Investigating Groundwater Systems on Regional and National Scales (2000) / Chapter Skim
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Scientific Issues
Scientific Issues
Pages 66-98
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From page 66... ...
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 decisionOne 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 groundwater problems have a significant geochemical component and that geochemistry can often provide important insights into hydrogeologic processes.
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From page 67... ...
AQUIFER MANAGEMENT Scientific and Management Issues Water managers have the very real problem of trying to project water 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.
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From page 68... ...
In addition to modeling, methods used to investigate freshwatersaltwater interactions include tracers (Box 4.~) and geophysical tools, especially electrical methods that are sensitive to conductive water (Rozycki, 1996~.
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From page 69... ...
Scientific Issues 69
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From page 70... ...
USGS Roles in Aquifer Management The role of the USGS in aquifer management includes collecting, inventorying, and analyzing data on groundwater levels, developing improved techniques for acquiring such data, and developing and improving analytical and numerical tools for aquifer management. Potentiometric and water-level maps are a key tool in assessing the effects of regional water use.
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From page 71... ...
Borehole tilt-meters or seismographs can be deployed in high-risk areas. The NRC identified the need to analyze links between water resources and climate change as one of eight key areas for USGS WRD research (NRC, 1991a)
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From page 72... ...
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.
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From page 73... ...
For example, using a water-table rise as evidence of recharge 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 optimizing parameter estimates can be brought to bear on the problem of nonuniqueness, but describing the aquifer heterogeneity is still critical.
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From page 74... ...
Improved knowledge of groundwater recharge will help water managers protect aquifer health under stresses imposed by increasing withdrawals 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.
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From page 75... ...
Scientific Issues 75 agement. If decisions about water or land use affect citizens preferentially, the map must be detailed enough to resolve local variations in soil, topography, and drainage perceived by an observant citizen.
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From page 76... ...
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 legislation at all levels of government to protect aquifers from contamination by land use, much of it under welIhead protection clauses.
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From page 77... ...
Public-sector managers need water quality and water-level monitoring information in areas geologically susceptible to degradation or characterized by high-risk land-use practices, and they need the educational tools to enlist the support of the public in their own self-interest. Where prevention fails or contamination has already occurred, the managers need tools to restore the water supply.
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From page 78... ...
Early views of groundwaterstream interactions often treated streams simply as areas of recharge to groundwater or recipients of discharge from groundwater; groundwater flow paths in the vicinity of surface water bodies were generally twodimensional and simplistic (Woessner, 1998~. Interactions between groundwater and streams or lakes were often characterized in terms of base flow or bank storage, the latter being recognized as an important storage and flood-wave attenuation mechanism (Whiting and Pomeranets, 19971.
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From page 79... ...
The NAWQA Program, a nationwide study of surface water and groundwater quality trends and their cause-and-effect relationships, seeks to treat groundwater and surface water as a single unit, where appropriate. The USGS also has contributed greatly to stream-groundwater interaction.
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From page 80... ...
Wetlands, as places where groundwater and surface water interact, are similar to hyporheic zones with respect to their being ecological and hydrologic interfaces between freshwater- and marine water-rich and water-poor landscapes. Compared to nonwetIand areas, the unique chemical characteristics of wetland soils, their ecological community compositions, and the relative degree to which their soils are saturated vary widely across the nation's climatic regimes.
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From page 81... ...
Together, the detailed studies of wetland hydrology, geochemistry, and biota and the more regional studies of wetland function in the landscape have fostered an almost entirely new scientific discipline of multidisciplinary study, complete with new journals such as Wetlands, published by the Society of Wetland Scientists.
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From page 82... ...
In many basins, regional permitting and planning decisions need to accurately incorporate the complex interactions between surface and subsurface flows in water-availability modeling. This growing need represents a timely opportunity for the USGS to integrate its expertise in process-based regional groundwater science with regional assessments of sustainable surface water and groundwater resources.
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From page 83... ...
Groundwater-surface water interactions embody critical processes, essential for understanding basin hydrology and watershed-scare fate and transport, developing watershed-scare management plans, and calculating total maximum daily Toads. GROUNDWATER IN KARST AND FRACTURED AQUIFERS Scientific and Management Issues Unlike many of the nation's major aquifers that owe their productiv~ty to primary (i.e., intergranular)
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From page 84... ...
Effective management means protecting the actual source area for potential water supplies, as is done in Florida by creation of conservancy recharge parks. Mapping recharge areas, however, is not straightforward.
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From page 85... ...
The bulk hydraulic conductivity of a fractured aquifer may be Tow, but flow velocities and contaminant transport rates in fractures or solution cavities can be high, resulting in early contaminant appearance at target wells or streams and in poor filtration of pathogens. Locations of fracture zones are extremely important to the development of water supplies in areas with generally unproductive aquifers.
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From page 86... ...
Determining reactions along groundwater flow paths requires a sound conceptualization of the hydrogeochemical system, including velocities, residence times, surface area, oxidation state, availability of particles such as clay and organic matter, and microbes. Advection dominates transport in fractures; diffusion dominates in blocks (Moench, 1995~.
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From page 87... ...
This test underscores the need for testing and developing field protocols for fractured aquifers. Methods are needed to locate transmissive fractures.
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From page 88... ...
Classic hydrogeology has often described aquifers only in teas of bulk hydraulic characteristics (transmissivity, storage coefficient, and porosity) that are relevant to groundwater resources issues.
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From page 89... ...
For example, a smallscale contamination study might collect field data and interpret heterogeneity based on wells located only a few meters or tens of meters apart. For a subregional groundwater model (for example, for a small town)
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From page 90... ...
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.
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From page 91... ...
It should be noted that the Cape Cod and Borden tests, which have become literature classics, were both conducted at relatively uniform sites. NUMERICAL MODELING Scientific and Management Issues During the last two decades, numerical modeling has become standard practice in most groundwater studies.
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From page 93... ...
USGS Roles in Numerical Modeling The USGS has a strong history of innovation and achievement in the
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From page 94... ...
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)
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From page 95... ...
· Effectively manage groundwater and surface-water resources for domestic, agricultural, commercial, industrial, recreational, and ecological uses. · Protect and enhance water resources for human health, aquatic health, and environmental quality.
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From page 96... ...
For example, saturated hydraulic conductivity data may be available from permeameter 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.
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From page 97... ...
. Water-use allocation takes into consideration not only scientific knowIedge about water resources, but also public policy options, and can be accomplished with the help of models that integrate the two areas ~C, 1991a)
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From page 98... ...
As part of the GroundWater 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.
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