Developing Potentiometric Surfaces and Flow Fields with a Head‐Specified MODFLOW Model

Investigating changes in an aquifer system often involves comparison of observed heads from different synoptic measurements, generally with potentiometric surfaces developed by hand or a statistical approach. Alternatively, head‐specified MODFLOW models, in which constant head cells simulate observed heads, generate gridded potentiometric surfaces that explicitly account for Darcy's Law and mass balance. We developed a transient head‐specified MODFLOW model for the stratified Cambrian‐Ordovician sandstone aquifer system of northeastern Illinois to analyze flow within its 275 m deep cone of depression. Potentiometric surfaces were developed using static heads from production wells regardless of open interval; hence assuming no vertical head difference. This assumption was tested against steady‐state, head‐specified models of each sandstone strata for 1980 and 2014. The results indicate that the original conceptual model was appropriate in 1980 but not 2014, where a vertical head difference had developed at the center of the cone of depression. For earlier years, when the head difference was minimal, the transient head‐specified model compared well with a traditional, flow‐specified model. In later years, the transient head‐specified model overestimated removal of water from storage. MODFLOW facilitates the development of a time‐series of potentiometric surfaces and can easily be modified to test the impacts of different conceptual models, such as assumptions on vertical head differences. For this study of a deep confined aquifer, MODFLOW also offers advantages in generating potentiometric surfaces and flow fields over statistical interpolation techniques, although future research is needed to assess its performance in other settings.     

Pressure Buildup and Brine Migration During CO2 Storage in Multilayered Aquifers

Carbon dioxide injection into deep saline formations may induce large‐scale pressure increases and migration of native fluid. Local high‐conductivity features, such as improperly abandoned wells or conductive faults, could act as conduits for focused leakage of brine into shallow groundwater resources. Pressurized brine can also be pushed into overlying/underlying formations because of diffuse leakage through low‐permeability aquitards, which occur over large areas and may allow for effective pressure bleed‐off in the storage reservoirs. This study presents the application of a recently developed analytical solution for pressure buildup and leakage rates in a multilayered aquifer‐aquitard system with focused and diffuse brine leakage. The accuracy of this single‐phase analytical solution for estimating far‐field flow processes is verified by comparison with a numerical simulation study that considers the details of two‐phase flow. We then present several example applications for a hypothetical CO₂ injection scenario (without consideration of two‐phase flow) to demonstrate that the new solution is an efficient tool for analyzing regional pressure buildup in a multilayered system, as well as for gaining insights into the leakage processes of flow through aquitards, leaky wells, and/or leaky faults. This solution may be particularly useful when a large number of calculations needs to be performed, that is, for uncertainty quantification, for parameter estimation, or for the optimization of pressure‐management schemes.     

Groundwater flow and implications for groundwater contamination north of Prewitt,New Mexico,U.S.A.

In the southern San Juan Basin, New Mexico, strata of Permian and younger age dip gently toward the center of the basin. Most previous investigators believed that recharge to these strata occurred by precipitation on the outcrops and groundwater flowed downdip to the north and northeast. Recent water-level measurements in an undeveloped part of the basin near Prewitt, New Mexico, show that groundwater at shallow depths in alluvium and bedrock flows southward, opposite to the dip direction, and toward a major ephemeral drainage in a strike valley. North of this area, groundwater in deep bedrock aquifers does appear to flow northward. This information suggests that there are two groundwater circulation patterns; a shallow one controlled by topography and a deeper one controlled by geologic structure.Significant amounts of recharge to sandstone aquifers by infiltration through outcrops is unlikely due to the near-vertical exposures on cliffs, the gentle dip of the strata, and small annual precipitation. Numerical model results suggest that recharge to bedrock aquifers may be from downward leakage via aquitards over large areas and leakage from narrow alluvial aquifers in the subcrop area. The recharge mechanism is controlled by the hydraulic conductivity of the strata.As the flow path is controlled by hydraulic conductivity contrasts, geologic structure, and topography, contamination movement from surface impoundments is likely to be difficult to predict without a thorough hydrogeological site investigation.     

Evaluation of Hydraulic Properties of Aquitards Using Earthquake‐Triggered Groundwater Variation

The hydraulic properties of aquitards are not easily obtained because monitoring wells are usually installed in aquifers for groundwater resources management. Earthquake‐induced crust stress (strain) triggers groundwater level variations over a short period of time in a large area. These groundwater anomalies can be used to investigate aquifer systems. This study uses a poroelastic model to fit the postseismic variations of groundwater level triggered by the Chi‐Chi earthquake to evaluate the hydraulic properties of aquitards in the Jhoushuei River alluvial fan (JRAF), Taiwan. Six of the adopted eight wells with depths of 70 to 130 m showed good agreement with the recovery theory. The mean hydraulic conductivities (K) of the aquifers for the eight wells are 1.62 × 10^?4 to 9.06 × 10^?4 m/s, and the thicknesses are 18.8 to 46.1 m. The thicknesses of the aquitards are 11.3 to 42.0 m. Under the isotropic assumption for K, the estimated values of K for the aquitards are 3.0 × 10^?8 to 2.1 × 10^?6 m/s, corresponding to a silty medium. The results match the values obtained for the geological material of the drilling core and those reported in previous studies. The estimated values were combined with those given in previous studies to determine the distribution of K in the first two aquitards in the JRAF. The distribution patterns of the aquitards reflect the sedimentary environments and fit the geological material. The proposed technique can be used to evaluate the K value of aquitards using inverse methods. The inversion results can be used in hydrogeological analyses, contaminant modeling, and subsidence evaluation.     

Modeling Steady Sea Water Intrusion with Single‐Density Groundwater Codes

Steady interface flow in heterogeneous aquifer systems is simulated with single‐density groundwater codes by using transformed values for the hydraulic conductivity and thickness of the aquifers and aquitards. For example, unconfined interface flow may be simulated with a transformed model by setting the base of the aquifer to sea level and by multiplying the hydraulic conductivity with 41 (for sea water density of 1025 kg/m³). Similar transformations are derived for unconfined interface flow with a finite aquifer base and for confined multi‐aquifer interface flow. The head and flow distribution are identical in the transformed and original model domains. The location of the interface is obtained through application of the Ghyben‐Herzberg formula. The transformed problem may be solved with a single‐density code that is able to simulate unconfined flow where the saturated thickness is a linear function of the head and, depending on the boundary conditions, the code needs to be able to simulate dry cells where the saturated thickness is zero. For multi‐aquifer interface flow, an additional requirement is that the code must be able to handle vertical leakage in situations where flow in an aquifer is unconfined while there is also flow in the aquifer directly above it. Specific examples and limitations are discussed for the application of the approach with MODFLOW. Comparisons between exact interface flow solutions and MODFLOW solutions of the transformed model domain show good agreement. The presented approach is an efficient alternative to running transient sea water intrusion models until steady state is reached.     

The effect of low‐permeability fault zones on groundwater flow in a compartmentalized system. Experimental evidence from a carbonate aquifer (Southern Italy)

The hydrogeological behaviour of fault zones in carbonate aquifers is often neglected in conceptual and numerical models. Furthermore, no information is available regarding the relationships between piezometric levels when significant compartmentalization occurs due to the occurrence of low‐flow fault zones. The aim of this study was to refine the conceptualization of subsurface flow in faulted carbonate aquifers and to analyse relationships between sub‐basins within a compartmentalized aquifer system in Southern Italy. The interactions between compartments that straddle low‐flow faults were investigated over four hydrologic years using a statistical approach to compare (i) the hydraulic heads within two wells located up‐ and down‐gradient of tectonic discontinuities as well as (ii) the rainfall and piezometric levels. The results of this study suggest that a set of barriers exists between the wells, and, therefore, the total head loss observed between the wells (approximately 80 m) should be distributed across several aquitards, with one aquitard exhibiting a relatively high permeability or low degree of integrity. Due to slight differences in permeability, transient conditions in aquitards can occur over relatively short periods, which is in agreement with the results of the statistical data analysis. Consequently, rather than being caused by pure aquitards, aquifer system compartmentalization likely results from slight differences in the permeability between lower‐permeability fault zones and adjacent higher‐permeability protoliths. Copyright © 2014 John Wiley & Sons, Ltd.     

Boundary depletion rate and drawdown in leaky wedge‐shaped aquifers

This study presents analytical solutions of the three‐dimensional groundwater flow to a well in leaky confined and leaky water table wedge‐shaped aquifers. Leaky wedge‐shaped aquifers with and without storage in the aquitard are considered, and both transient and steady‐state drawdown solutions are derived. Unlike the previous solutions of the wedge‐shaped aquifers, the leakages from aquitard are considered in these solutions and unlike similar previous work for leaky aquifers, leakage from aquitards and from the water table are treated as the lower and upper boundary conditions. A special form of finite Fourier transforms is used to transform the z‐coordinate in deriving the solutions. The leakage induced by a partially penetrating pumping well in a wedge‐shaped aquifer depends on aquitard hydraulic parameters, the wedge‐shaped aquifer parameters, as well as the pumping well parameters. We calculate lateral boundary dimensionless flux at a representative line and investigate its sensitivity to the aquitard hydraulic parameters. We also investigate the effects of wedge angle, partial penetration, screen location and piezometer location on the steady‐state dimensionless drawdown for different leakage parameters. Results of our study are presented in the form of dimensionless flux‐dimensionless time and dimensionless drawdown‐leakage parameter type curves. The results are useful for evaluating the relative role of lateral wedge boundaries and leakage source on flow in wedge‐shaped aquifers. This is very useful for water management problems and for assessing groundwater pollution. The presented analytical solutions can also be used in parameter identification and in calculating stream depletion rate and volume. Copyright © 2011 John Wiley & Sons, Ltd.     

Nonlinear Flow Process: A New Package to Compute Nonlinear Flow in MODFLOW

A new MODFLOW package (Nonlinear Flow Process; NLFP) simulating nonlinear flow following the Forchheimer equation was developed and implemented in MODLFOW‐2005. The method is based on an iterative modification of the conductance calculated and used by MODFLOW to obtain an effective Forchheimer conductance. The package is compatible with the different layer types, boundary conditions, and solvers as well as the wetting capability of MODFLOW. The correct implementation is demonstrated using four different benchmark scenarios for which analytical solutions are available. A scenario considering transient flow in a more realistic setting and a larger model domain with a higher number of cells demonstrates that NLFP performs well under more complex conditions, although it converges moderately slower than the standard MODFLOW depending on the nonlinearity of flow. Thus, this new tool opens a field of opportunities to groundwater flow simulation with MODFLOW, especially for core sample simulation or vuggy karstified aquifers as well as for nonlinear flow in the vicinity of pumping wells.     

Aquifer Vulnerability Assessment Based on Sequence Stratigraphic and 39Ar Transport Modeling

A large‐scale groundwater flow and transport model is developed for a deep‐seated (100 to 300 m below ground surface) sedimentary aquifer system. The model is based on a three‐dimensional (3D) hydrostratigraphic model, building on a sequence stratigraphic approach. The flow model is calibrated against observations of hydraulic head and stream discharge while the credibility of the transport model is evaluated against measurements of ³⁹Ar from deep wells using alternative parameterizations of dispersivity and effective porosity. The directly simulated 3D mean age distributions and vertical fluxes are used to visualize the two‐dimensional (2D)/3D age and flux distribution along transects and at the top plane of individual aquifers. The simulation results are used to assess the vulnerability of the aquifer system that generally has been assumed to be protected by thick overlaying clayey units and therefore proposed as future reservoirs for drinking water supply. The results indicate that on a regional scale these deep‐seated aquifers are not as protected from modern surface water contamination as expected because significant leakage to the deeper aquifers occurs. The complex distribution of local and intermediate groundwater flow systems controlled by the distribution of the river network as well as the topographical variation (Tóth 1963) provides the possibility for modern water to be found in even the deepest aquifers.     

Application of an integrated surface water‐groundwater model to multi‐aquifers modeling in Choushui River alluvial fan,Taiwan

This research proposes a combination of SWAT and MODFLOW, MD‐SWAT‐MODFLOW, to address the multi‐aquifers condition in Choushui River alluvial fan, Taiwan. The natural recharge and unidentified pumping/recharge are separately estimated. The model identifies the monthly pumping/recharge rates in multi‐aquifers so that the daily streamflow can be simulated correctly. A multi‐aquifers condition means a subsurface formation composed of at least the unconfined aquifer, the confined aquifer, and an in‐between aquitard. In such a case, the variation of groundwater level is related to pumping/recharge activities in vertically adjacent aquifer and the river‐aquifer interaction. Both factors in turn affect the streamflow performance. Results show that MD‐SWAT‐MODFLOW performs better than SWAT alone in terms of simulated streamflow, especially during low flow period, when pumping/recharge rates are properly estimated. A sensitivity analysis of individual parameter suggests that the vertical leakance may be the most sensitive among all investigated MODFLOW parameters in terms of the estimated pumping/recharge among aquifers, and the Latin‐Hypercube‐One‐factor‐At‐a‐Time sensitivity analysis indicates that the hydraulic conductivity of channel is the most sensitive to the model performance. It also points out the necessity to simultaneously estimate pumping/recharge rates in multi‐aquifers. The estimated net pumping rate can be treated as a lower bound of the actual local pumping rate. As a whole, the model provides the spatio‐temporal groundwater use, which gives the authorities insights to manage groundwater resources. Copyright © 2012 John Wiley & Sons, Ltd.     

Predicting Water Resource Impacts of Unconventional Gas Using Simple Analytical Equations

The rapid expansion in unconventional gas development over the past two decades has led to concerns over the potential impacts on groundwater resources. Although numerical models are invaluable for assessing likelihood of impacts at particular sites, simpler analytical models are also useful because they help develop hydrological understanding. Analytical approaches are also valuable for preliminary assessments and to determine where more complex models are warranted. In this article, we present simple analytical solutions that can be used to predict: (1) the spatial extent of drawdown from horizontal wells drilled into the gas‐bearing formation, and rate of recovery after gas production ceases; (2) the potential for upward transport of contaminants from the gas‐bearing formation to shallow aquifers during hydraulic fracturing operations when pressures in the gas‐bearing formation are greatly increased; and (3) the potential downward leakage of water from shallow aquifers during depressurization of gas‐bearing formations. In particular, we show that the recovery of pressure after production ceases from gas‐bearing shale formations may take several hundred years, and we present critical hydraulic conductivity values for intervening aquitards, below which the impact on shallow aquifers will be negligible. The simplifying assumptions inherent in these solutions will limit their predictive accuracy for site‐specific assessments, compared to numerical models that incorporate knowledge of spatial variations in formation properties and which may include processes not considered in the simpler solutions.     

Numerical Modeling of Slug Tests with MODFLOW Using Equivalent Well Blocks

Slug tests are a widely used technique to estimate aquifer hydraulic parameters and the test data are generally interpreted with analytical solutions under various assumptions. However, these solutions are not convenient when slug tests are required to be analyzed in a three‐dimensional model for complex aquifer‐aquitard systems. In this study, equivalent well blocks (EWB) are proposed in numerical modeling of slug test data with MODFLOW. Multi‐well slug tests in partially penetrating wells with skin zones can be simulated. Accuracy of the numerical method is demonstrated by benchmarking with analytical solutions. The EWB method is applied in a case study on slug tests in aquitards in the Pearl River Delta, China.     

The vertical hydraulic conductivity of an aquitard at two spatial scales

Aquitards protect underlying aquifers from contaminants and limit recharge to those aquifers. Understanding the mechanisms and quantity of ground water flow across aquitards to underlying aquifers is essential for ground water planning and assessment. We present results of laboratory testing for shale hydraulic conductivities, a methodology for determining the vertical hydraulic conductivity (K(v)) of aquitards at regional scales and demonstrate the importance of discrete flow pathways across aquitards. A regional shale aquitard in southeastern Wisconsin, the Maquoketa Formation, was studied to define the role that an aquitard plays in a regional ground water flow system. Calibration of a regional ground water flow model for southeastern Wisconsin using both predevelopment steady-state and transient targets suggested that the regional K(v) of the Maquoketa Formation is 1.8 x 10(-11) m/s. The core-scale measurements of the K(v) of the Maquoketa Formation range from 1.8 x 10(-14) to 4.1 x 10(-12) m/s. Flow through some additional pathways in the shale, potential fractures or open boreholes, can explain the apparent increase of the regional-scale K(v). Based on well logs, erosional windows or high-conductivity zones seem unlikely pathways. Fractures cutting through the entire thickness of the shale spaced 5 km apart with an aperture of 50 microns could provide enough flow across the aquitard to match that provided by an equivalent bulk K(v) of 1.8 x 10(-11) m/s. In a similar fashion, only 50 wells of 0.1 m radius open to aquifers above and below the shale and evenly spaced 10 km apart across southeastern Wisconsin can match the model K(v).     

Commingled Fluids in Abandoned Boreholes: Proximity Analysis of a Hidden Liability

The interactions between old abandoned wellbores of suspect well integrity with hydraulic fracturing (HF), enhanced oil recovery (EOR), or salt water disposal (SWD) operations can result in upward leakage of deep aqueous liquids into overlying aquifers. This potential for upward fluid migration is largely unquantified as monitoring abandoned wells is rarely done, and leakage may go unnoticed especially when in deeper aquifers. This study performs a proximity analysis between old abandoned wells and HF, EOR, and SWD wells, and identifies commingled old abandoned wellbores, which are those wells where groundwater may flow from one aquifer to one or more other aquifers, to identify the locations with the greatest potential for upward aqueous fluid migration at three study sites in the Western Canadian Sedimentary Basin. Our analysis indicates that at all three study sites there are several locations where HF, EOR, or SWD operations are located in close proximity to a given old abandoned well. Much of this overlap occurs in formations above typically produced hydrocarbon reservoirs but below exploited potable aquifers, otherwise known as the intermediate zone, which is often connected between abandonment plugs in old abandoned wells. Information on the intermediate zone is often lacking, and this study suggests that unanticipated alterations to groundwater flow systems within the intermediate zone may be occurring. Results indicate the need for more field-based research on the intermediate zone.     

Field Investigation of a New Recharge Approach for ASR Projects in Near‐Surface Aquifers

Aquifer storage and recovery (ASR) is the artificial recharge and temporary storage of water in an aquifer when water is abundant, and recovery of all or a portion of that water when it is needed. One key limiting factor that still hinders the effectiveness of ASR is the high costs of constructing, maintaining, and operating the artificial recharge systems. Here we investigate a new recharge method for ASR in near‐surface unconsolidated aquifers that uses small‐diameter, low‐cost wells installed with direct‐push (DP) technology. The effectiveness of a DP well for ASR recharge is compared with that of a surface infiltration basin at a field site in north‐central Kansas. The performance of the surface basin was poor at the site due to the presence of a shallow continuous clay layer, identified with DP profiling methods, that constrained the downward movement of infiltrated water and significantly reduced the basin recharge capacity. The DP well penetrated through this clay layer and was able to recharge water by gravity alone at a much higher rate. Most importantly, the costs of the DP well, including both the construction and land costs, were only a small fraction of those for the infiltration basin. This low‐cost approach could significantly expand the applicability of ASR as a water resources management tool to entities with limited fiscal resources, such as many small municipalities and rural communities. The results of this investigation demonstrate the great potential of DP wells as a new recharge option for ASR projects in near‐surface unconsolidated aquifers.     

Numerical Modeling of Methane Leakage from a Faulty Natural Gas Well into Fractured Tight Formations

Horizontal drilling and hydraulic fracturing have enabled hydrocarbon recovery from unconventional reservoirs, but led to natural gas contamination of shallow groundwaters. We describe and apply numerical models of gas‐phase migration associated with leaking natural gas wells. Three leakage scenarios are simulated: (1) high‐pressure natural gas pulse released into a fractured aquifer; (2) continuous slow leakage into a tilted fractured formation; and (3) continuous slow leakage into an unfractured aquifer with fluvial channels, to facilitate a generalized evaluation of natural gas transport from faulty natural gas wells. High‐pressure pulses of gas leakage into sparsely fractured media are needed to produce the extensive and rapid lateral spreading of free gas previously observed in field studies. Transport in fractures explains how methane can travel vastly different distances and directions laterally away from a leaking well, which leads to variable levels of methane contamination in nearby groundwater wells. Lower rates of methane leakage (≤1 Mcf/day) produce shorter length scales of gas transport than determined by the high‐pressure scenario or field studies, unless aquifers have low vertical permeabilities (≤1 millidarcy) and fractures and bedding planes have sufficient tilt (～10°) to allow a lateral buoyancy component. Similarly, in fractured rock aquifers or where permeability is controlled by channelized fluvial deposits, lateral flow is not sufficiently developed to explain fast‐developing gas contamination (0‐3 months) or large length scales (～1 km) documented in field studies. Thus, current efforts to evaluate the frequency, mechanism, and impacts of natural gas leakage from faulty natural gas wells likely underestimate contributions from small‐volume, low‐pressure leakage events.     

Electromagnetic‐Induction Logging to Monitor Changing Chloride Concentrations

Water from the San Joaquin Delta, having chloride concentrations up to 3590 mg/L, has intruded fresh water aquifers underlying Stockton, California. Changes in chloride concentrations at depth within these aquifers were evaluated using sequential electromagnetic (EM) induction logs collected during 2004 through 2007 at seven multiple‐well sites as deep as 268 m. Sequential EM logging is useful for identifying changes in groundwater quality through polyvinyl chloride‐cased wells in intervals not screened by wells. These unscreened intervals represent more than 90% of the aquifer at the sites studied. Sequential EM logging suggested degrading groundwater quality in numerous thin intervals, typically between 1 and 7 m in thickness, especially in the northern part of the study area. Some of these intervals were unscreened by wells, and would not have been identified by traditional groundwater sample collection. Sequential logging also identified intervals with improving water quality—possibly due to groundwater management practices that have limited pumping and promoted artificial recharge. EM resistivity was correlated with chloride concentrations in sampled wells and in water from core material. Natural gamma log data were used to account for the effect of aquifer lithology on EM resistivity. Results of this study show that a sequential EM logging is useful for identifying and monitoring the movement of high‐chloride water, having lower salinities and chloride concentrations than sea water, in aquifer intervals not screened by wells, and that increases in chloride in water from wells in the area are consistent with high‐chloride water originating from the San Joaquin Delta rather than from the underlying saline aquifer.