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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. 76

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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. 77

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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). 78

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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). 79

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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 80

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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 81

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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 82

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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? 83

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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. 84

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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 85