Detecting Drawdowns Masked by Environmental Stresses with Water‐Level Models

Detecting and quantifying small drawdown at observation wells distant from the pumping well greatly expands the characterized aquifer volume. However, this detection is often obscured by water level fluctuations such as barometric and tidal effects. A reliable analytical approach for distinguishing drawdown from nonpumping water‐level fluctuations is presented and tested here. Drawdown is distinguished by analytically simulating all pumping and nonpumping water‐level stresses simultaneously during the period of record. Pumping signals are generated with Theis models, where the pumping schedule is translated into water‐level change with the Theis solution. This approach closely matched drawdowns simulated with a complex three‐dimensional, hypothetical model and reasonably estimated drawdowns from an aquifer test conducted in a complex hydrogeologic system. Pumping‐induced changes generated with a numerical model and analytical Theis model agreed (RMS as low as 0.007 m) in cases where pumping signals traveled more than 1 km across confining units and fault structures. Maximum drawdowns of about 0.05 m were analytically estimated from field investigations where environmental fluctuations approached 0.2 m during the analysis period.     

Identification of Pumping Influences in Long‐Term Water Level Fluctuations

Identification of the pumping influences at monitoring wells caused by spatially and temporally variable water supply pumping can be a challenging, yet an important hydrogeological task. The information that can be obtained can be critical for conceptualization of the hydrogeological conditions and indications of the zone of influence of the individual pumping wells. However, the pumping influences are often intermittent and small in magnitude with variable production rates from multiple pumping wells. While these difficulties may support an inclination to abandon the existing dataset and conduct a dedicated cross‐hole pumping test, that option can be challenging and expensive to coordinate and execute. This paper presents a method that utilizes a simple analytical modeling approach for analysis of a long‐term water level record utilizing an inverse modeling approach. The methodology allows the identification of pumping wells influencing the water level fluctuations. Thus, the analysis provides an efficient and cost‐effective alternative to designed and coordinated cross‐hole pumping tests. We apply this method on a dataset from the Los Alamos National Laboratory site. Our analysis also provides (1) an evaluation of the information content of the transient water level data; (2) indications of potential structures of the aquifer heterogeneity inhibiting or promoting pressure propagation; and (3) guidance for the development of more complicated models requiring detailed specification of the aquifer heterogeneity.     

Effects of Linking a Soil‐Water‐Balance Model with a Groundwater‐Flow Model

A previously published regional groundwater‐flow model in north‐central Nebraska was sequentially linked with the recently developed soil‐water‐balance (SWB) model to analyze effects to groundwater‐flow model parameters and calibration results. The linked models provided a more detailed spatial and temporal distribution of simulated recharge based on hydrologic processes, improvement of simulated groundwater‐level changes and base flows at specific sites in agricultural areas, and a physically based assessment of the relative magnitude of recharge for grassland, nonirrigated cropland, and irrigated cropland areas. Root‐mean‐squared (RMS) differences between the simulated and estimated or measured target values for the previously published model and linked models were relatively similar and did not improve for all types of calibration targets. However, without any adjustment to the SWB‐generated recharge, the RMS difference between simulated and estimated base‐flow target values for the groundwater‐flow model was slightly smaller than for the previously published model, possibly indicating that the volume of recharge simulated by the SWB code was closer to actual hydrogeologic conditions than the previously published model provided. Groundwater‐level and base‐flow hydrographs showed that temporal patterns of simulated groundwater levels and base flows were more accurate for the linked models than for the previously published model at several sites, particularly in agricultural areas.     

Water‐bottom multiple attenuation by Kirchhoff extrapolation

Despite being less general than 3D surface‐related multiple elimination (3D‐SRME), multiple prediction based on wavefield extrapolation can still be of interest, because it is less CPU and I/O demanding than 3D‐SRME and moreover it does not require any prior data regularization. Here we propose a fast implementation of water‐bottom multiple prediction that uses the Kirchhoff formulation of wavefield extrapolation. With wavefield extrapolation multiple prediction is usually obtained through the cascade of two extrapolation steps. Actually by applying the Fermat’s principle (i.e., minimum reflection traveltime) we show that the cascade of two operators can be replaced by a single approximated extrapolation step. The approximation holds as long as the water bottom is not too complex. Indeed the proposed approach has proved to work well on synthetic and field data when the water bottom is such that wavefront triplications are negligible, as happens in many practical situations.     

Discrete‐Storm Water‐Table Fluctuation Method to Estimate Episodic Recharge

We have developed a method to identify and quantify recharge episodes, along with their associated infiltration‐related inputs, by a consistent, systematic procedure. Our algorithm partitions a time series of water levels into discrete recharge episodes and intervals of no episodic recharge. It correlates each recharge episode with a specific interval of rainfall, so storm characteristics such as intensity and duration can be associated with the amount of recharge that results. To be useful in humid climates, the algorithm evaluates the separability of events, so that those whose recharge cannot be associated with a single storm can be appropriately lumped together. Elements of this method that are subject to subjectivity in the application of hydrologic judgment are values of lag time, fluctuation tolerance, and master recession parameters. Because these are determined once for a given site, they do not contribute subjective influences affecting episode‐to‐episode comparisons. By centralizing the elements requiring scientific judgment, our method facilitates such comparisons by keeping the most subjective elements openly apparent, making it easy to maintain consistency. If applied to a period of data long enough to include recharge episodes with broadly diverse characteristics, the method has value for predicting how climatic alterations in the distribution of storm intensities and seasonal duration may affect recharge.     

Simulation of Water‐Table Aquifers Using Specified Saturated Thickness

Simulating groundwater flow in a water‐table (unconfined) aquifer can be difficult because the saturated thickness available for flow depends on model‐calculated hydraulic heads. It is often possible to realize substantial time savings and still obtain accurate head and flow solutions by specifying an approximate saturated thickness a priori, thus linearizing this aspect of the model. This specified‐thickness approximation often relies on the use of the “confined” option in numerical models, which has led to confusion and criticism of the method. This article reviews the theoretical basis for the specified‐thickness approximation, derives an error analysis for relatively ideal problems, and illustrates the utility of the approximation with a complex test problem. In the transient version of our complex test problem, the specified‐thickness approximation produced maximum errors in computed drawdown of about 4% of initial aquifer saturated thickness even when maximum drawdowns were nearly 20% of initial saturated thickness. In the final steady‐state version, the approximation produced maximum errors in computed drawdown of about 20% of initial aquifer saturated thickness (mean errors of about 5%) when maximum drawdowns were about 35% of initial saturated thickness. In early phases of model development, such as during initial model calibration efforts, the specified‐thickness approximation can be a very effective tool to facilitate convergence. The reduced execution time and increased stability obtained through the approximation can be especially useful when many model runs are required, such as during inverse model calibration, sensitivity and uncertainty analyses, multimodel analysis, and development of optimal resource management scenarios.     

Homotopy Perturbation Method‐Based Analytical Solution for Tide‐Induced Groundwater Fluctuations

The groundwater variations in unconfined aquifers are governed by the nonlinear Boussinesq's equation. Analytical solution for groundwater fluctuations in coastal aquifers under tidal forcing can be solved using perturbation methods. However, the perturbation parameters should be properly selected and predefined for traditional perturbation methods. In this study, a new dimensional, higher‐order analytical solution for groundwater fluctuations is proposed by using the homotopy perturbation method with a virtual perturbation parameter. Parameter‐expansion method is used to remove the secular terms generated during the solution process. The solution does not require any predefined perturbation parameter and valid for higher values of amplitude parameter A/D, where A is the amplitude of the tide and D is the aquifer thickness.     

Water‐quality response to a high‐elevation wildfire in the Colorado Front Range

Water quality of the Big Thompson River in the Front Range of Colorado was studied for 2 years following a high‐elevation wildfire that started in October 2012 and burned 15% of the watershed. A combination of fixed‐interval sampling and continuous water‐quality monitors was used to examine the timing and magnitude of water‐quality changes caused by the wildfire. Prefire water quality was well characterized because the site has been monitored at least monthly since the early 2000s. Major ions and nitrate showed the largest changes in concentrations; major ion increases were greatest in the first postfire snowmelt period, but nitrate increases were greatest in the second snowmelt period. The delay in nitrate release until the second snowmelt season likely reflected a combination of factors including fire timing, hydrologic regime, and rates of nitrogen transformations. Despite the small size of the fire, annual yields of dissolved constituents from the watershed increased 20–52% in the first 2 years following the fire. Turbidity data from the continuous sensor indicated high‐intensity summer rain storms had a much greater effect on sediment transport compared to snowmelt. High‐frequency sensor data also revealed that weekly sampling missed the concentration peak during snowmelt and short‐duration spikes during rain events, underscoring the challenge of characterizing postfire water‐quality response with fixed‐interval sampling. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrical Resistance Heating of Clay Layers in Water‐Saturated Sand

Electrical resistance heating (ERH) experiments were performed in a two‐dimensional water‐saturated porous medium comprising an electrically conductive, low‐permeability clay lens embedded within a less electrically conductive, higher permeability silica sand. These were compared to experiments performed in homogeneous silica sand. All experiments were performed in the absence of a non‐aqueous phase liquid (NAPL) or dissolved volatile organic compound (VOC). Temperature monitoring showed preferential heating in the clay lens and higher overall heating rates throughout the test cell compared to the homogeneous case. Gas production was localized around the sand–clay interface due to high temperature and low capillary displacement pressure. Above the clay lens, unexpected temperature plateaus were observed, similar to those observed in previous experiments during NAPL–water co‐boiling. A conceptual model based on the consumption of thermal energy as latent heat of vaporization in the highly localized heating and gas production region adjacent to the clay lens is proposed to explain the temperature plateaus. Supporting data is drawn from images of the gas phase and electric current measurements. These results show that the use of co‐boiling plateaus as an indicator of NAPL–water co‐boiling could be misleading during applications of ERH at sites containing electrically conductive, low‐permeability clay lenses embedded within less electrically conductive, higher‐permeability sands.     

Water‐level dynamics in natural and artificial pools in blanket peatlands

Perennial pools are common natural features of peatlands, and their hydrological functioning and turnover may be important for carbon fluxes, aquatic ecology, and downstream water quality. Peatland restoration methods such as ditch blocking result in many new pools. However, little is known about the hydrological function of either pool type. We monitored six natural and six artificial pools on a Scottish blanket peatland. Pool water levels were more variable in all seasons in artificial pools having greater water level increases and faster recession responses to storms than natural pools. Pools overflowed by a median of 9 and 54 times pool volume per year for natural and artificial pools, respectively, but this varied widely because some large pools had small upslope catchments and vice versa. Mean peat water‐table depths were similar between natural and artificial pool sites but much more variable over time at the artificial pool site, possibly due to a lower bulk specific yield across this site. Pool levels and pool‐level fluctuations were not the same as those of local water tables in the adjacent peat. Pool‐level time series were much smoother, with more damped rainfall or recession responses than those for peat water tables. There were strong hydraulic gradients between the peat and pools, with absolute water tables often being 20–30 cm higher or lower than water levels in pools only 1–4 m away. However, as peat hydraulic conductivity was very low (median of 1.5 × 10^?5 and 1.4 × 10^?6 cm s^?1 at 30 and 50 cm depths at the natural pool site), there was little deep subsurface flow interaction. We conclude that (a) for peat restoration projects, a larger total pool surface area is likely to result in smaller flood peaks downstream, at least during summer months, because peatland bulk specific yield will be greater; and (b) surface and near‐surface connectivity during storm events and topographic context, rather than pool size alone, must be taken into account in future peatland pool and stream chemistry studies.     

Assessment of Primary Production by Statistical Analysis of Water‐quality Data

Time series of weekly water‐quality data at Schnackenburg on the Elbe River (1985—2000) were subjected to principal component analysis (PCA). Considering the amplitudes of composite patterns of variables is a step towards a process‐oriented interpretation of waterquality data. One specific objective was to investigate the impact of improved water quality after the German reunification in 1990 on primary production and the oxygen budget. To discriminate anthropogenic signals from natural fluctuations a separation of the impact of discharge was attempted based on a linear regression approach. A dominant pattern of co‐variation in the residual data could be attributed to biological activity (primary production). The most relevant variables of this 'biomode' are oxygen saturation, pH, and orthophosphate. We conclude that multivariate statistical analysis of water‐quality data can help to estimate primary production when direct observations of algal concentrations are missing. In the years from 1998—2000 the trend of the ‘biomode’ indicates an increased load of oxygen consuming biomass caused by enhanced primary production in the middle stretches of the Elbe River which corresponds with the observation of more severe oxygen deficits in the tidal section of the river.     

Numerical Analysis of Groundwater Ridging Processes Considering Water‐Air Flow in a Hillslope

In this study, a water‐air two‐phase flow model was employed to investigate the formation, extension, and dissipation of groundwater ridging induced by recharge events in a hypothetical hillslope‐riparian zone, considering interactions between the liquid and gas phases in soil voids. The simulation results show that, after a rain begins, the groundwater table near the stream is elevated instantaneously and significantly, thereby generating a pressure gradient driving water toward both the stream (the discharge of groundwater to the stream) and upslope (the extension of groundwater ridging into upslope). Meanwhile, the airflow upslope triggered by the advancing wetting front moves downward gradually. Therefore, the extension of groundwater ridging into upslope and the downward airflow interact within a certain region. After the rain stops, groundwater ridging near the stream declines quickly while the airflow in the lower part of upslope is still moving into the hillslope. Thus, the airflow upslope mitigates the dissipation of groundwater ridging. Additionally, the development of groundwater ridging under different conditions, including rain intensity, intrinsic permeability, capillary fringe height, and initial groundwater table, was analyzed. Changes in intrinsic permeability affect the magnitude of groundwater ridging near the stream, as well as the downward speed of airflow, thereby generating highly complex responses. The capillary fringe is not a controlling factor but an influence factor on the formation of groundwater ridging, which is mainly related to the antecedent moisture. It was demonstrated that groundwater ridging also occurs where an unsaturated zone occurs above the capillary fringe with a subsurface lateral flow.     

Screening‐Level Sensitivity Analysis for the Design of Pump‐and‐Treat Systems

Pump‐and‐treat systems can prevent the migration of groundwater contaminants and candidate systems are typically evaluated with groundwater models. Such models should be rigorously assessed to determine predictive capabilities and numerous tools and techniques for model assessment are available. While various assessment methodologies (e.g., model calibration, uncertainty analysis, and Bayesian inference) are well‐established for groundwater modeling, this paper calls attention to an alternative assessment technique known as screening‐level sensitivity analysis (SLSA). SLSA can quickly quantify first‐order (i.e., main effects) measures of parameter influence in connection with various model outputs. Subsequent comparisons of parameter influence with respect to calibration vs. prediction outputs can suggest gaps in model structure and/or data. Thus, while SLSA has received little attention in the context of groundwater modeling and remedial system design, it can nonetheless serve as a useful and computationally efficient tool for preliminary model assessment. To illustrate the use of SLSA in the context of designing groundwater remediation systems, four SLSA techniques were applied to a hypothetical, yet realistic, pump‐and‐treat case study to determine the relative influence of six hydraulic conductivity parameters. Considered methods were: Taguchi design‐of‐experiments (TDOE); Monte Carlo statistical independence (MCSI) tests; average composite scaled sensitivities (ACSS); and elementary effects sensitivity analysis (EESA). In terms of performance, the various methods identified the same parameters as being the most influential for a given simulation output. Furthermore, results indicate that the background hydraulic conductivity is important for predicting system performance, but calibration outputs are insensitive to this parameter (K_BK). The observed insensitivity is attributed to a nonphysical specified‐head boundary condition used in the model formulation which effectively “staples” head values located within the conductivity zone. Thus, potential strategies for improving model predictive capabilities include additional data collection targeting the K_BK parameter and/or revision of model structure to reduce the influence of the specified head boundary.     

Spatial Patterns of Sea Level Variability Associated with Natural Internal Climate Modes

Sea level rise (SLR) can exert significant stress on highly populated coastal societies and low-lying island countries around the world. Because of this, there is huge societal demand for improved decadal predictions and future projections of SLR, particularly on a local scale along coastlines. Regionally, sea level variations can deviate considerably from the global mean due to various geophysical processes. These include changes of ocean circulations, which partially can be attributed to natural, internal modes of variability in the complex Earth’s climate system. Anthropogenic influence may also contribute to regional sea level variations. Separating the effects of natural climate modes and anthropogenic forcing, however, remains a challenge and requires identification of the imprint of specific climate modes in observed sea level change patterns. In this paper, we review our current state of knowledge about spatial patterns of sea level variability associated with natural climate modes on interannual-to-multidecadal timescales, with particular focus on decadal-to-multidecadal variability. Relevant climate modes and our current state of understanding their associated sea level patterns and driving mechanisms are elaborated separately for the Pacific, the Indian, the Atlantic, and the Arctic and Southern Oceans. We also discuss the issues, challenges and future outlooks for understanding the regional sea level patterns associated with climate modes. Effects of these internal modes have to be taken into account in order to achieve more reliable near-term predictions and future projections of regional SLR.     

Tide‐Induced Air Pressure Fluctuations in a Coastal Unsaturated Zone: Effects of Thin Low‐Permeability Pavements

This paper presents an analytical case study to explore one‐dimensional subsurface air pressure variation in a coastal three‐layered unsaturated zone. The upper layer is thin and much less permeable than the middle layer, and water table is located in the very permeable lower layer. An analytical solution was derived to describe the air pressure variation caused by tide‐induced water table fluctuations. We revisited the case study at Hong Kong International Airport conducted by Jiao and Li (2004) who used a two‐dimensional numerical model. The analytical prediction using the parameter values equivalent to the two‐dimensional numerical model agreed very well with the observed air pressure, indicating the validity and applicability of our one‐dimensional model in approximating the actual situation in this coastal zone with adequate accuracy. The analysis revealed that the asphalt pavement played an important role in causing air pressure fluctuations below it. Abnormally high air pressure can be caused beneath the surface pavement when the air permeability decreases due to rainfall infiltration, which may lead to heaving problems during rising tides.     

A Stream‐Based Methane Monitoring Approach for Evaluating Groundwater Impacts Associated with Unconventional Gas Development

Gaining streams can provide an integrated signal of relatively large groundwater capture areas. In contrast to the point‐specific nature of monitoring wells, gaining streams coalesce multiple flow paths. Impacts on groundwater quality from unconventional gas development may be evaluated at the watershed scale by the sampling of dissolved methane (CH₄) along such streams. This paper describes a method for using stream CH₄ concentrations, along with measurements of groundwater inflow and gas transfer velocity interpreted by 1‐D stream transport modeling, to determine groundwater methane fluxes. While dissolved ionic tracers remain in the stream for long distances, the persistence of methane is not well documented. To test this method and evaluate CH₄ persistence in a stream, a combined bromide (Br) and CH₄ tracer injection was conducted on Nine‐Mile Creek, a gaining stream in a gas development area in central Utah. A 35% gain in streamflow was determined from dilution of the Br tracer. The injected CH₄ resulted in a fivefold increase in stream CH₄ immediately below the injection site. CH₄ and δ¹³C_CH4 sampling showed it was not immediately lost to the atmosphere, but remained in the stream for more than 2000 m. A 1‐D stream transport model simulating the decline in CH₄ yielded an apparent gas transfer velocity of 4.5 m/d, describing the rate of loss to the atmosphere (possibly including some microbial consumption). The transport model was then calibrated to background stream CH₄ in Nine‐Mile Creek (prior to CH₄ injection) in order to evaluate groundwater CH₄ contributions. The total estimated CH₄ load discharging to the stream along the study reach was 190 g/d, although using geochemical fingerprinting to determine its source was beyond the scope of the current study. This demonstrates the utility of stream‐gas sampling as a reconnaissance tool for evaluating both natural and anthropogenic CH₄ leakage from gas reservoirs into groundwater and surface water.