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Decadal Change in Groundwater Quality

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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.

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Evaluating Decadal Changes in Groundwater Quality

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 Program. 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 2022 (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 seventh 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 76 networks, changes in water quality between the 1st and 3rd sampling events for 63 networks, and changes across all 3 sampling events for 60 networks. 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.

Decadal changes in groundwater quality were assessed for these constituents:

Inorganics
Arsenic
Boron
Chloride
Fluoride
Iron
Lithium
Manganese
Molybdenum
Nitrate
Orthophosphate
Radium
Sodium
Strontium
Sulfate
Total Dissolved Solids
Uranium

Organics
Atrazine
Chloroform
Deethylatrazine
Dieldrin
Methyl tert-butyl ether
Metolachlor
Prometon
Simazine
Tetrachlorothene
Toluene
Trichloroethene
1,2-Dibromo-3-chloropropane (DBCP)

For more information about constituents and why they were chosen click here.

Online Mapper Showing Decadal Changes in Groundwater Quality

Decadal changes in concentrations of nutrients, metals, and pesticides and other organic contaminants in groundwater were evaluated in a total of 89 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 probability of the test is ≦ 0.10 and 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.

Decadal Groundwater Quality Networks

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 2022 and at least one previous sampling event are shown in orange. Networks sampled in the first and second sampling events but are no longer being sampled are shown in gray.

Related Resources

This software has been approved for release by the U.S. Geological Survey (USGS). Although the software has been subjected to rigorous review, the USGS reserves the right to update the software as needed pursuant to further analysis and review. No warranty, expressed or implied, is made by the USGS or the U.S. Government as to the functionality of the software and related material nor shall the fact of release constitute any such warranty. Furthermore, the software is released on condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from its authorized or unauthorized use.

Version 4.0.1

Updates to Decadal Changes in Groundwater Quality

This web mapping application is based on a data release that updates Decadal changes in groundwater quality: U.S. Geological Survey Web page, Lindsey and others, 2018. The initial data release documented decadal changes in groundwater quality for 67 networks that had been sampled by the National Water-Quality Program. The results were based on analysis of 1,520 wells in the 67 networks that had been sampled during 1988-2001 and again between 2002-2012. The U.S. Geological Survey's National Water-Quality Assessment Project ended in 2021, but monitoring is continuing as a part of the National Water Quality Program. The table below shows the numbers of wells and networks that can be compared across any 2 or 3 time periods:

	Sample pairs in Decade 1 and 2	Sample pairs in Decade 1 and 3	Sample pairs in Decade 2 and 3	Sample sets in Decade 1, 2, and 3	Number of wells or networks with at least 2 decadal sampling pairs
Numbers of wells sampled	1,520	1,202	1,659	1,617	2,277
Numbers of Networks sampled	67	63	76	60	90

New data added from the time period 2022 include: 34 wells sampled in 2 networks that could be compared to the same wells and networks sampled during 1988-2001; 65 wells sampled in 3 networks that could be compared to the same wells and networks sampled during 2002-2012; and 52 wells in 2 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 2012-2021, 2002-2012 to 2012-2022, and also results for data collected across all three time periods. The updated map has results for 2,277 wells in 90 networks. The list of constituents analyzed was updated in 2020 to include radium, which is now included on the map for the 2nd to 3rd time interval. Previous time intervals lack sufficient data to evaluate changes in radium concentrations. 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.

Change from versions prior to 2022:

The network sanasus1 result for orthophosphate was previously incorrectly reported as a small decrease from the first to the second decade. This should have been reported as a small increase and is correctly reported in this update.

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), and Decade 3 (2013-2021) of the National Water-Quality Assessment (NAWQA) Project, which transitioned into the National Water Quality Program in 2022. Mapped constituents met one of the four following criteria:

Constituents that exceeded a Maximum Contaminant Level or other human-health benchmark in more than 1 percent of public- or domestic-supply wells¹ ² ³, or
Constituents that exceeded a Secondary Maximum Contaminant Level or taste threshold in more than 1 percent of public- or domestic-supply wells¹ ² ³, or
The five most frequently detected pesticides and volatile organic comounds (VOCs) in groundwater⁴ ⁵, or
Constituents of special or regional interest.

Table 1. Constituents meeting analysis criteria, results mapped

[μg/L, micrograms per liter; mg/L, milligrams per liter; SMCL, Secondary Maximum Contaminant Level]

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
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
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

Table 2. Constituents met criteria, not mapped due to insufficient data

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
Radon	inorganic	1300 (Alternate 4,000)	picocuries per liter	Exceeded human-health benchmark in more than 1 percent of domestic or public-supply wells

References Cited:

DeSimone, L.A., Hamilton, P.A., and Gilliom, R.J., 2009, Quality of water from domestic wells in principal aquifers of the United States, 1991-2004-Overview of major findings: U.S. Geological Survey Circular 1332, 48 p. [Also available at http://pubs.usgs.gov/circ/circ1332/.]
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 http://pubs.usgs.gov/circ/1346/.]
Ayotte, J.D., Gronberg, J.M., and Apodaca, L.E., 2011, Trace elements and radon in groundwater across the United States: U.S. Geological Survey Scientific Investigations Report 2011-5059, 115 p. [Also available at http://water.usgs.gov/nawqa/trace/pubs/sir2011-5059/.]
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 http://pubs.usgs.gov/circ/circ1292/.]
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 http://pubs.usgs.gov/circ/2005/1291/.]

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.
Network code	The network name acronym used by the USGS as described in the supporting data release.
Sample dates (1^st, 2^nd, 3^rd)	The year of the first, second, and third sampling event.

Decadal Change in Groundwater Quality

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