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A first of its kind, national assessment of an unseen, valuable resource
About 140 million people—almost one-half of the Nation's population—rely on groundwater for public supply, and demand for groundwater for public supply, irrigation and agriculture continues to increase.
This mapper shows how concentrations of pesticides, nutrients, metals, and organic contaminants in groundwater are changing during decadal periods across the Nation.
Tracking changes in groundwater quality and investigating the reasons for these changes is crucial for informing management decisions to protect and sustain our valuable groundwater resources.
IMAGE: An image of a USGS scientist collecting data.
Groundwater-quality data were collected from 5,000 wells between 1988-2001 (first decadal sampling event) by the U.S. Geological Survey's National Water-Quality Assessment Project. Samples are collected in groups of 20-30 wells with similar characteristics called networks. About 1,500 of these wells in 67 networks were sampled again approximately 10 years later between 2002-2012 (second sampling event) to evaluate decadal changes in groundwater quality. Between 2012 and 2018 (third sampling event), a subset of these networks was sampled again, allowing additional results to be displayed on the web page: Decadal changes in groundwater quality. This is the fourth iteration of data added to the website. With the additional data, it is possible to evaluate changes in water quality between the 2nd and 3rd sampling events for 21 additional networks (56 total), changes in water quality between the 1st and 3rd sampling events for 18 additional networks (45 total), and changes across all 3 sampling events for 18 additional networks (45 total). A total of 83 networks have been sampled at least twice. Samples were obtained from monitoring wells, domestic-supply wells, and some public-supply wells before any treatment on the system.
Groundwater samples used to evaluate decadal change were collected from networks of wells with similar characteristics. Some networks, consisting of domestic or public supply wells, were used to assess changes in the quality of groundwater used for drinking water supply. Other networks, consisting of monitoring wells, assessed changes in the quality of shallow groundwater underlying key land-use types such as agricultural or urban lands. Networks were chosen based on geographic distribution across the Nation and to represent the most important water-supply aquifers and specific land-use types.
For more information about constituents and why they were chosen click here.
Decadal changes in concentrations of nutrients, metals, and pesticides and other organic contaminants in groundwater were evaluated in a total of 83 networks across the Nation by comparing changes between selected sampling events.
Decadal changes in median concentrations for a network are classified as large, small, or no change in comparison to a benchmark concentration. For example, a large change in chloride concentrations indicates that the median of all differences in concentrations in a network is greater than 5 percent of the chloride benchmark per decade. For chloride, which has a Secondary Maximum Contaminant level of 250 milligrams per liter, this would mean the change in concentration exceeded 12.5 milligrams per liter (mg/L), or 5 percent of the benchmark.
230 networks were sampled from 1988 to 2001 to assess the status of the Nation's groundwater quality. Each dot on the above map represents the center point of a network of about 20 to 30 wells. Networks sampled in the first sampling event only are shown in green. There were 67 networks resampled from 2002 to 2012 to assess decadal changes in groundwater quality. Networks sampled from 2012 to 2018 and at least one previous sampling event are shown in orange and trend networks that have not yet been resampled in the third decadal sampling event are shown in blue. Networks sampled in the first and second sampling events but are no longer being sampled are shown in gray.
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U.S. Environmental Protection Agency (USEPA) homepage
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USEPA Pesticide Information
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Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
|Sampled in Decade 1 and 2||Sampled in Decade 1 and 3||Sampled in Decade 2 and 3||Sampled in Decade 1, 2, and 3||Number of wells or networks with at least 2 decadal sampling events|
|Numbers of wells sampled||1,509||891||1,239||1,213||2,057|
|Numbers of Networks sampled||67||45||56||45||83|
New data added from the time period 2017-18 include: 362 wells sampled in 18 networks that could be compared to the same wells and networks sampled during 1988-2001; 473 wells sampled in 21 networks that could be compared to the same wells and networks sampled during 2002-2012; and 492 wells in 18 networks with sufficient data to evaluate changes across all three events. The results across multiple time periods allow the user to select the time period of interest for display. The comparison of 1988-2001 to 2002-2012 displays the same results as all previous versions, with exception of the errata listed below. The new options allow the user to select comparisons for new time periods: 1988-2001 to 2012-2018, 2002-2012 to 2012-2016, and also results for data collected across all three time periods. The updated map has results for 2,057 wells in 83 networks. Health-based screening levels were updated in 2018 to include lithium, which is now included on the map for the 2nd to 3rd time interval. Previous time intervals lack sufficient data to evaluate changes in lithium concentrations. Changes in lithium concentrations were evaluated for 1,157 wells in 53 networks. The criteria for mapping, benchmarks, data preparation, and statistical analysis are described in the Technical Information tab. In order to make the evaluations of the magnitude of change comparable, the magnitude of change is displayed based the change in concentration per decade. For comparisons between the first to third sampling period (approximately two decades), the median change is divided by two. For comparisons across three or more time periods, the statistical test result of change in concentration per year is multiplied by ten.
Errata from V1, corrected in V2: The following corrections have been made in version 2 of the web site, and thus appear as a change from version 1 for comparison of changes between Decade 1 and Decade 2:
Errata from V2: The following corrections have been made in the previous data release (version 3), and thus appear as a change from version 2:
|Constituent||Statistically significant finding of increase, decrease, or no change in concentration for this network. Details of statistical analysis are available in the documentation. Small and large changes are defined in the explanation.|
|Network type||Major aquifer studies target drinking water wells in a selected aquifer. Land use studies target wells that underlie areas of urban or agricultural land use in a selected aquifer, and are typically shallower than wells sampled in major aquifer studies.|
|Types of wells||If no qualifier is listed, network is entirely of one well type. 'Predominantly' indicates that 80 percent of the wells in the network are of that type. Mixed networks list those well types making up at least 75 percent of the wells in the network. Possible well types include commercial, domestic, industrial, irrigation, monitoring, public-supply, stock, recreational, and other.|
|Typical depth range||Range of well depths listed are first and third quartile for the network.|
|Principal aquifer||Aquifer names are from the map of the principal aquifers of the United States (U.S. Geological Survey, 2003, http://water.usgs.gov/ogw/aquifer/map.html|
|Regional aquifer||The local or regional name for the aquifer sampled.|
|Aquifer material||Aquifer materials are from the map of the principal aquifers of the United States (U.S. Geological Survey, 2003, http://water.usgs.gov/ogw/aquifer/map.html|
|Additional information||Lists the specific land use activity, if available.|
|NAWQA network code||The network name acronym used by the USGS NAWQA project.|
|Sample dates (1st, 2nd, 3rd)||The year of the first, second, and third sampling event.|
SearchingThe search option allows the user to search for a location such as a city, state, zip code, or general place name.
Navigating the mapThe scroll wheel on the mouse can be used to zoom, and in the upper left area of the tool are "+" and "-" icons to zoom in and out, respectively.
Home LocationThe crosshair button will zoom the map to your location, and the home button will center the US on the map.
Print MapThe print map tab creates a .pdf file of the current map extent, including an explanation.
Explanation displayThe Explanation box can be collapsed to display more of the mapped area.
Using the ExplanationThe expanded Explanation box explains the arrows and circles that appear at the network centroid, displaying the statistical results.
The size of the arrow indicates the magnitude of change for networks with statistically significant changes.
Networks without any statistically significant change are displayed as a solid circle.
Networks that have been sampled, but have insufficient data for the selected trend period- constituent combination are not displayed unless the "all networks" box in the map layers menu is selected. When that option is selected, networks with no data or insufficient data for the trend period-constituent combination are shown with a gray circle; this could be because fewer than 10 pairs were available or because a constituent was sampled in one sampling period but not in the other.
Changing the basemap styleThe "Basemaps" sidebar allows a user to show various geographic information or landscape imagery as a background layer beneath the trend results.
Changing or adding map layersThe "Map Layers" sidebar allows a user to toggle laysers that sit on top of the basemap.
Inorganic vs OrganicSelect either "Inorganic" or "Organic" based on the Constituent you would like to view.
Selecting a constituentThe Constituent group selection allows a user to choose a constituent group and then choose a specific constituent from within that group. See About -> Learn More for a list of constituents.
Toggling Decades of the Trend PeriodThe Trend Period selection allows a user to choose a Constituent and then view the decades within each Constituent.
Mapping Criteria and Benchmarks The NAWQA Project has developed an interactive mapping tool that displays decadal changes in concentrations. From among the more than 300 constituents sampled during the two decades, up to 27 constituents were prioritized for analysis for decadal change based on the following criteria:
Data Preparation Steps In preparation for statistical analysis, environmental water-quality data for selected sites are retrieved from the USGS National Water Information System database. A common assessment reporting level (CAL) as described by Gilliom and others (2006) is chosen for the data analysis, usually the lowest reporting level that still retains the maximum amount of data for analysis. Analysis is completed for networks with at least 10 pairs of samples. Once a CAL is selected for a given constituent, all nondetections with a reporting level greater than the CAL are deleted from the dataset. All nondetections and reported values less than the CAL are recoded to a unique value selected to specifically represent values below the CAL. The value used for recoding is typically slightly less than the CAL, but its exact value does not affect the statistical analysis, which calculates results using the ranking of values relative to each other rather than using the actual values themselves. The selection of a CAL and the recoding are done to make correct comparisons among nondetections and between nondetections and low-level detections; for example, if a CAL of 0.2 is selected, reported results of < 0.1 and < 0.2 are recoded as 0.19 before statistical analysis so that the statistical program does not interpret these two nondetections as different values and also to distinguish both from a reported value of 0.2. Reported results of 0.17 (a detection) and < 0.2 (a nondetection) are also recoded as 0.19 before statistical analysis because it is not possible to determine if the two values differ. The CAL and the value used for recoding nondetections are reported for each constituent in the "readme" tab of the data files. Data for the pesticide compounds atrazine, prometon, metolachlor, simazine, dieldrin and deethylatrazine (a degradate of atrazine) are prepared using a different method, as described in Toccalino and others (2014). Differences in data preparation for pesticide compounds and degradates include that concentrations were adjusted for recovery and that nondetections were replaced with a single value less than the lowest detection (rather than a value less than the CAL). For methyl tert-butyl ether, the CAL was determined for each pair rather than for the entire data set. The CAL that was selected for use in Versions 1.0 and 2.0 was also used for analysis of the data used in Version 3.0 so that the results are comparable.
Two statistical tests were used to evaluate changes. The Wilcoxon-Pratt signed-rank test (Pratt, 1959) was used to make comparisons between pairs of sampling events, and the Regional Kendall test was used to make comparisons across three or more sampling events. The use of the Wilcoxon-Pratt signed-rank test is described in Lindsey and Rupert (2012). The method first calculates changes in concentrations at individual wells and then uses the pattern of those changes to determine whether or not there has been a statistically significant change for a well network as a whole. Because the R-statistical program cannot analyze networks if all the data are tied (no differences in any pair), networks with all ties are assumed to have no significant change. The Regional Kendall test (Helsel and others, 2006) evaluates trends for individual sites across three or more sampling events, then evaluates whether a consistent trend is evident at the network level. The test evaluates whether the changes are statistically significant, and also determines the slope, or the rate of change in concentration. For comparisons across three or more time periods that were evaluated using the Regional Kendall test, the annual change in concentration - which is calculated by the test - is multiplied by 10 to determine the decadal change in concentration. For both of these statistical tests, a 90-percent confidence level, or a p-value of less than 0.10, is used to signify a statistically significant change.
Results of the statistical analysis for each well network were classified as indicating a statistically significant increase, a statistically significant decrease, or no significant change. Results were further classified as being large or small changes to provide context for the results. A statistically significant change for an individual network is displayed on the mapping tool by an arrow pointing up or down. Each symbol on the map represents a network of multiple wells. To provide context, the median change between the first and second sampling events was calculated for each well network with a statistically significant change, and the median was compared to the benchmark (MCL, SMCL, or HBSL).
The changes in concentration across a decade, or near-decadal period were used to evaluate the magnitude of change. For comparisons that used the Wilcoxon-Pratt signed-rank test, the median of differences in concentrations are calculated for networks with statistically significant changes. For comparisons across three or more time periods that were evaluated using the Regional Kendall test, the annual change in concentration - which is calculated by the test - is multiplied by 10 to determine the decadal change in concentration.
For inorganic constituents, if the median of all differences in concentrations was greater than 5 percent of the benchmark, the change was considered large. If the change was less than or equal to 5 percent of the benchmark, then the change was considered to be small. For organic compounds, if the median of all differences in concentrations was greater than 1 percent of the benchmark, the change was considered large, and if the change was less than or equal to 1percent of the benchmark, then the change was considered to be small. This approach provides a way to distinguish very small but statistically significant changes from changes that are of a larger magnitude.
Organic constituents are treated differently than inorganic constituents because the organic constituents are generally introduced to the environment as a result of human activity, whereas most of the inorganic constituents are found naturally at some level. In some cases, networks had statistically significant changes, but the median change between sampling events was zero. In those cases, the data were analyzed graphically to determine if the change was a decrease or an increase, and the magnitude of change was considered to be small.
Networks that have been sampled, but have insufficient data for the selected trend period- constituent combination are not displayed unless the "all networks" box in the map layers menu is selected. When that option is selected, networks with no data or insufficient data for the trend period-constituent combination are show with a gray circle; this could be because fewer than 10 pairs were available or because a constituent was sampled in one sampling period but not in the other.
DeSimone, L.A., 2009, Quality of water from domestic wells in principal aquifers of the United States, 1991-2004: U.S. Geological Survey Scientific Investigations Report 2008-5227, 139 p. Also available at https://pubs.usgs.gov/sir/2008/5227/.]
DeSimone, L.A., McMahon, P.B., and Rosen, M.R., 2014, The quality of our Nation's waters-Water quality in Principal Aquifers of the United States, 1991-2010: U.S. Geological Survey Circular 1360, 151 p., [Also available at https://dx.doi.org/10.3133/cir1360.]
Eberts, S.M., Thomas, M.A., and Jagucki, M.L., 2013, The quality of our Nation's waters-Factors affecting public-supply-well vulnerability to contamination-nderstanding observed water quality and anticipating future water quality: U.S. Geological Survey Circular 1385, 120 p. [Also available online at https://pubs.usgs.gov/circ/1385/.]
Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton, P.A., Martin, J.D., Nakagaki, Naomi, Nowell, L.H., Scott, J.C., Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 2006, The quality of our Nation's waters-Pesticides in the Nation's streams and ground water, 1992-2001: U.S. Geological Survey Circular 1291, 172 p. [Also available at https://pubs.usgs.gov/circ/2005/1291/
Helsel, D.R., Mueller, D.K., and Slack, J.R., 2006, Computer program for the Kendall family of trend tests: U.S. Geological Survey Scientific Investigations Report 2005-5275, 4 p. [Also available at https://pubs.usgs.gov/sir/2005/5275/pdf/sir2005-5275.pdf
Lapham, W.W., Wilde, F.D., and Koterba, M.T., 1995, Ground-water data-collection protocols and procedures for the National Water-Quality Assessment Program-Selection, installation, and documentation of wells, and collection of related data: U.S. Geological Survey Open-File Report 95-398, 71 p. [Also available at https://pubs.usgs.gov/of/1995/ofr-95-398/.]
Lindsey, B.D., and Rupert, M.G., 2012, Methods for evaluating temporal groundwater quality data and results of decadal-scale changes in chloride, dissolved solids, and nitrate concentrations in groundwater in the United States, 1988-2010: U.S. Geological Survey Scientific Investigations Report 2012-5049, 46 p. [Also available at https://pubs.usgs.gov/sir/2012/5049/.]
Maupin, M.A., Kenny, J.F., Hutson, S.S., Lovelace, J.K., Barber, N.L., and Linsey, K.S., 2014, Estimated use of water in the United States in 2010: U.S. Geological Survey Circular 1405, 56 p. [Also available at https://pubs.usgs.gov/circ/1405/.]
Pratt, J.W., 1959, Remarks on zeros and ties in the Wilcoxon signed rank procedures: American Statistical Association Journal, v. 54, no. 287, p. 655-667. [Also available at https://www.jstor.org/stable/2282543.]
Rosen, M.R., and Lapham, W.W., 2008, Introduction to the U.S. Geological Survey National Water-Quality Assessment (NAWQA) of ground-water quality trends and comparison to other national programs: the Journal of Environmental Quality, v. 37, no. 5, Supplement, p. S-190-S-198. [Also available at https://dx.doi.org/10.2134/jeq2008.0049.]
Toccalino, P.L., and Hopple, J.A., 2010, The quality of our Nation's waters-Quality of water from public supply wells in the United States, 1993-2007-Overview of major findings: U.S. Geological Survey Circular 1346, 58 p. [Also available at https://pubs.usgs.gov/circ/1346/.]
Toccalino, P.L., Gilliom, R.J., Lindsey, B.D., and Rupert, M.G., 2014, Pesticides in groundwater of the United States-Decadal-scale changes, 1993-2011: Groundwater, v. 52, Supplement S1, p. 112-125. [Also available at https://dx.doi.org/10.1111/gwat.12176.]
Norman, J.E., Toccalino, P.L., Morman, S.A., 2018, Health-Based Screening Levels for evaluating water-quality data (2d ed.). U.S. Geological Survey web page, accessible at https://water.usgs.gov/water-resources/hbsl/, doi:10.5066/F71C1TWP.
U.S. Environmental Protection Agency, 2012, 2012 edition of the drinking water standards and health advisories: U.S. Environmental Protection Agency, Office of Water, EPA 822-S-12-001, 20 p. [Also available at https://nepis.epa.gov/Exe/ZyPDF.cgi/P100N01H.PDF?Dockey=P100N01H.PDF.]
U.S. Environmental Protection Agency, 2013, Human health benchmarks for pesticides-2013 update: U.S. Environmental Protection Agency, Office of Water, EPA-820-F-13-019, 2 p. [Also available at https://www.epa.gov/sites/production/files/2015-10/documents/hh-benchmarks-factsheet.pdf.]
U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. A1-A10, accessed July 31, 2009, at https://water.usgs.gov/owq/FieldManual/.]
Zogorski, J.S., Carter, J.M., Ivahnenko, Tamara, Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., and Toccalino, P.L., 2006, The quality of our Nation's waters-Volatile organic compounds in the Nation's ground water and drinking-water supply wells: U.S. Geological Survey Circular 1292, 101 p. [Also available at https://pubs.usgs.gov/circ/circ1292/
Table 1 lists the chemical constituents that met the criteria for a statistical analysis of decadal-scale changes in concentrations in groundwater between Decade 1 (1988-2001) and Decade 2 (2002-2012) of the National Water-Quality Assessment (NAWQA) Project. Mapped constituents met one of the four following criteria:
|Constituent name||Constituent class||Benchmark||Units||Why study|
|Arsenic||inorganic||10||μg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Boron||inorganic||6,000||μg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Chloride||inorganic||250||mg/L||Exceeded SMCL in more than 1 percent of domestic-supply or public-supply wells|
|Fluoride||inorganic||4||mg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Iron||inorganic||300||μg/L||Exceeded SMCL in more than 1 percent of domestic-supply or public-supply wells|
|Lithium||inorganic||10||μg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Manganese||inorganic||300||μg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Molybdenum||inorganic||40||μg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Nitrate (measured as nitrite plus nitrate)||inorganic||10||mg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Orthophosphate||inorganic||None||mg/L||Constituent of special or regional interest: Possible source of discharge to surface water bodies|
|Sodium||inorganic||30||mg/L||Exceeded taste threshold in more than 1 percent of domestic-supply or public-supply wells|
|Strontium||inorganic||4000||μg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Sulfate||inorganic||250||mg/L||Exceeded SMCL in more than 1 percent of domestic-supply or public-supply wells|
|Total Dissolved Solids||inorganic||500||mg/L||Exceeded SMCL in more than 1 percent of domestic-supply or public-supply wells|
|Uranium||inorganic||30||μg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Atrazine||organic||3||μg/L||One of the five most frequently detected pesticide compounds in groundwater in the Nation|
|Chloroform||organic||80||μg/L||One of the five most frequently detected pesticide compounds in groundwater in the Nation|
|Deethylatrazine||organic||None||μg/L||One of the five most frequently detected volatile organic compounds in groundwater in the Nation|
|1,2-Dibromo-3-chloropropane (DBCP)||organic||0.2||μg/L||Constituent of special or regional interest|
|Dieldrin||organic||0||μg/L||Exceeded human-health benchmark in more than 1 percent of public-supply wells|
|Methyl tert-butyl ether||organic||20||μg/L||One of the five most frequently detected volatile organic compounds in groundwater in the Nation|
|Metolachlor||organic||700||μg/L||One of the five most frequently detected volatile organic compounds in groundwater in the Nation|
|Simazine||organic||4||μg/L||One of the five most frequently detected pesticide compounds in groundwater in the Nation|
|Prometon||organic||400||μg/L||One of the five most frequently detected pesticide compounds in groundwater in the Nation|
|Tetrachloroethene||organic||5||μg/L||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells, and one of the five most frequently detected volatile organic compounds in groundwater in the Nation|
|Toluene||organic||1,000||μg/L||One of the five most frequently detected volatile organic compounds in groundwater in the Nation|
|Trichloroethene||organic||5||μg/L||One of the five most frequently detected volatile organic compounds in groundwater in the Nation|
|Constituent name||Constituent class||Benchmark||Units||Why study|
|Gross alpha (α)||inorganic||15||picocuries per liter||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Radium 226 plus Radium 228||inorganic||5||picocuries per liter||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|
|Radon||inorganic||1300 (Alternate 4,000)||picocuries per liter||Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells|