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4
Scientific :Issues
In Chapter 1, some of the nation's most pressing groundwater issues,
along with their social importance, were introduced. This chapter pres-
ents most of the same issues, with their corresponding tools or methods,
as potential research topics for incorporation into the Ground-Water Re-
sources Program (GWRP), and provides recommended actions for the
USGS. The issues are the following:
making.
aquifer management,
natural groundwater recharge,
groundwater quality and movement in surficial materials,
groundwater-surface water interactions,
groundwater in karst and fractured aquifers,
characterization of subsurface heterogeneity,
modeling of flow, transport, and management, and
facilitating the use of groundwater information in decision-
One common thread that connects all the topics discussed below is
the necessity of integrating geochemical investigations into many, if not
most, groundwater studies. The committee recognizes that most ground-
water problems have a significant geochemical component and that geo-
chemistry can often provide important insights into hydrogeologic proc-
esses. Historically, most regional groundwater investigations by the
USGS have emphasized physical hydrogeology at the expense of geo-
66
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67
chemical hydrogeology. Yet physical and geochemical problems are
usually intertwined, and both affect sustainability.
AQUIFER MANAGEMENT
Scientific and Management Issues
Water managers have the very real problem of trying to project wa-
ter use and water supply for a future that includes population growth,
climate variability, and unknown technological breakthroughs. They
must make decisions about curbing growth, investing in technology, and
balancing the various needs of stakeholders and ecosystems. Funda-
mental to these decisions are water-budget issues. How much water can
be used without drawing down the water table or potentiometric surface,
thereby causing Toss of storage, salt-water intrusion, or property damage
due to aquifer subsidence? How can managers avoid drying up streams
or draining wetlands many of which retain suspended sediment, excess
nutrients, and pesticides and maintain wildlife habitat? The sustain-
ability of human communities, including the ecosystems that support
them, needs to be considered as an integral part of aquifer management.
Climate change over decades can also have a major effect on water re-
sources, independent of local human influence. Changes in global
weather patterns can cause marked changes in precipitation and evapo-
transpiration rates and distribution, resulting in changes in recharge,
streamflow, flooding, and drought patterns. Although fully understand-
ing climate change is a global issue, the USGS has a useful role in as-
sisting those predicting climate change at the regional level in the United
States.
Excessive pumping of groundwater for irrigation and other uses has
caused water-level declines of greater than 100 feet in some regions. In
addition to causing resource depletion, this reduces pore pressures and
raises the effective stress on the aquifer, often leading to irreversible
consolidation. Differential settling cracks foundations, which may not
only be costly for structures such as roads or buildings, but also hazard-
ous to dams, power plants, or pipelines. In some cases, subsidence
caused by irrigation pumping has lowered the land surface to the point
where rivers have changed course and have flooded agricultural lands.
Subsidence caused by overpumping of both water and hydrocarbons has
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Investigating Groundwater Systems
submerged coastal areas below sea level, causing ecological damage to
coastal wetlands and exacerbating hurricane damage (White et al., 1993;
Kreitler, 1977~.
Excessive pumping has caused salt-water intrusion in the majority of
U.S. coastal states, including Massachusetts (Person et al., 1998), New
Jersey (Pope and Gordon, 1999), South Carolina (Smith, 1994), Florida
(Merritt, 1996), Louisiana (Tomaszewski, 1996), and California (Izbicki,
1996~. Even inland areas underlain by formations containing saline wa-
ter are susceptible (Sophocleous and Ma, 1998~. Saline groundwater is
present in most of the major basins of the United States, and as coastal
cities grow, this problem may be expected to get worse.
It may take decades before salinity is noticeable in well water and,
by then, years may be needed to purge the saline plume even if pumping
halts. Injection of freshwater hastens the purging process only slightly,
assuming a fresh supply can be found (Kazmann, 1972~. For this reason,
it is in the national interest to thoroughly understand the process of salt-
water intrusion to assess and manage the risk before damage occurs.
The position of the freshwater-saltwater contact can often be esti-
mated using the Ghyben-Herzberg principle (Baybon-Ghyben, 1888),
which predicts a density-controlled floating lens of freshwater with a
"root" approximately 40 times the elevation of the water table above sea
level, thinning toward the coastline. However, most salt-water intrusion
problems are too complex for simplistic approaches. Wedges of relict
freshwater occur far offshore, sandwiched between saline water. Tidal
forcing, rainfall events, and storm surges are transient short-term proc-
esses that influence salinity. Pockets of relict seawater and intrusion of
salt water through failed well casings, joints, or sinkholes complicate the
interpretation of salinity in well water. Heterogeneity in aquifer material
properties also affects the location of the saltwater-freshwater interface.
Such systems are generally studied today using either a sharp interface
model (e.g., SHARP; Essaid, 1990) or a variable-density solute transport
model (e.g., SUTRA; Voss, 1984~.
In addition to modeling, methods used to investigate freshwater-
saltwater interactions include tracers (Box 4.~) and geophysical tools,
especially electrical methods that are sensitive to conductive water
(Rozycki, 1996~. Uncertainty in and scale dependence of material
properties and processes plague virtually all measurements of the salt-
water-groundwater flux.
Aquifer storage and recovery (ASR) technology has recently gained
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Investigating Ground water Systems
increased interest as a means for aquifer management. Aquifer storage
ant] recovery projects involve the artificial storage of water in under-
ground aquifers during times of water availability and the recovery of
that water when the water is needed (Pyne, 1995~. Most projects involve
the subsurface injection of water into aquifers and later extraction of the
same water. Example ASR applications to meet aquifer management
needs include, among others, seasonal storage of water, emergency stor-
age of water, the prevention of salt-water intrusion, enhanced wellfield
production, and hydraulic control of contaminant plumes. ASR technol-
ogy has been used in various parts of the nation since the late 1960s.
The use of ASR poses many technical challenges. These include as-
sessing the hydraulic performance of the systems and determining the
effects on nearby wells, the Tong-term geochemical changes caused by
mixing waters of different chemical compositions in the subsurface, and
contaminant migration away from ASR sites.
USGS Roles in Aquifer Management
The role of the USGS in aquifer management includes collecting,
inventorying, and analyzing data on groundwater levels, developing im-
proved techniques for acquiring such data, and developing and improv-
ing analytical and numerical tools for aquifer management.
Potentiometric and water-level maps are a key tool in assessing the
effects of regional water use. Such maps were made for the major re-
gional aquifer systems as part of the Regional Aquifer-System Analysis
(RASA) Program. An unmet need is a national effort to track water lev-
els over time in order to monitor water-level declines (Sun and Johnston,
1994~. This is being done on an ad hoc basis by individual states, but
the creation of regional potentiometric maps is the responsibility of the
federal government. Data to support potentiometric surface mapping are
likely to be available from non-USGS entities, especially state geological
surveys; the USGS must collaborate with these entities in sharing and
interpreting water-level data.
Traditional groundwater resources projects are still required in many
areas. A growing U.S. population, especially in the arid and semiarid
regions, points to the need for the USGS to explore and characterize al-
ternative groundwater supplies in areas such as the Great Basin in Ne-
vada.
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71
The exact location and rates of subsidence depend on geology and
duration of pumping. Although it is not possible to make exact predic-
tions with respect to settlement, it should be possible to improve our per-
formance in that area. More effective management will first require
better definition of geologic heterogeneity. The USGS should continue
to study the relationship between water levels and subsidence, modeling
interactions among Ethology, clay content, recharge, pumping, storage,
and subsidence. The goal should be to keep subsidence within safety
limits for the strain of structures and to identify the critical pumping rate
at which there is no permanent strain.
For tracking regional subsidence, techniques such as synthetic aper-
ture radar interferometry (Massonnet and Feig1, 1998; Amelung et al.,
1999) and global positioning systems (GPS) should be fully exploited.
These do not require a fixed datum, as does high-precision leveling, and
are more cost-effective for large geographic areas. Likewise, arrays of
piezometers with transducers should be used to track long-te~ regional
changes in the potentiometric surface as an early warning system. Bore-
hole tilt-meters or seismographs can be deployed in high-risk areas.
The NRC identified the need to analyze links between water re-
sources and climate change as one of eight key areas for USGS WRD
research (NRC, 1991a). The difficulty of scaling hydrologic models to
be compatible with coarse-meshed global circulation models, or vice
versa, is a limitation that must be overcome. Predictions under a variety
of scenarios must be wedded to decision-making models they must be
presenter! in a form useful to water managers and decision-makers.
Salt-water intrusion modeling has far to go before it reaches the
stage where it can be used effectively by water resources managers.
Surprisingly, few test cases exist for independently "verifying" the
groundwater codes used for such modeling (Simmons et al., 1999~.
Also, although three-dimensional models exist, computation time still
limits most real-worId simulations to two-dimensional analysis. A fur-
ther challenge is that the nonlinear coupling of the flow and transport
equations creates difficulties in their numerical solution. Finally, inte-
grated optimization tools are generally lacking, as are linkages to geo-
graphic information systems (GIS).
Geochemical methods can also use refinement. The radium tracer
technique of Box 4.1 has potential for field ground-truthing. Various
ratios—e.g., CI:Br (Davis et al., 1998) and CI:F (Vengosh and Pankra-
tov, 1998) have also shown promise in distinguishing CT from modern
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Investigating Groundwater Systems
seawater, "connate" water, wastewater, road salt, and domestic water-
conditioning recharge effluent.
It is likely that the USGS will become involved with ASR projects
as they influence regional aquifer management. Appropriate roles for
the USGS include regional modeling of ASR impacts, investigations of
geochemical and hydraulic processes associated with ASR projects, and
determination of aquifer properties (transmissivity, storage, heterogene-
ity) relevant to ASR performance and design.
NATURAL GROUNDWATER RECHARGE
Ciroundwater recharge is a critical part of the water budget, and it is
arguably the hardest component to quantify. The difficulty in measuring
this "income" term in the water budget makes it no less important, espe-
cially in arid and semiarid areas. It is also important in coastal areas,
where lowering of the water table induces salt-water intrusion into water
supplies, and in surficial aquifers, where recharge can carry surface and
soil contamination into shallow water supplies.
Scientific and Management Issues
The critical attributes of recharge are its rate and spatial distribution.
Combining the two yields volumetric recharge to an aquifer. In the past,
the estimation of recharge rate, particularly in arid areas, was given the
most attention. Recharge rates can be estimated using hydroclimatologi-
cal approaches requiring measurement or estimation of rainfall,
evapotranspiration, soil moisture, and runoff and treating recharge as
the residual.
Groundwater recharge rates and their spatial distribution can also be
estimated using environmental tracers such as dyes, chloride, bromide,
nitrogen-15, and chiorofluorocarbons (CFCs), the stable isotopes deute-
rium and oxygen-1 S. and the radioisotopes carbon-14, tritium and chio-
rine-36 (Clark and Fritz, 1997, pp. 80-99~. These techniques can be ap-
plied on both the small scale (Cook et al., 1994) and large scale (Cam-
pana and Boyer, 1996~. Agricultural chemicals for which application
records exist can also serve as tracers.
Recharge can also be estimated by intensive study of infiltration and
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73
moisture redistribution in the unsaturated zone. Obviously, the scales of
these two approaches are drastically different, from tens of kilometers to
centimeters. The infiltration approach is physically based and rigorous;
however, extrapolation to large scale presents an obstacle. Moreover,
although the centimeter-scale process can be modeled using physically
based models of the unsaturated zone, linking these models to aquifer
models with a resolution of tens of meters or kilometers continues to be
difficult. The lack of data to support centimeter-scare modeling of vast
areas provides a strong disincentive to reconciling the two scales.
Methods of measuring recharge directly have the advantage of inte-
grating the sub-centimeter-scare changes to the meter scale. Although
the controlling processes occur at the pore scale, they result in a percep-
tible movement of moisture that can be measured at the field scale with
appropriate field instruments. Noninvasive surficial methods include
geophysical methods and remote sensing. Time-domain refractometry
and micro-gravity surveys show promise in determining recharge rates
(e.g., Young et al., 1997~. The USGS is monitoring micro-gravity at the
University of Arizona's network near Tucson in the first basinwide ap-
plication of micro-gravity methods to the measurement of changes in
groundwater storage. Long-term monitoring, including two El Nino
events already, will permit correlation of storage changes with climatic
events, facilitating water-use planning and management. Recharge has
been interpreted from remotely sensed data with some success. For ex-
ample, high-resolution radar images, filtered by principal components
analysis, show promise for quantifying the dependence of recharge on
climate and topography (Verhoest et al., 1998~.
Although the USGS and others have been researching various meth-
ods of estimating recharge, the goal of straightforward regional applica-
tion has yet to be achieved in most cases.
For example, the use of ground-penetrating radar to determine travel
times for establishing depth to water is confounded by variations in soil
moisture. Unfortunately, surficial methods of recharge estimation will
always be difficult because of spatial variability of hydrogeologic mate-
rials and soils. For example, using a water-table rise as evidence of re-
charge may be misleading if elevated areas are actually areas of lower
hydraulic conductivity, because there is not a unique relationship among
hydraulic conductivity, head, and recharge. Statistical methods of opti-
mizing parameter estimates can be brought to bear on the problem of
nonuniqueness, but describing the aquifer heterogeneity is still critical.
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Investigating Groundlwater Systems
In most watersheds, recharge is not spatially uniform because of
variations in rainfall, evapotranspiration, infiltration, and runoff. Dis-
charge, or negative recharge, may be a natural process occurring in wet-
lands or stream valleys, or it may be a result of pumping. In any case,
predicting the flow of water within aquifers requires specifying the
fluxes of water into and out of the system. Until recently, standard
practice was to assume a uniform recharge rate over an entire watershed,
and for some purposes this assumption yielded practical results. The
assumption becomes increasingly restrictive with decreasing scale and
with increasing need for resolution. In 1991, the NBC recommended the
development of methods to identify critical recharge areas on small spa-
tial scales ~C, 199lb). The need for mapping recharge remains, de-
spite locally notable efforts such as Sophocleous (1992~. Although
methods have been proposed, they have not been widely used and are
complicated by problems of scale.
USGS Roles in Groundwater Recharge
For regional studies of groundwater, it is essential that the USGS
continue to develop and test methods that define recharge at scales
ranging from local to regional (Box 4.2~. The required knowledge base
includes (1) an improved understanding of basic controlling processes
such as evapotranspiration and infiltration, (2) new modeling methods
integrating centimeter-scale processes and linking them to large-scare
models, including numerical methods for handling nonlinearity in satu-
rated-unsaturated models, and (3) methods to measure or average or sta-
tistically represent centimeter-scale heterogeneity.
Improved knowledge of groundwater recharge will help water man-
agers protect aquifer health under stresses imposed by increasing with-
drawals or by drought, and it will help them avoid recharging aquifers
with poor-quality (contaminated or salines water. From the point of
view of health of aquifers regionally, it is critical that studies of recharge
make the leap from local, intensive "case" studies to general principles,
determining what controls recharge regionally and mapping those factors
with a GIS to provide a basis for aquifer management. As appealing as
this concept is, efforts to map groundwater vulnerability regionally for
management have not always produced practical results. It is important
that maps not be too generalized if they are to be useful in local man-
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75
agement. If decisions about water or land use affect citizens preferen-
tially, the map must be detailed enough to resolve local variations in soil,
topography, and drainage perceived by an observant citizen.
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Investigating Groundwater Systems
GROUNDWATER QUALITY AND MOVEMENT
IN SURFICIAL MATERIALS
Over broad areas of the United States, groundwater occurs in shal-
low surficial materials. These materials include glacial, alluvial, and
lacustrine deposits as well as weathered bedrock residuum. In general,
such materials are a few tens to a few hundreds of feet thick and often lie
above deeper bedrock aquifers. Surficial materials can be quite discon-
tinuous, as exemplified by eskers in the Northeast, or they can be very
extensive, such as the tit! sheets in the northern Midwest. Where such
materials are composed of permeable sand and gravel, they often form
important aquifers. However, materials of Tower permeability, such as
clayey till or silty lacustrine deposits, also contain and transport
groundwater ant! have important functions in the overall water cycle.
Scientific and Management Issues
Occurring near the land surface, groundwater in shallow surficial
materials is particularly vulnerable to contamination (see Chapter I, Box
1.~) by the hundreds of thousands of reported releases of gasoline from
leaking underground fuel tanks nationwide, and the nation is currently
spending hundreds of millions of dollars remediating contaminated sites
in these materials. The USGS National Water-Quality Assessment
(NAWQA) Program discovered many instances of nitrate and pesticide
contamination of shallow groundwater in agricultural areas (USGS,
1999b). Likewise, onsite septic systems and lawn fertilization can also
contaminate groundwater. Shallow groundwater contamination can
move to adjacent lakes, rivers, and wetlands as well as to underlying
deep aquifers used for water supply.
Concern for the integrity of groundwater supplies has led to legisla-
tion at all levels of government to protect aquifers from contamination
by land use, much of it under welIhead protection clauses. Despite great
effort expended on predicting how water and contaminants move under-
ground, it is still difficult to state with confidence that a given land use
will have a specific impact on a particular water supply. Clearly,
though, regional deterioration of shallow water supplies has occurred
and can be linked to land-use practices, with an example being agricul-
tural fertilizers causing high nitrate levels in rural water supplies. Much
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Investigating Groundwater Systems
~ determination of flow paths, tracers are a promising method for
determining fracture-flow. Isotopes, dyes, dissolved chemicals, bacte-
ria, and even lanthanide-labeled clay have been used successfully. Ma-
trix porosity and fracture aperture can be determined with accuracy and
are relatively insensitive to type of tracer experiment (Himrnelsbach et
al., 1998~. There is a need for tracer-test protocol. For example, in-
duced-gradient tracer tests may underestimate the importance of disper-
sion relative to advection because under low-velocity and Tong-resi-
dence-time natural conditions, dispersion dominates transport (Raven et
al., 1988~. There also needs to be a testing and cataloging of suitable
tracers, including natural or isotopic tracers. Parameter-estimation mod-
els of fractured systems will help direct data collection; these models
have shown that permeability is a poor estimator of fracture aperture, but
that flow velocities and tracer breakthrough times are good estimators of
aperture (Tsang et al., 1988~.
CHARACTERIZATION OF SUBSURFACE
HETEROGENEITY
Aquifer heterogeneity arises from the complex history of geologic
deposition, erosion, lithification, and tectonic deformation of rocks. The
importance of heterogeneity to groundwater occurrence and movement is
apparent in the wide range of hydraulic conductivities commonly ob-
served from ~ 0~~ ~ to 1 o2 cm/see (Freeze and Cherry, ~ 979~. Given this
range, the determining characteristic of an aquifer in controlling fluid
movement is its hydraulic conductivity distribution, or heterogeneity.
Despite its importance, characterizing heterogeneity remains elusive.
Scientific and Management Issues
The need for better characterization of heterogeneous aquifers is
driven by scientific and public needs for groundwater protection and
remediation. Classic hydrogeology has often described aquifers only in
teas of bulk hydraulic characteristics (transmissivity, storage coeffi-
cient, and porosity) that are relevant to groundwater resources issues.
The RASA models, which combined many complex stratigraphic units
into a few conceptual layers, are examples of this approach. However,
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~9
bulk properties are rarely, if ever, adequate to determine flow paths and
travel times necessary for contaminant transport studies or welIhead
protection. instead, a more detailed knowledge of the distribution of
hydraulic properties is critical.
Efforts to cope with heterogeneity fall into three categories. First,
there have been attempts to map heterogeneity by intensive drilling and
geophysical surveying. Second, some researchers have attempted to
logically relate rock or soil properties to the depositional process, using
geologic facies architecture. Facies models—conceptual models of the
expected distribution of facies based on the geologic depositional history
of an area—can be used to define hydrostratigraphic units (Maxey,
1964; Seaber, ~ 988; Anderson, ~ 989~. The petroleum industry interprets
relatively scarce borehole data and abundant "soft" data such as three-
dimensional seismograms using facies models. Third, heterogeneity has
been treated as a stochastic process, initially as a purely random distri-
bution of properties, more recently adding realism with correlation, non-
stationarity, and nonrandomness.
Predicted hydraulic conductivities Took increasingly plausible with
these advanced methods, but they still need to be conditioned with in-
formation including "soft" data (electrical resistance tomography, seis-
mic tomography, radar tomography, etc.~. An abundance of small-scale
data are required to detect the underlying stochastic processes for a vari-
ety of geologic settings. Detailed studies are needed at heterogeneous
sites such as those at the MADE (MAcroDispersion Experiment) site in
Mississippi. Stochastic process models will have to be incorporated into
facies models to cope with the nonstationarity that appears at the large
scale. In the past, detailed characterization usually was not attempted
because numerical models, the fundamental too] of modern hydro-
geologic prediction, were largely unable to handle this complexity. This
situation has changed with the advent of fast, inexpensive computers and
improved modeling codes.
Many hydrogeologists have encountered the so-called "scale effect"
of hydraulic conductivity (K), which suggests that the effective K of a
given material varies with the scale of either the testing method used or
the field problem being addressed (Hsieh, 1998~. For example, a small-
scale contamination study might collect field data and interpret hetero-
geneity based on wells located only a few meters or tens of meters apart.
For a subregional groundwater model (for example, for a small town),
heterogeneity might be studied on the scale of hundreds of meters. What
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Investigating Groun~lwater Systems
is the effective K in these two cases? The question pertains to both the
method of measuring K (aquifer test vs. slug test, for example) and the
appropriate K to use when simulating aquifer behavior with a numerical
model. The number of papers published on the topic of scale since the
early 1990s shows there is growing interest in this topic. Different in-
vestigators have examined possible causes of the scale effect in several
ways, including field-testing and modeling studies. However, there is no
consensus on the causes of the effect or on factors that might control its
magnitude. Indeed, some hydrogeologists claim there is no physical ba-
sis for the scale effect (Butler et al., 1996~.
Because heterogeneity results from small-scare (and larger) proc-
esses, understanding these processes requires a microscale investigation.
Paradoxically, the results will eventually be applied at a larger scale,
especially in numerical modeling. So in addition to needing methods to
define small-scale features, methods are needed to realistically represent
these processes at larger scales.
USGS Roles in Characterization of Subsurface Heterogeneity
The USGS should continue studies of groundwater in a variety of
complex settings to reveal important principles and processes controlling
water supply and quality. The Survey should also continue its inventory
of aquifer properties in order to develop regional databases. The science
is by no means complete, as is evident from new developments in the
understanding of natural attenuation of contaminants. Translating
lithostratigraphy to hydrostratigraphy rests on a foundation of detailed
hydrogeologic studies at representative sites such as the Cape Cod toxic
waste research site. Detailed studies at sites representative of important
(common or especially susceptible to damage) hydrogeologic settings
should continue and should be encouraged. It is important, however,
that the significance of these studies for generalizing the results to
broader areas be understood and emphasized by the USGS and stressed
in its reports to the public. The USGS must justify the investment of
resources at these intensive-study sites.
In terms of regional groundwater investigations, there is a need for
better integration among geologic disciplines: hydrogeology, stratigra-
phy, sedimentology, and structural geology. The USGS should continue
to develop methods of deducing hydrologic information from geologic
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91
models and geophysical methods (Jorgensen, 19884. The current Middle
Rio Grande basin projects illustrate how this integration can be done
successfully. Currently, however, there are no generally accepted
"rules" or measures of heterogeneity and its importance; developing
such measures would be a fruitful area for research. There are many
possible research directions for the improved simulation of the spatial
heterogeneity of aquifers (geostatistical models, fractal methods, and
process models).
Other areas for investigation include better integration of subsurface
stratigraphy with hydrogeology, innovative geophysical tools (down-
hole logging, geotomography, flowmeters, radar, etc.), measurements of
hydraulic parameters such as hydraulic conductivity at a variety of
scales, and correlation of these measurements with stratigraphic facies.
Tracer experiments, especially experiments that test/verify fieldwork and
modeling experiments in heterogeneous aquifers, are needed. It should
be noted that the Cape Cod and Borden tests, which have become litera-
ture classics, were both conducted at relatively uniform sites.
NUMERICAL MODELING
Scientific and Management Issues
During the last two decades, numerical modeling has become stan-
dard practice in most groundwater studies. Better modeling codes, faster
and cheaper computers, and user-friendly interfaces have put sophisti-
cated modeling within the facilities and budgets of most groundwater
projects (Figure 4.2~. However, these advances are a mixed blessing. A
1983 editorial titled "Groundwater Modeling: The Emperor Has No
Clothes" (Anderson, 1983) examined the pitfalls of using sophisticated
groundwater-flow models without a clear understanding of the modeling
process and/or without proper data and model calibration. A follow-up
abstract titled "Modeling Complexity: Does the Emperor Have Too
Many Clothes?" (Anderson and Hunt, 1998) discussed what has hap-
pened to groundwater modeling in the intervening 15 years. The prolif-
eration of model add-one such as pre- and postprocessors and various
optional packages (transport, streamflow routing, lake interactions,
evapotranspiration, etc.) has made extremely complex models compara-
tively easy to construct. Such complex models offer a false sense of ac-
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92
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Scientific Issues
93
curacy and precision if the model complexity cannot be supported with
appropriate field information and the model uncertainty is not quanti-
f~ed.
The ability to evaluate uncertainty and sensitivity an important re-
cent trend in mode] development—addresses concerns about misreading
model results. Parameter estimation codes such as UCODE and
MODFLOWP (Poeter and Hill, 1998) allow modelers to estimate opti-
mum sets of model parameters, consistent with field data, and they also
provide rigorous measures of the sensitivity of the model solution to
changes in parameters. Such uncertainty analyses improve models as
tools for decision-making.
Aquifer management-optimization codes—e.g., AQMAN (Lefkoff
and Gorelick, 1986), AQMAN3D (Puig et al., 1990), and various com-
mercial products are a significant step forward in decision-making.
Such codes provide optimal groundwater-management solutions, such as
the most favorable well placement or pumping rates, under various
physical and economic scenarios. They enable, for example, a munici-
pality to maximize groundwater extraction subject to the limitation that
heads near a sensitive stream or hazardous waste site do not fall below a
threshold value.
Traditional methods of measuring and modeling flow in porous me-
dia are being used only cautiously in fractured-rock systems. Significant
advances have occurred in the understanding of fractured-rock hydro-
geology (NRC, 1996~. Most water movement occurs through open
fractures, while most storage occurs in the porous matrix. A number of
analytical models (e.g., Moench, 1995) now exist for such dual-porosity
systems, while sophisticated numerical codes such as FracMan/MAFTC
(Golder Associates, 1987) allow evaluation and simulation of discrete
fracture networks using stochastic techniques. Field methods are also
being developed to characterize these aquifers. One of the major im-
pediments to progress in fractured-rock hydrogeology is a lack of well-
characterized field sites for model evaluation. The USGS fractured-rock
hydrology research site at Mirror Lake, New Hampshire (Shapiro and
Hsieh, 1996), is one of only a few such sites in the United States.
USGS Roles in Numerical Modeling
The USGS has a strong history of innovation and achievement in the
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Investigating Groun~lwater Systems
development of fundamental groundwater models such as MODFLOW.
Efforts should continue in conceptual and theoretical aspects of numeri-
cal modeling (flow, reactive chemical transport, management), espe-
cially in the near-surface environment, so that increasingly sophisticated
models will be available to help diagnose cause-and-effect relationships
and perform predictive simulations. However, the committee strongly
feels that, in the context of flow modeling, the USGS should devote its
efforts to conceptual and theoretical breakthroughs rather than fine-
tuning or developing graphical interfaces for codes like MODFLOW.
Such work is already being done by the private sector (e.g., Visual
MODFLOW, Groundwater Vistas).
In the context of regional groundwater investigations, the USGS
should continue to develop appropriate conceptual and numerical
"framework" models covering large geographic areas, and it should de-
velop the means for focusing or telescoping these models to smaller
scales. The recent work on telescopic mesh refinement (Leake and
CIaar, 1999) provides examples of such techniques. In addition, analyti-
cal element (AK) models can be used for scaling from regional to local
simulation. A far-field AE model can be used to develop boundary con-
ditions for a local finite-difference model. Analytical element models
have the added advantage of allowing exploration of a model's sensitiv-
ity to boundary conditions, an important step that is rarely done (Hunt et
al., 1998~.
FACILITATING USE OF GROUNDWATER
INFORMATION IN DECISION-MAKING
Investigation of these regional issues must provide useful infor-
mation to water resources managers and decision- or policy-makers.
This section discusses three ways that the USGS can assist in this pro-
cess: (1) by promoting the use of information from USGS studies in
decision-making by quantifying and reducing uncertainty in predictions,
(2) by scaling results of local studies to the regional level, and (3) by
assisting in the development of decision-making and risk models that
incorporate groundwater information. The WRD's mission statement
clearly emphasizes the need to actively disseminate hydrogeologic data
and reports to the public:
The mission of USGS Water Resources Division (WRD) is "to pro-
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Scientific Issues
95
vice reliable, impartial, timely information that is needed to understand
the nation's water resources. WRD actively promotes the use of this
information by decision-makers to
· Minimize the loss of life and property as a result of water-related
hazards, such as floods, droughts, and land movement.
· Effectively manage groundwater and surface-water resources for
domestic, agricultural, commercial, industrial, recreational, and ecologi-
cal uses.
· Protect and enhance water resources for human health, aquatic
health, and environmental quality.
· Contribute to the wise physical and economic development of the
nation's resources for the benefit of present and future generations."
(USGS, 1999c).
Quantifying and Reducing Uncertainty in Predictions
Predictions about groundwater systems are always subject to uncer-
tainty as a result of spatial and temporal variability in subsurface prop-
erties and processes. Additional uncertainty arises from attempts to
characterize the subsurface based on limited and possibly imprecise
measurements. Although uncertainty is an integral part of groundwater
systems, past models, measurements, and predictions have not always
explicitly identified the associated error.
Future groundwater predictions should specifically include an asso-
ciated quantitative error. One benefit of estimating error is an improve-
ment in decision-making. Error estimates allow decision-makers and
others to understand that hydrologic variables can take on a range of
values, facilitating the development of options that will meet objectives
under various scenarios. Thus, reporting errors in hydrologic variables
should lead to more robust decisions.
Associating uncertainties with predictions and measurements also
provides a rational basis for future data collection efforts. Understand-
ing uncertainty and its source allows development of sampling plans that
will result in the greatest reductions in uncertainty subject to fiscal and
other constraints.
Parameter-estimation modeling provides a measure of uncertainty in
predictions that is badly needed. Parameter-estimation modeling should
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Investigating Groundwater Systems
become standard practice, especially when models are used as a basis for
water resources decisions. For example, what is the probability that
monitoring will detect contamination? If the uncertainty in model re-
sults is unacceptable, as it may well be, strategies are needed to diminish
that uncertainty.
Scaling Available Information to the Regional Level
How can information that has already been collected at a variety of
scales be used in regional-scale studies? Data from past studies are
likely to be available on many different scales in new regions of interest
to the USGS. Data from smaller-scale or local groundwater studies are
likely to have been collected in the past by the Survey and others, and
some regional-scare information may be available as well. For example,
saturated hydraulic conductivity data may be available from permeame-
ter tests on sediment samples, slug tests, and aquifer tests.
Regional studies will require data collection on regional scales, since
many hydrogeologic variables depend upon the measurement scale.
However, it makes sense for a regional study to incorporate smaller-
scale data previously collected within the region. In regions or parts of
regions where hydrogeologic variables are statistically stationary, small-
scaTe parameter values may be representative of larger-scare effective
values (Neuzil, 1994~. For example, researchers have observed similar
values for effective flow parameters on multiple scales at the Mirror
Lake site in New Hampshire (Hsieh, 1998~. However, as scale changes,
new geologic features (fractures, stratigraphic changes) may become
important, resulting in regional effective properties that differ from those
observed in smaller-scare studies. At some sites, very large changes in
permeability have been seen with observation scale (e.g., Bredehoeft et
al., 1983~.
More research is needed to determine if there are situations in which
upscaTing (i.e., using data collected on smaller scales to derive informa-
tion at larger scales) is possible and to develop upscaling methods.
Many studies have collected hydrogeologic data on multiple scales;
however, researchers may not have taken the further step of developing
relationships between the scales. General methods for upscaling have
not been established. Indeed, researchers will probably need different
methods of upscaling depending on the region's characteristics (homoge-
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Scientific Issues
neons, stationary, trend/pattern, etc.~.
97
Upscaling parameter estimates
may not be possible at sites with markedly nonstationary parameter
fields unless an observable trend exists (e.g., a linear decrease in perme-
ability with depth).
When a pattern of variability is observed at a number of small-scare
studies, researchers sometimes assume that pattern for the larger study.
If a number of subregional studies have been conducted in a region, the
small-scare studies have clear value as indicators of subregional vari-
ability. This information may be particularly important for regional
transport studies. where larae-scale dispersion is dependent upon small-
scaTe permeability variation.
"7 . . . .
. . .
We recommend that the USGS incorporate into its regional model-
ing efforts relevant and reliable data collected during previous studies
within the regions. Using data from previous studies is particularly im-
portant in the groundwater field because of the spatial and temporal
variability inherent in subsurface data sets. Because subsurface proper-
ties and processes vary in space and time, it may be useful to character-
ize modeled variables as random or stochastic. Given the impossibility
of collecting data everywhere at all times, the properties and processes
of interest are always uncertain. In this context, every additional piece
of information is valuable in reducing uncertainty in modeling efforts.
High data collection costs for the subsurface further increase the value of
data available from past studies.
Developing Decision-Making and Risk Models
for Groundwater Use
As noted earlier, the WRD should be involved not only in collecting
data on water supply, but also in facilitating the use of this information
by decision-makers, who have to contend with competing uses (domes-
tic, agricultural, commercial, industrial, recreational, and ecological).
Water-use allocation takes into consideration not only scientific knowI-
edge about water resources, but also public policy options, and can be
accomplished with the help of models that integrate the two areas ~C,
1991a). The WRD may, by working in partnership with other national
or regional agencies, have a role in analyzing how various policies or
laws affect water use regionally. Models could, for example, explore
system behavior in response to changes in management or policy, where
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Investigating Ground water Systems
variables might include cost of pumping water, cost of crop production,
income from crops, tax revenues, etc.
CONCLUSIONS
Numerous important advances in hydrogeology have occurred in the
past two decades, but serious challenges remain. As part of the Ground-
Water Resources Program (GWRP) and associated programs, the USGS
WRD should investigate groundwater occurrence and movement in
complex hydrogeologic environments such as fractured rock and karst
and in heterogeneous media. Advances in theory should be supported by
the creative application of field methods and should lead to more reaTis-
tic models backed by sensitivity and uncertainty analysis.
Surficial aquifers and their boundaries should also receive consider-
able attention, even in regional studies. Aside from being vulnerable to
contamination, shallow aquifers are the focus of research on the spatial
and temporal distribution of recharge and discharge and on interactions
of groundwater and ecosystems. Many scientific disciplines, including
ecology, limnology, chemistry, hydrology, and meteorology, have some-
thing to contribute to such groundwater investigations. Regional
groundwater studies thus provide ideal opportunities for collaboration
within WRD programs and with other USGS divisions and external or-
ganizations. Collaboration should also facilitate the development of
water-management models, which incorporate legal, economic, ecologi-
cal, and other constraints.
Finally, changing technology is creating opportunities for innovative
approaches to the dissemination of the groundwater information and re-
sults generated by such projects. Chapter 5 is devoted to these data is-
sues.
Representative terms from entire chapter:
surface water
