Capture Versus Capture Zones: Clarifying Terminology Related to Sources of Water to Wells

The term capture, related to the source of water derived from wells, has been used in two distinct yet related contexts by the hydrologic community. The first is a water‐budget context, in which capture refers to decreases in the rates of groundwater outflow and (or) increases in the rates of recharge along head‐dependent boundaries of an aquifer in response to pumping. The second is a transport context, in which capture zone refers to the specific flowpaths that define the three‐dimensional, volumetric portion of a groundwater flow field that discharges to a well. A closely related issue that has become associated with the source of water to wells is streamflow depletion, which refers to the reduction in streamflow caused by pumping, and is a type of capture. Rates of capture and streamflow depletion are calculated by use of water‐budget analyses, most often with groundwater‐flow models. Transport models, particularly particle‐tracking methods, are used to determine capture zones to wells. In general, however, transport methods are not useful for quantifying actual or potential streamflow depletion or other types of capture along aquifer boundaries. To clarify the sometimes subtle differences among these terms, we describe the processes and relations among capture, capture zones, and streamflow depletion, and provide proposed terminology to distinguish among them.     

Geophysical Techniques to Delineate Saturated Alluvial Zones for the Siting of HVDC Power Transmission Return Electrodes

As high-voltage, direct-current (HVDC) electrical power transmission technology advances, the siting and design of the grounded return electrodes become critical factors. Electrode sites should be chosen where surrounding geologic conditions will not unfavorably channel large earth currents and harmfully affect nearby populations. An optimum site allows direct contact between the electrode and a large volume of a stable, low-resistivity material. A saturated alluvial zone between dry surface soils and an underlying crystalline bedrock is an ideal material for containing the current flow from the electrode.
A hydrogeologic investigation was conducted to delineate the saturated zone in several alluvial valleys in the southwestern desert region. Regional ground water flow directions, water quality, and seasonal fluctuations of the water table elevations were determined. The structural shape and topography of the crystalline basement beneath the valleys were interpreted from gravity data. Iterative forward-modeling of the data provided a low-cost means of deducing the volume of the lightweight alluvium. Electrical resistivity surveys were completed to measure the lateral and vertical variations in soil resistivities throughout the valleys. The geophysical data were used to extend the known hydrologic information into areas with no wells.
Composite models of the saturated zones in each valley were produced by integrating gravity interpretations with hydrologic and resistivity results. The composite models were used to predict current density in the ground that would be associated with a HVDC electrode. This methodology proved to be a cost-effective means of siting electrodes in a suitable geologic environment that would minimize their detrimental effects.     

Evaluation of Bias Associated with Capture Maps Derived from Nonlinear Groundwater Flow Models

The impact of groundwater withdrawal on surface water is a concern of water users and water managers, particularly in the arid western United States. Capture maps are useful tools to spatially assess the impact of groundwater pumping on water sources (e.g., streamflow depletion) and are being used more frequently for conjunctive management of surface water and groundwater. Capture maps have been derived using linear groundwater flow models and rely on the principle of superposition to demonstrate the effects of pumping in various locations on resources of interest. However, nonlinear models are often necessary to simulate head‐dependent boundary conditions and unconfined aquifers. Capture maps developed using nonlinear models with the principle of superposition may over‐ or underestimate capture magnitude and spatial extent. This paper presents new methods for generating capture difference maps, which assess spatial effects of model nonlinearity on capture fraction sensitivity to pumping rate, and for calculating the bias associated with capture maps. The sensitivity of capture map bias to selected parameters related to model design and conceptualization for the arid western United States is explored. This study finds that the simulation of stream continuity, pumping rates, stream incision, well proximity to capture sources, aquifer hydraulic conductivity, and groundwater evapotranspiration extinction depth substantially affect capture map bias. Capture difference maps demonstrate that regions with large capture fraction differences are indicative of greater potential capture map bias. Understanding both spatial and temporal bias in capture maps derived from nonlinear groundwater flow models improves their utility and defensibility as conjunctive‐use management tools.     

Analytical Solutions for Determination of Non-Steady- State and Steady-State Capture Zones

This paper presents analytical solutions for determining non-steady-state capture zones produced by a single recovery well and steady-state capture zones produced by multiple recovery wells. Analysis of non-steady-slate capture zones is based on the lime-dependent location of caplure zone stagnation points and the geometric similarity between steady-slate and non-steady-state capture zones. The analytical solution of steady-state capture zones is obtained from spatial variations of discharge potential across the capture zone boundary. Both capture zone analyses are based on the assumptions of uniform flow field with a constant hydraulic conductivity, the Dupuit assumption of insignificant vertical flow, a negligible delayed yield, and a fully penetrating well with a constant pumping rate. For a ground water pump-and-trcat remediation program, the pumping rate and well location design variables can be adjusted to ensure containment of the ground water contaminant plume.     

Issues and Options in the Delineation of Well Capture Zones under Uncertainty

The delineation of wellhead protection areas (WHPAs) under uncertainty is still a challenge for heterogeneous porous media. For granular media, one option is to combine particle tracking (PT) with the Monte Carlo approach (PT‐MC) to account for geologic uncertainties. Fractured porous media, however, require certain restrictive assumptions under this approach. An alternative for all types of media is the capture probability (CP) approach, which is based on the solution of the standard advection‐dispersion equation in a backward mode, making use of the analogy between forward and backward transport processes. Within this context, we review the current controversy about the correct form of the conceptual model for transport, finding that the advection‐diffusion model, which represents the diffusive interchange between streamtubes with differing velocities, is more physically realistic than the conventional advection‐dispersion model. For mildly to moderately heterogeneous materials, stochastic theories and simulation experiments show that this process converges at the field scale to an effective advection‐dispersion process that can be simulated with conventional transport models using appropriate macrodispersivity values. For highly heterogeneous materials, stochastic theories do not yet exist but there is no reason why the process should not converge naturally as well. Macrodispersivities appear to be formation‐specific. The advection‐dispersion model can be used for capture zone delineation in heterogeneous granular media. For fractured porous systems, hybrid equivalent porous medium and discrete fracture network or CP‐based approaches may have potential. In general, capture zones delineated by PT without MC will always be too small and should not be used as a basis for land‐use decisions.     

A Finite Layer Formulation for Groundwater Flow to Horizontal Wells

A finite layer approach for the general problem of three‐dimensional (3D) flow to horizontal wells in multilayered aquifer systems is presented, in which the unconfined flow can be taken into account. The flow is approximated by an integration of the standard finite element method in vertical direction and the analytical techniques in the other spatial directions. Because only the vertical discretization is involved, the horizontal wells can be completely contained in one specific nodal plane without discretization. Moreover, due to the analytical eigenfunctions introduced in the formulation, the weighted residual equations can be decoupled, and the formulas for the global matrices and flow vector corresponding to horizontal wells can be obtained explicitly. Consequently, the bandwidth of the global matrices and computational cost rising from 3D analysis can be significantly reduced. Two comparisons to the existing solutions are made to verify the validity of the formulation, including transient flow to horizontal wells in confined and unconfined aquifers. Furthermore, an additional numerical application to horizontal wells in three‐layered systems is presented to demonstrate the applicability of the present method in modeling flow in more complex aquifer systems.     

Altering Dissolved Oxygen to Determine Flow Conditions in Fractured Bedrock Wells

A new approach to locate transmissive fractures and decipher vertical borehole flow conditions in fractured crystalline bedrock wells is presented, which uses dissolved oxygen (DO) as a benign tracer. The method was tested in two fractured crystalline bedrock wells previously characterized by televiewer and flow meter logging under both ambient and stressed (slug test) conditions. The method entailed elevating wellbore DO concentrations by circulating water through showerheads or injection of compressed air. The DO dilution was used to locate inflowing fractures. Changes in the DO concentration with time were used to ascertain flow within the borehole and to locate outflowing fractures and stagnant zones. Flow rates were also estimated. Fractures detected by the method corresponded to those observed by televiewer logging and for the most part were comparable to flow meter results. Given the effectiveness, time‐efficiency and low cost, the method is a promising alternative to other methods currently in use to characterize transmissive fractures in wells.     

A New Capture Fraction Method to Map How Pumpage Affects Surface Water Flow

All groundwater pumped is balanced by removal of water somewhere, initially from storage in the aquifer and later from capture in the form of increase in recharge and decrease in discharge. Capture that results in a loss of water in streams, rivers, and wetlands now is a concern in many parts of the United States. Hydrologists commonly use analytical and numerical approaches to study temporal variations in sources of water to wells for select points of interest. Much can be learned about coupled surface/groundwater systems, however, by looking at the spatial distribution of theoretical capture for select times of interest. Development of maps of capture requires (1) a reasonably well-constructed transient or steady state model of an aquifer with head-dependent flow boundaries representing surface water features or evapotranspiration and (2) an automated procedure to run the model repeatedly and extract results, each time with a well in a different location. This paper presents new methods for simulating and mapping capture using three-dimensional groundwater flow models and presents examples from Arizona, Oregon, and Michigan.     

Automatic Delineation of Capture Zones for Pump and Treat Systems: A Case Study in Piedmont,Italy

The design of a pump and treat (P&T) system for the hydraulic control of a contaminated plume in a confined aquifer is presented here. Being the system designed for the emergency containment of a nonaqueous phase liquid plume, the evaluation of the system’s short-term efficiency was considered an important issue. For this reason, both time-related and ultimate capture zones were defined. They were traced using the automatic protection area (APA) model, a capture-zone delineation tool based on a hybrid forward-backward particle tracking algorithm, that provides an automatic post-processing encirclement of capture zones. Two simple indexes are here proposed for the evaluation of the performance of the hydraulic barrier, that is, the efficacy and efficiency indexes, calculated from the capture areas provided by APA. The discharge rates of the wells were dimensioned applying the APA algorithm, maximizing efficacy and efficiency of the barrier. Results proved both visually (via plotting of capture zones) and numerically (via calculation of the indexes) that the P&T system can provide a complete capture of the contaminated area and minimizes the volume of extracted water. Consequently, the APA algorithm was proved to be a useful tool in capture zone delineation. As a future perspective, it could be coupled with the real-time measurement of pumping rates and water levels and be implemented as a part of a tuning tool for the management of the hydraulic barrier.