× Frequently Asked Questions

Study scope

1. What is the purpose of this study?

The purpose of this study is to determine if the quality of the Nation's groundwater has become better, worse, or stayed the same during 1988-2016. Evaluating changes in groundwater quality at the decadal scale is one component of a larger effort to understand the quality of the Nation's water resources and how water quality conditions are changing with time.

2. Who completed this study?

The U.S. Geological Survey's (USGS) National Water-Quality Assessment (NAWQA) Project completed this study. Sampling teams in USGS Water Science Centers across the country collected the samples and project staff analyzed the data.

3. Why evaluate changes in water quality?

Groundwater is the source of drinking water for about one-half of the population of the United States and has many other important uses, such as for irrigation. It is important to document how the quality of this vital resource is changing in order to understand how management practices and ongoing environmental stresses may be affecting groundwater quality. Degradation of this resource can happen during a long time period, and recovery from historic contamination can also take a long time.

4. Does this map represent all changes in groundwater resources across the country?

The samples used to create this map come from 27 principal aquifers that account for more than 90 percent of the groundwater used for public water supply in the United States; however, the wells that were sampled are from selected geographic parts of each aquifer and, in some cases, also represent a specific land-use type. In addition, most of the samples are from monitoring wells and domestic-supply wells, which represent the shallower parts of the aquifer. As such, these results apply to the depth zone and setting where the samples were collected but also provide insight on possible future changes in water quality in the deeper zones of these aquifers that is used for public supply.

5. What percentage of the population of the United States relies upon groundwater for their drinking water?

Groundwater is the source of drinking-water supply for 140 million people - nearly one-half of the Nation's population (Maupin and others, 2014)

6. How many wells were sampled for this study, and what time period is represented?

The dataset consists of of 1,881 wells in 78 groups of wells called networks in various principal aquifers across the country from the U.S. Geological Survey National Water-Quality Assessment project. A network typically is a group of 20-30 wells representing an aquifer (major aquifer study), or a specific depth and (or) land use (land-use study) (Lapham and others, 1995) . Each network has been sampled at least two times during the periods of 1988-2001, 2002-12, and 2012-16.

7. What does the term "Cycle" mean as used in the "Trend Period" selector?

The National Water-Quality Assessment project evaluates changes groundwater quality at the network level on approximately a 10-year time scale. The first time period, or cycle, was from 1988 to 2001 (including pilot studies). The second time period was between 2001 and 2012. And the third time period is from 2012 to 2016. Additional data will be added to the third cycle as data are approved and statistical analysis completed.

8. How did you determine which constituents to display on the map?

9. Where can I find help navigating this web page?

An explanation of the arrows and icons on the map and what they mean is here . A description of how to activate map layers is here . A description of the search, zoom, and print capabilities is here

Features of the Mapper

10. Are the data available for download?

The data used to make these maps are available for download here: https://www.sciencebase.gov/catalog/item/5a0cac1ae4b09af898cd4307

11. What are groundwater networks and what do they represent?

Networks are groups of wells with similar characteristics. Some are designed to give a broad overview of groundwater quality in an aquifer used as a source of drinking-water supply and others are designed to examine the factors that affect the quality of shallow groundwater underlying key types of land use. Networks were chosen for decadal-scale water-quality sampling based on geographic distribution across the Nation and to represent the most important aquifers and specific land-use types. A network typically is a group of 20-30 wells representing an aquifer (major aquifer study), or a specific depth and (or) land use (land-use study) (Lapham and others, 1995) . The same wells from each of the selected networks are sampled on a decadal-scale interval (Rosen and Lapham, 2008)

Study methods

12. Why are you sampling only once every decade?

Sampling one time per decade allows the project to collect repeated samples in all of the well networks across the Nation. If we sampled more frequently, we would have to sample fewer wells. Because changes in groundwater quality typically happen relatively slowly, sampling once per decade can be sufficient to capture these slow changes. Other ongoing USGS studies evaluate changes in groundwater quality at time intervals greater than and less than a decadal scale.

13. How do you determine whether or not changes in concentrations with time are statistically significant?

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. Significant changes for each constituent are determined by a statistical test called the Wilcoxon-Pratt signed rank test (Pratt, 1959) as described in Lindsey and Rupert (2012) using the R-statistical software. The method 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. 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 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. More on statistics.

14. How do you calculate statistics when reporting levels changed with time?

More on data preparation. Because reporting levels vary with time, a maximum common assessment level (CAL) is chosen for each constituent prior to the statistical analysis. Data for the pesticide compounds atrazine, prometon, metolachlor, simazine, dieldrin, and deethylatrazine are prepared using the method described in Toccalino and others (2014a)

Study results

15. Which benchmarks were used to provide context for the results?

Two human-health benchmarks and one other water-quality benchmark were used in this study. The human-health benchmarks were Maximum Contaminant Levels (MCLs) developed by EPA,s Office of Water for compounds that are regulated in drinking water under the Safe Drinking Water Act and nonenforceable Health-Based Screening Levels (HBSLs ) developed by the USGS for unregulated compounds without MCLs. These benchmarks are concentrations below which contaminants are not anticipated to cause adverse human-health effects from a lifetime of exposure. Secondary Maximum Contaminant Levels (SMCLs ) are nonenforceable guidelines regarding cosmetic effects such as tooth or skin discoloration or aesthetic effects such as taste, odor, or color of drinking water.

16. Do the findings indicate if water from the wells is safe to drink?

No. The NAWQA Project did not assess the safety of drinking water. The quality of finished drinking water is regulated by the EPA under the Safe Drinking Water Act. All of the samples included in this study, however, were collected prior to any treatment or blending that potentially could alter contaminant concentrations. As a result, the sampled groundwater represents the quality of the source water and not necessarily the quality of finished water ingested by the people served by these domestic and public wells. In addition, the sampling included some monitoring wells and other types of wells, which are not used as a source of drinking water.

17. What does it mean when large decadal increases in concentrations were identified on these maps with upwardly facing red arrows, and why do the concentration ranges for large and small arrows vary by constituent?

When there was a statistically significant change in the concentration of a constituent with time, the magnitude of the change was classified as being "large" or "small" as compared to a benchmark to provide context for the results. "large" changes in concentrations over time mean that the magnitude of the change was more than 1 percent or 5 percent of the benchmark concentration, depending on the type of constituent, meaning that concentrations in the overall group of wells are approaching a benchmark more quickly than areas having "small" changes. It does not mean that concentrations exceed a benchmark. Each assessment is for a group of 10-30 individual wells in a similar setting. Individual wells within a network may or may not have concentrations that exceed a benchmark. Within each group some concentrations are likely to be decreasing and others increasing, regardless of the direction of change for the overall group. Files that include concentrations for individual wells are available for download.

18. Where can I learn more about contaminants in public-supply wells and domestic wells?

19. What are the causes of these changes in groundwater quality?

The exact causes of changes in groundwater quality are not evaluated for every constituent and every network. Some nationwide changes, such as the banning of a chemical or introduction of a new chemical, can be documented on a large scale. Changes at the level of an individual network, however, require a focused evaluation of hydrologic conditions, groundwater age, and history of use of the contaminant. Some of these studies are here

20. What human factors are contributing to changes in groundwater quality?

21. What natural features influence water quality?

22. Will other constituents be added to this tool?

The 26 constituents displayed were selected as the most important for statistical analysis as described in the 'Criteria for analyzing constituents' table. Constituents that meet those criteria in future sampling events will be added to the map. Because of changes in data collection and analytical methods, some constituents that meet those criteria cannot be analyzed for statistical change across all time periods.

23. The data show results through 2016. Are data still being collected and will those results be displayed?

A third decade of data collection is underway, and these results will be compared to results from the previous two decades. Data collected after 2016 will be analyzed and the results will be added to this map on an annual basis after quality checks on the data.

24. Are some areas likely to see faster changes in groundwater quality than others? Why?

Wells in areas where groundwater arrives at a well shortly after entering the aquifer will likely see concentrations change more rapidly than wells in areas where groundwater travels more slowly. Factors such as well depth and the type of aquifer material control these rates. See more information on factors affecting vulnerability to contamination

25. What can we do to improve groundwater quality? How long will it take?

The EPA has resources on source water protection. Source water protection strategies that rely on changes in human activities and practices at the land surface to achieve water-quality objectives can take many decades to affect the quality of water in some deeper wells.

For more information on groundwater quality

26. Where is more information available about water-quality testing guidelines for domestic wells?





U.S. Environmental Protection Agency, Private Drinking Water Wells



https://www.epa.gov/privatewells.



U.S. Center for Disease Control and Prevention, Division of Parasitic Diseases, Drinking Water, Private Well Resources

https://www.cdc.gov/ncidod/dpd/healthywater/privatewell.htm



U.S. Department of Agriculture and U.S. Environmental Protection Agency, National Farm*A*Syst/Home*A*Syst Program

https://extension.psu.edu/programs/nutrient-management/farm-a-syst



American Ground Water Trust

https://www.agwt.org



Ground Water Protection Council

https://www.gwpc.org



National Ground Water Association





National Rural Water Association

https://www.nrwa.org



Water Systems Council

https://www.watersystemscouncil.org There are many sources of information about water-quality testing of domestic wells. Many state environmental or public-health agencies provide information and recommendations for homeowners about testing and water-quality of domestic wells. The EPA also provides such information, and provides links to many state agency Web sites on private wells. The U.S. Centers for Disease Control and Prevention provides information on water-quality testing and the health effects of selected contaminants in private wells. The U.S. Department of Agriculture, in cooperation with EPA, provides information and resources for domestic-well owners through its Farm*A*Syst/Home*A*Syst and Cooperative State Research, Education, and Extension (CREES) Program. Local health departments, in many cases, are a source of information about private wells. Recommendations for water-quality testing and other information about domestic wells also are provided by several nongovernmental organizations. Sources of information available on the internet from some of these agencies and organizations are listed below:U.S. Environmental Protection Agency, Private Drinking Water Wells https://www.epa.gov/safewater/privatewells/index2.html andU.S. Center for Disease Control and Prevention, Division of Parasitic Diseases, Drinking Water, Private Well ResourcesU.S. Department of Agriculture and U.S. Environmental Protection Agency, National Farm*A*Syst/Home*A*Syst ProgramAmerican Ground Water TrustGround Water Protection CouncilNational Ground Water Association http://www.ngwa.org/Pages/default.aspx and https://www.wellowner.org National Rural Water AssociationWater Systems Council

27. Where can I learn more information about other NAWQA water-quality assessments?

Access https://water.usgs.gov/nawqa for information about the USGS NAWQA Project and other assessments in the Project about the water resources of the Nation.

Citation and Contacts

28. How should these Web pages be cited?





Lindsey, B.D., Levitt, J.P., and Johnson, T.D., 2019, Data from Decadal Change in Groundwater Quality Web Site, 1988-2016, U.S. Geological Survey data release, Lindsey, B.D., Johnson, T.D., Privette, L.M., and Estes, N.J., 2018, Decadal changes in groundwater quality: U.S. Geological Survey Web page, https://nawqatrends.wim.usgs.gov/Decadal/ Lindsey, B.D., Levitt, J.P., and Johnson, T.D., 2019, Data from Decadal Change in Groundwater Quality Web Site, 1988-2016, U.S. Geological Survey data release, https://doi.org/10.5066/P9VAULHG