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The Importance of Monitoring To Groundwater Management
William M. Alley,
US Geological Survey
Introduction
Monitoring is an essential element of any effort to integrate groundwater science with
water-management decisions. Monitoring provides important data that serve as a key
input into the decision-making process. Groundwater monitoring can:
1. Track changes in groundwater levels to help decision-makers better
understand the long-term sustainability of an aquifer as a source of water
supply and make appropriate policy choices.
2. Provide groundwater contamination information, such as identification of
groundwater contaminants and measurements of contaminant levels, and help
in identification of sources of groundwater contamination. Such information
can help decision-makers better understand the aquifer’s water quality,
potential effects on public health and the ecosystem, and which sources most
need to be addressed.
3. Identify existing or potential changes in flow due to groundwater withdrawal.
This information can help decision-makers to make appropriate policy
decisions to prevent damage such as saltwater intrusion or movement of
contaminants towards a pumping station or well.
4. Assess the effects of climate on groundwater levels, enabling decision-makers
to issue timely drought warnings or declarations and take appropriate
mitigation measures.
This paper illustrates the value of long-term monitoring as described above through four
case studies that highlight the broad applicability of monitoring data to water-resource
issues. The discussion focuses on regional monitoring of groundwater levels and
groundwater quality. The paper then provides a brief review of some key choices in the
design of monitoring programs.
Case Studies on the Use of Long-term Monitoring Data
The utility of long-term monitoring is illustrated by four examples from the United
States. The examples illustrate the value of long-term water-level monitoring, water-
quality monitoring of springs, the combined use of water-level and water-quality
monitoring, and the potential utility of real-time monitoring.
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Groundwater Depletion in the High Plains Aquifer
The High Plains is a 450,000-square-kilometer area of flat to gently rolling terrain in the
central U.S. that is characterized by moderate precipitation but in general has a low rate
of natural recharge. The underlying High Plains aquifer consists of unconsolidated
alluvial deposits that form a water-table aquifer. Irrigation water pumped from the aquifer
has made the High Plains one of the most important agricultural areas in the United
States.
Changes in groundwater levels in the High Plains aquifer are tracked annually through
the cooperative effort of the U.S. Geological Survey (USGS) and State and local agencies
in the High Plains region (McGuire et al., 2003). Typically, water-level measurements are
collected from about 7,000 wells distributed throughout the aquifer. Water levels are
measured in the spring prior to the start of the irrigation season to provide consistency
across the region. Information gathered in this multi-State cooperative effort reveals
information that is important to decision-makers, such as how changes in water stored in
the aquifer vary from place to place depending on: 1) soil type, 2) recharge from
precipitation, 3) irrigation practices, and 4) the areal extent and magnitude of water
withdrawals.
In the case of the High Plains, monitoring shows that over the years the intense use of
groundwater for irrigation in the area has caused major water-level declines (Figure 1)
and decreased the saturated thickness of the aquifer significantly in some areas. For
example, in parts of Kansas, New Mexico, Oklahoma, and Texas, groundwater levels
have declined more than 100 feet (30 meters). Decreases in saturated thickness of the
aquifer exceeding 50 percent of the predevelopment saturated thickness have occurred in
some areas. The multi-State groundwater-level monitoring program revealed such
changes and has allowed these changes to be tracked over time for the entire High Plains
region. The data provided by the program are critical to evaluating different options for
groundwater management, such as well permitting, pumping, and spacing limitations and
to document the effects of conservation efforts. This level of coordinated groundwater-
level monitoring is unique among major multi-State regional aquifers in the United
States.
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Figure 1 Water-level changes in the High Plains aquifer from predevelopment to 2000
(Modified from McGuire and et al, 2003).
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Nitrate Contamination of Florida Springs
There are numerous springs in Florida, particularly in the northern half of the State.
Large demands for water from a rapidly growing population and large influx of visitors
have resulted in reductions in discharge from many of the springs. Likewise, a steady
increase in nitrate concentrations has been observed in many of the spring waters as
documented by water-quality monitoring over the past 30 or more years (Figure 2). The
karst terrain of Florida and thin cover of highly permeable sands facilitate the movement
of nitrate to the subsurface.
Figure 2 Trends in nitrate concentrations in three major springs in northern Florida
(B.G. Katz, U.S. Geological Survey, written communication., 2004).
The increasing concentrations have resulted in concerns about human health impacts and
ecological impacts, including the potential effects on the extensive aesthetic, cultural, and
recreational value of these springs. Some potential sources of nitrate contamination
include fertilizer used in agriculture, livestock waste, and sewage. In Florida, many
springs are located in agricultural areas with row crops, poultry, and dairy farms, all of
which are key nitrate sources. The results of long-term monitoring of nitrate have
spurred considerable public interest in restoration and protection of the springs (Florida
Springs Task Force, 2000) and in scientific investigations using a variety of techniques
including geochemical tracers, age-dating, nitrogen isotopes, and reconstructions of
fertilizer application rates to understand the sources of nitrate and their transport
processes and timescales (Katz et al., 2001).
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Saline Water Intrusion in New Jersey
Since the 1800’s, the principal source of public-water supply in the Coastal Plain of New
Jersey has been groundwater obtained from wells in 10 major confined aquifers arranged
in a layered groundwater system. Because of large groundwater withdrawals, regional
cones of depression have developed in each of the aquifers. By 1978, the potentiometric
surfaces of most of the aquifers had been lowered below sea level, and natural flow
directions in some areas were reversed. Consequently, saline water that is naturally
present in the deeper parts of the aquifers was induced to migrate toward pumping
centers.
As an example, pumping by public-supply wells completed in the Upper Potomac-
Raritan-Magothy aquifer near the New Jersey coastline resulted in sharply rising chloride
concentrations for the Union Beach well field as shown in Figure 3. Concentrations
increased significantly above background levels beginning in about 1970 and increased
steadily after that time. Although pumping was curtailed in the 1980's, degradation of the
aquifer by saline water was sufficiently extensive that the well field was later abandoned
and replaced by wells farther inland.
Figure 3 A composite graph of chloride concentration in water samples from wells
screened at about the same depth in the Union Beach well field, New Jersey (Schaefer
and Walker, 1981).
Because of the continued potential threat of degradation of the freshwater parts of the
aquifers, groundwater withdrawals are now carefully monitored and regulated by the
New Jersey Department of Environmental Protection (NJDEP). In addition, the NJDEP
and USGS have developed a cooperative program to monitor changes in water levels and
chloride concentrations at five-year intervals in each of the confined aquifers. As part of
this monitoring program, water-level hydrographs are prepared from continuous
measurements collected in 99 long-term observation wells to assess seasonal trends in
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groundwater recharge and storage. Water-level measurements are made in approximately
1,000 additional observation wells and used to construct potentiometric maps showing
any significant changes in the size of the cones of depressions developed in the aquifers.
Water samples are collected from selected observation wells for analysis of chloride and
dissolved-solids concentrations, and these data are compiled to monitor changes in the
relation between hydraulic heads, groundwater-flow directions, and groundwater quality.
Using this combined water-level and water-quality monitoring program, the NJDEP can
evaluate the effects of water-management decisions on the aquifers and carefully monitor
the improvement or further degradation of water quality in the aquifers.
Drought Monitoring in Pennsylvania
More than 40 million people in the United States supply their own drinking water from
domestic wells. Many of these wells are shallow and vulnerable to extended droughts.
Yet, relatively few observation wells are measured regularly to provide an indication of
the response of groundwater to climatic conditions. Wells for such purposes are needed in
relatively undeveloped recharge areas where water-level fluctuations primarily reflect
climatic variation rather than groundwater withdrawals or human-induced recharge. The
timeliness of water-level data also is critical to understanding the effects of climate.
Most wells are measured monthly or less frequently. Even if wells are equipped with a
digital water-level recorder, the data must be retrieved and processed before they are
available. As a result, available water-level data commonly lag behind current conditions
by several months or more, limiting their use to portray current conditions.
In response to concerns about groundwater-level declines caused by a severe drought in
1930, a statewide well network was established in Pennsylvania in 1931 to monitor
water-level fluctuations. Today, this network consists of about 70 wells operated by the
USGS in cooperation with the Pennsylvania Department of Environmental Protection.
The primary purpose of the observation-well network is to monitor groundwater
conditions for indications of drought. The State uses data from the wells when
categorizing counties for a drought declaration. Water levels for the network wells are
transmitted by satellite telemetry and displayed on the USGS Web pages for
Pennsylvania, providing direct access to the information by the public (see
http://pa.water.usgs.gov/monitor/gw/index.html ). Such continuous collection,
processing, and transmittal of water-level data for display of “real-time” groundwater
conditions on the Internet is increasing in many parts of the United States (Cunningham,
2001). The data can be transmitted by land-line telephone, cellular telephone, land-based
radio frequency (RF) technology, satellite telemetry, or a combination of these
technologies. Advantages of this approach include not only improved timeliness of the
data, but also improved quality from continual review of the data and increased interest in
groundwater conditions by the public. Drought monitoring in Pennsylvania provides an
example of the use of real-time monitoring that builds on a long-term data collection
network to place current water levels in a long-term climatic context. This baseline
understanding of climatic effects and frequent measurement can enable timely drought
warnings and declarations and facilitate the adoption of mitigation techniques. “Real
time” conveyance of these data allows the public to take appropriate measures. An
additional benefit of collecting and analyzing these data is that by knowing the baseline
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changes caused by climate variation, scientists can better distinguish and understand
levels of groundwater withdrawal and recharge from humans and determine their total
effect on groundwater levels.
Design of monitoring programs
Key choices in the design of a groundwater-monitoring program include site selection,
documentation of sites, selection of measurement types, the frequency and timeframe of
measurements, quality assurance, and data reporting. These and other technical
considerations are briefly reviewed below and discussed in more detail by Alley (1993)
with respect to regional groundwater quality and by Taylor and Alley (2001) with respect
to water-level monitoring.
Site Selection: Decisions about the number and locations of monitoring sites are crucial
to any groundwater data collection program. Site selection depends first and foremost on
the purpose of the monitoring program. Ideally, the sites chosen will provide data
representative of various topographic, geologic, climatic, and land-use environments.
Decisions about the areal distribution and depth of completion of monitoring wells also
should consider the physical boundaries and geologic complexity of aquifers under study.
Monitoring programs for complex, multilayer aquifer systems may require measurements
in wells completed at multiple depths in different geologic units.
Documentation of Sites: Documentation of each monitored site is an essential part of any
groundwater study. Unfortunately, it is all too easily neglected. In establishing criteria
for the suitability of existing wells for inclusion in a monitoring program, one should
consider, among other factors, the well construction, the condition of the well, the
existing pumping equipment, aspects of land use, degree of disturbance upstream from
site, sources of potential contamination, and accessibility for sampling and water-level
measurement.
Types of Measurements: The selection of water-quality constituents and methods for
measurement of water quality and water levels are obvious important choices in
establishing a monitoring program. Commonly overlooked is the need to collect other
types of hydrologic information. For example, meteorological data, such as precipitation
data, aid in the interpretation of water-level, and possibly, water-quality data. In addition,
data on pumping rates can greatly enhance the interpretation of trends observed in water
levels and explain changes in the storage of groundwater over time.
Frequency of Measurements--The frequency of measurements is among the most
important components of a groundwater-monitoring program. Groundwater systems are
dynamic and adjust continually to changes in climate, groundwater withdrawals, and
land-use activities. Although often influenced by economic considerations, the frequency
of measurements should be determined to the extent possible with regard to the
anticipated data variability and the amount of detail needed to fully characterize the
hydrologic behavior of the aquifer.
Timeframe of Measurements—Initial data collected for an aquifer provide critical
baseline information. Monitoring data collected over one or more decades are required to
compile a hydrologic record that encompasses the potential range of aquifer conditions
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and to track trends with time. Systematic, long-term data collection offers the greatest
likelihood that variability caused by variations in climatic conditions and trends caused
by changes in land-use or water-management practices will be observed or detected.
Therefore, monitoring sites typically should be selected with an emphasis on those sites
for which measurements can be made for some time into the future.
Quality Assurance--Good quality-assurance practices help to maintain the accuracy and
precision of measurements, ensure that monitoring wells reflect conditions in the aquifer
being monitored, and provide data that can be relied upon for many intended uses.
Therefore, field and office practices that will provide the needed levels of quality
assurance should be carefully thought out and consistently employed. Good quality-
assurance practices include proper use and cleaning of field instruments, and use of
blanks, replicates, and other means to ensure water-quality samples are representing the
aquifer conditions.
Data Reporting--Data reporting techniques vary greatly depending on the intended use of
the data, but too often measurements are simply tabulated and recorded in a paper file.
The accessibility of monitoring data is greatly enhanced by the use of electronic
databases, especially those that incorporate Geographic Information System (GIS)
technology to visually depict the locations of monitoring sites relative to pertinent
geographic, geologic, or hydrologic features. The availability of electronic information
transfer on the Internet greatly enhances the capability for rapid retrieval and transmittal
of monitoring data to potential users.
Management Decisions – If the purpose of the monitoring system is, in part, to inform
key management decisions, consideration of what management decisions and which
locations would influence and/or be influenced by those decisions, could be an important
element of an effective monitoring system. Taking likely management decisions into
consideration when developing monitoring programs could result in data that is much
likelier to inform decisions.
Concluding Remarks
Systematic, long-term monitoring data are crucial to the resolution of many complex
water-resources issues. A comprehensive monitoring program should include monitoring
of: 1) aquifers substantially affected by groundwater pumping, 2) areas of future
groundwater development, and 3) surficial aquifers that serve as major areas of
groundwater recharge. To ensure that adequate data are being collected for present and
anticipated future uses, monitoring programs need to be evaluated periodically. In the
course of these evaluations, several questions might be asked. Are data being collected
from areas that represent the full range in variation in topographic, hydrogeologic,
climatic, and land-use environments? Who are the principal users of the data, and are the
needs of these users being met? Are plans to ensure long-term viability of data-collection
programs being made? How the data are stored, accessed, and made available to
scientists, decision-makers, and the public?
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Many of the applications of monitoring data involve the use of computer models. It is
often not until development of these models that the limitations of existing data are fully
recognized. Furthermore, enhanced understanding of the groundwater-flow system and
data limitations identified by calibrating groundwater models provide insights into the
most critical needs for collection of future data. These aspects suggest an ongoing,
iterative process of data collection, application of models or other interpretive techniques,
and fine-tuning of monitoring programs over time.
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References
Alley, W.M. 1993. Regional Groundwater Quality: van Nostrand Reinhold, New York.
Cunningham, W.L. 2001. Real-time groundwater data for the Nation: U.S. Geological
Survey Fact Sheet 090-01.
Florida Springs Task Force. 2000. Florida’s springs: Strategies for protection and
restoration: Report prepared for the Florida Department of Environmental protection.
Katz, B.G., Böhlke, J.K., and Hornsby, H.D. 2001. Timescales for nitrate contamination
of spring waters, northern Florida, USA: Chemical Geology, v. 179, p. 167-186.
McGuire, V.L., and others. 2003. Water in storage and approaches to groundwater
management, High Plains aquifer, 2000: U.S. Geological Survey Circular 1243, Also
available on the World Wide Web at http://pubs.water.usgs.gov/circ1243
Schaefer, F.L., and Walker, R.L. 1981. Saltwater intrusion into the Old Bridge aquifer in
the Keyport-Union Beach area of Monmouth County, New Jersey: U.S. Geological
Survey Water-Supply Paper 2184.
Taylor, C.J., and Alley, W.M. 2001. Groundwater-level monitoring and the importance
of long-term water-level data: U.S. Geological Survey Circular 1217, Also available
on the World Wide Web at http://pubs.water.usgs.gov/circ1217
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Representative terms from entire chapter:
monitoring data
