A zonal evaluation of intrinsic susceptibility in selected principal aquifers of the United States
Highlights
► A novel particle tracking scheme was developed to evaluate aquifer susceptibility (AE). ► AE was compared in 11 aquifers to examine regional and temporal patterns. ► Similarities were identified by geographic location of east vs. west study locations. ► Climate, geology, and aquifer geometry were the strongest distinguishing factors. ► Mechanisms of slow and rapid transport were ubiquitous to all aquifers.
Introduction
Groundwater is the main source of potable water for approximately one-half the population of the United States, including nearly all users served by domestic water-supply systems (Alley et al., 1999). Anthropogenic constituents, such as herbicides, pesticides, and volatile organics, and heavy metals can enter the groundwater system with recharge as a result of activities on the land surface (Fig. 1, credits: http://en.wikipedia.org/). Groundwater-contaminant concentrations are frequently greatest in shallow parts of an aquifer system beneath agricultural and urban land uses, and contaminants from the overlying land surface are often least concentrated in deep parts of an aquifer system (Lapham et al., 2005). Deep parts of an aquifer may be devoid of contaminants from the land surface because of (a) low permeability layers that can slow or perhaps prevent groundwater flow and thus disable contaminants from arriving by the time of analysis, or (b) long groundwater-flow paths and associated transport times that allow for chemical constituents to be transformed by geochemical and microbiological processes as they are transported through the groundwater system.
Several definitions of vulnerability and susceptibility are provided in the literature. This study falls within the broad definition of vulnerability as termed by the National Research Council (1993): the tendency or likelihood for contaminants to reach a specified position in the groundwater system after introduction at some location above the uppermost aquifer. The goal of this investigation is to track a conservative solute that enters the aquifer through recharge and stream leakage, which is termed herein as intrinsic groundwater susceptibility. Susceptibility assessment in this investigation does not target specific types of contamination that would constitute an analysis of aquifer vulnerability, but instead focuses on the physical factors affecting conservative transport. Susceptibility of groundwater may depend on the geologic properties of the aquifer such as lithology, aquifer structure and geometry, porosity, and permeability, as well as the hydrologic conditions such as local and regional hydraulic gradients, depth to water, net recharge, surface-water interaction, and pumping from domestic and municipal wells (Focazio et al., 2002).
Reviews of aquifer susceptibility can be found in a variety of publications. Notable contributions include US Environmental Protection Agency, 1993, National Research Council, 1993, and Focazio et al. (2002). Methods for determining susceptibility generally fall within three categories: (1) categorical-index methods, (2) process-based model simulations, and (3) empirical statistical approaches. Susceptibility is typically defined by methods that describe levels of intensity using delineations at the land surface and therefore do not account for the dynamical influences occurring in a groundwater system, variations in concentrations with depth, or diffuse processes that cause contaminants to persist in aquifers on time scales from days to millennia. Assessments that do not account for groundwater-flow patterns may exhibit significant limitations, particularly in areas of shallow depth to bedrock (Ross et al., 2004). Moreover, simple ranking methods of susceptibility have shown only mixed success under rigorous evaluation (e.g. Rupert et al., 1991, Koterba et al., 1993, Barbash and Resek, 1996). Sources of contaminants of anthropogenic or natural origin, fate, and transport should be considered with the inherent interconnection of the groundwater-flow system to identify potential influences on water quality (Back et al., 1993, Focazio et al., 2002). The ongoing challenge associated with the use of groundwater models as tools for assessing aquifer susceptibility is to simulate the important hydrologic processes accurately at a variety of scales in both space and time while incorporating measures of uncertainty into the assessment.
In this investigation, three-dimensional groundwater-flow models of 11 study areas are integrated with a customized particle-tracking analysis to investigate spatial and temporal patterns of intrinsic groundwater susceptibility that are based on simulations of conservative solute transport and groundwater age. The technique considers a simplified representation of groundwater flow processes, which allows for the assessment of multiple scenarios in applied studies, and thus builds on existing methods that consider only the advection of groundwater for susceptibility assessment under a single and often uncertain prediction.
Section snippets
Study areas
Intrinsic groundwater susceptibility was evaluated using groundwater models of 11 study areas across the United States (Fig. 2). Selection criteria for each study area was based on the following model characteristics: (1) major hydrostratigraphic units are defined spatially within the model, (2) intermediate model size with an area less than 104 km2, and (3) published results from calibrated models using MODFLOW-2000 (Harbaugh et al., 2000) to simulate groundwater movement under steady-state
Analysis of intrinsic groundwater susceptibility
The assessment of intrinsic susceptibility using a single (deterministic) prediction of advective solute movement can be strengthened by accounting for additional transport processes and by addressing the uncertainty that is linked with parameterization. Conservative transport was simulated using the groundwater-flow model MODFLOW (Harbaugh et al., 2000) and the advection-based particle-tracking code MODPATH (Pollock, 1994) with modifications that incorporate additional capabilities under a
Results
A framework was developed in this investigation to examine intrinsic groundwater susceptibility in 11 study areas across the United States. An intermediate-scale groundwater-flow model was constructed for each study area in previous investigations to capture the basic hydrodynamics of each region to allow for a basic comparison between study areas. The comparison of results between multiple groundwater models, however, can be influenced by the differences in hydrogeologic characteristics
General implications
The study emphasizes that conservative solute migration from recharge to the groundwater can be detected in as early as a few days while the longest groundwater residence times can extend for a period of 102–106 years. Intrinsic groundwater susceptibility was shown to vary substantially between study areas and between spatially defined zones within a study area. Dominant environmental factors that control the level of intrinsic groundwater susceptibility were identified as recharge rate and
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