil moisture in response to tree transpiration.
The emergence of large language models (LLMs), such as ChatGPT, has garnered significant attention, particularly in academic and scientific circles. Researchers, scientists, and instructors hold ...varying perspectives on the advantages and disadvantages of using ChatGPT for research and teaching purposes. ChatGPT will be used by many scientists going forward for creating content and driving scientific progress. This commentary offers a brief explanation of the fundamental principles behind ChatGPT and how it can be applied in the fields of hydrology and other Earth sciences. The article examines the primary applications of this open artificial intelligence tool within these fields, specifically its ability to assist with writing and coding tasks, and highlights both the advantages and concerns associated with using such a model. Moreover, the study brings up some other limitations of the model, and the dangers of potential miss‐uses. Finally, we suggest that the academic community adapts its regulations and policies to harness the potential benefits of LLMs while mitigating its pitfalls, including establishing a structure for utilizing LLMs and presenting clear regulations for their implementation. We also outline some specific steps on how to accomplish this structure.
Key Points
Large language models (LLMs) such as ChatGPT, are new technological tools that might fundamentally change academia
ChatGPT can assist in academic writing but should not be relied on as the only source of information in hydrology and Earth Science studies
Authors should exhibit transparency in their utilization of LLMs and uphold ethical responsibility
This study integrates geochemical modeling, spatial analysis and several statistical methods including principal component analysis, multivariate regression and cluster analysis to investigate ...hydrogeologic controls of arsenic and uranium contamination within groundwater of the Arikaree aquifer on the Pine Ridge Reservation (PRR). Located in southwestern South Dakota, the PRR is largely rural and many people rely on domestic supply wells completed in the Arikaree aquifer as their primary drinking water source. Locally, the White River Group, which unconformably underlies the Arikaree Group, is enriched in arsenic and uranium related to volcanic ash deposits and acts as a geogenic metal source. Geochemical data from over 250 groundwater samples were obtained through collaboration with the Oglala Sioux Tribe. Cluster spatial statistics analyses delineated four regions of statistically significant variations in groundwater chemistry that represent upgradient, intermediate, and downgradient portions of the Arikaree aquifer. Groundwater evolves as it flows through the Arikaree aquifer with increasing alkalinity, sodium, and pH along flow paths. These chemical changes are likely due to dissolution of carbonate minerals and volcanic ash. Thermodynamic calculations suggest increasing supersaturation of the groundwater with respect to calcite; thus, volcanic ash dissolution may be an important secondary source of alkalinity. Elevated alkalinity and pH levels were found to be the driving factors of arsenic and uranium mobility, and downgradient sections of the aquifer in the northern portions of the PRR are most likely to be impacted by metal(loid) contamination, with 73% of the wells in this grouping failing a USEPA maximum contaminant level for arsenic, uranium, and/or gross alpha.
•Statistical tools parsed four geochemical environments within the Arikaree aquifer.•Groundwater quality within the Arikaree aquifer degrades along flowpaths.•73% of the furthest downgradient wells failed an MCL for As, U and/or gross alpha.
The advection-dispersion equation (ADE) often fails to predict solute transport, in part due to incomplete mixing in the subsurface, which the development of non-local models has attempted to deal ...with. One such model is dual-domain mass transfer (DDMT); one parameter that exists within this model type is called immobile porosity. Here, we explore the complexity of estimating immobile porosity under varying flow rates and density dependencies in a large-scale heterogeneous system. Immobile porosity is estimated experimentally and using numerical models in 3-D flow systems, and is defined by domains of comparatively low advective velocity instead of truly immobile regions at the pore scale. Tracer experiments were conducted in a mesoscale 3-D tank system with embedded large impermeable zones and the generated data were analyzed using a numerical model. The impermeable zones were used to explore how large-scale structure and heterogeneity affect parameter estimation of immobile porosity, assuming a dual-porosity model, and resultant characterization of the aquifer system. Spatially and temporally co-located fluid electrical conductivity (σf) and bulk apparent electrical conductivity (σb)—using geophysical methods—were measured to estimate immobile porosity, and numerical modeling (i.e., SEAWAT and R3t) was conducted to explore controls of the immobile zones on the experimentally observed flow and transport. Results showed that density-dependent flow increased the hysteresis between measured fluid and bulk electrical conductivity, resulting in larger interpreted immobile pore-space estimates. Increasing the dispersivity in the model simulations decreased the estimated immobile porosity; flow rate had no impact. Overall, the results of this study highlight the difficulty faced in determining immobile porosity values in field settings, where hydrogeologic processes may vary temporally. Our results also highlight that immobile porosity is an effective parameter in an upscaled model whose physical meaning is not necessarily clear and that may not align with intuitive interpretations of a porosity.
•Impermeable barriers and density dependence increase estimated immobile porosity.•Decreasing dispersivity leads to increased estimated immobile porosity.•Immobile porosity estimates were insensitive to changes in system flow rates.
•An empirical electrical resistivity-based method is proposed to estimate stem water content.•Variation of sapwood moisture reveals water replenishment in wet and loss in dry season.•Drooping sheoak ...sapwood can have moisture content as low as fibre saturation point in dry season.
Stem water content (θ) is an important state variable in the soil-plant-atmosphere continuum (SPAC), and varies temporally and spatially in response to environmental factors and plant growth stages. However, it is difficult to measure θ distribution in living trees in a manner that is not destructive. In this study, temporal and spatial variations in θ within living tree stems were examined based on minimally destructive electrical resistivity tomography. Measurements of tree bulk electrical resistivity (ρ), stem temperature, sap flow and stem water potential were taken on an Australian native tree species, Allocasuarina verticillata. The results show that θ estimated from adjusted resistivity (ρ*) based on a reference ρ-θ relationship approach agrees well with θ estimated from predawn stem water potential. Sapwood θ gradually increased during the wet season and substantially decreased during the dry season as predawn stem water potential and sap velocity increased and decreased, respectively, and ρ*-estimated θ reveals water replenishment and loss within the sapwood during the wet and dry seasons, respectively. During the dry period, mean sapwood θ for the study trees decreased to 0.23 cm3/cm3 (January), and daily maximum sap velocity was only about 10 % of that in wet season, suggesting that the trees were suffering heavy water stress. The spatially exhaustive method to estimate stem water content within living trees proposed here provides an additional approach to investigating tree response to water stress.
Methane leakage due to compromised hydrocarbon well integrity can lead to impaired groundwater quality. Here we use a three‐dimensional, multiphase (vapor and aqueous), multicomponent (methane, ...water, salt), numerical model (TOUGH2 EOS7C) to investigate hydrogeological conditions that could result in groundwater contamination from natural gas wellbore leakage that migrates upward toward a freshwater aquifer. The conceptual model used for the simulations assumes methane leakage at 20–30 m below groundwater. We perform 180 simulations for a sensitivity analysis, examining (1) multiphase flow parameters related to storage, capillarity, and relative permeability, including porosity (ϕ), initial fluid‐phase saturation (SL), and van Genuchten n and α, (2) geostatistical variations in intrinsic permeability (ki), and (3) methane source‐zone pressure. Simulated mean ki values are 10−18 and 10−13 m2 with variances of 1 and 5 m4. Simulated source‐zone pressures range from just over ambient hydrostatic pressure at the depth of leakage (100 kPa) to the maximum pressure that steel casings are commonly rated to withstand (20,340 kPa). ki, initial SL, ϕ, and van Genuchten's n and α were the most important parameters in determining the volume of methane reaching groundwater during a given time period. Multiphase parameterization of formations underlying freshwater aquifers and overlying hydrocarbon production zones is fundamental to assessing aquifer vulnerability to methane leakage.
Plain Language Summary
Methane leakage from oil and gas wellbores below freshwater aquifers may impact groundwater quality. The scope of the problem is such that millions of kilograms of methane could reach groundwater in the case of a long‐term, persistent leak. However, flow rates at the base of the aquifer are slow, and changes in methane concentrations may go undetected. In this paper, we use numerical modeling to investigate hydrogeological conditions that could result in groundwater contamination from natural‐gas wellbore leakage that migrates upward toward a freshwater aquifer. Measurement or careful estimation of parameters impacting both gas‐ and liquid‐phase flow and transport are needed in determining volumes and flow rates of methane reaching groundwater and, thus, aquifer vulnerability to methane leakage.
Key Points
Methane leakage from oil‐and‐gas wellbores below freshwater aquifers impacts groundwater quality
Multiphase analysis allows study of gas‐phase source‐zone pressure, capillarity and relative permeability, which influence methane migration
Multiphase parameters play a fundamental role in determining volumes and flow rates of methane reaching groundwater
•Current groundwater recharge in the western US is synthesized.•Recharge components are compared across the selected aquifers.•Climate-change is analyzed to determine impact on total recharge and ...mechanism.•Geographical patterns in total recharge and mechanism changes are described.•Knowledge gaps that limit predictions of future changes in recharge are identified.
Existing studies on the impacts of climate change on groundwater recharge are either global or basin/location-specific. The global studies lack the specificity to inform decision making, while the local studies do little to clarify potential changes over large regions (major river basins, states, or groups of states), a scale often important in the development of water policy. An analysis of the potential impact of climate change on groundwater recharge across the western United States (west of 100° longitude) is presented synthesizing existing studies and applying current knowledge of recharge processes and amounts. Eight representative aquifers located across the region were evaluated. For each aquifer published recharge budget components were converted into four standard recharge mechanisms: diffuse, focused, irrigation, and mountain-systems recharge. Future changes in individual recharge mechanisms and total recharge were then estimated for each aquifer. Model-based studies of projected climate-change effects on recharge were available and utilized for half of the aquifers. For the remainder, forecasted changes in temperature and precipitation were logically propagated through each recharge mechanism producing qualitative estimates of direction of changes in recharge only (not magnitude). Several key patterns emerge from the analysis. First, the available estimates indicate average declines of 10–20% in total recharge across the southern aquifers, but with a wide range of uncertainty that includes no change. Second, the northern set of aquifers will likely incur little change to slight increases in total recharge. Third, mountain system recharge is expected to decline across much of the region due to decreased snowpack, with that impact lessening with higher elevation and latitude. Factors contributing the greatest uncertainty in the estimates include: (1) limited studies quantitatively coupling climate projections to recharge estimation methods using detailed, process-based numerical models; (2) a generally poor understanding of hydrologic flowpaths and processes in mountain systems; (3) difficulty predicting the response of focused recharge to potential changes in the frequency and intensity of extreme precipitation events; and (4) unconstrained feedbacks between climate, irrigation practices, and recharge in highly developed aquifer systems.
We develop a new framework, hyporheic reaction potential (HRP), to predict the influence of oxidation-reduction reactions on metal fate and transport in streams using data from tracer studies and ...geochemical sampling. HRP, with energy flux units KJ m–2 s–1, is a metric calculated from both the physical and chemical properties of the hyporheic zone. We apply the HRP framework for iron reactions, using existing geochemical and geophysical data from two metal-impacted alpine streams at high and low flow. In these two systems, HRP delineates contrasting controls on iron fate and transport with biogeochemical controls in Mineral Creek and physical controls in Cement Creek. In both systems, HRP scales with discharge and hyporheic-zone extent as flows change seasonally, which demonstrates the ability of HRP to capture physical aspects of chemical reactions in the hyporheic zone. This paper provides a foundation on which HRP can be expanded to other solutes where chemical gradients in the hyporheic zone control reaction networks, making it broadly applicable to redox cycling in stream systems. This framework is useful in quantifying the role of the hyporheic zone in sourcing and storing metal(loid)s under varying hydrologic conditions with implications for water quality, mine remediation, and regional watershed management.
•High sap flows occur with decreases in ground conductivity and soil moisture.•Mean and variance of the ground conductivity decreases into the summer dry season.•Data suggest use of a water source ...>60cm to maintain sap flow through the summer.•Tree conductivity cycles daily due to changes in water content from transpiration.
The feedbacks among forest transpiration, soil moisture, and subsurface flowpaths are poorly understood. We investigate how soil moisture is affected by daily transpiration using time-lapse electrical resistivity imaging (ERI) on a highly instrumented ponderosa pine and the surrounding soil throughout the growing season. By comparing sap flow measurements to the ERI data, we find that periods of high sap flow within the diel cycle are aligned with decreases in ground electrical conductivity and soil moisture due to drying of the soil during moisture uptake. As sap flow decreases during the night, the ground conductivity increases as the soil moisture is replenished. The mean and variance of the ground conductivity decreases into the summer dry season, indicating drier soil and smaller diel fluctuations in soil moisture as the summer progresses. Sap flow did not significantly decrease through the summer suggesting use of a water source deeper than 60cm to maintain transpiration during times of shallow soil moisture depletion. ERI captured spatiotemporal variability of soil moisture on daily and seasonal timescales. ERI data on the tree showed a diel cycle of conductivity, interpreted as changes in water content due to transpiration, but changes in sap flow throughout the season could not be interpreted from ERI inversions alone due to daily temperature changes.
PDF
