Estimated Nitrate Concentrations in Groundwater Used for Drinking
About this indicator
Nitrate in groundwater drinking water systems is of concern because private self-supplied drinking water systems, which primarily draw from groundwater, are not federally regulated. It is the owner’s responsibility to test and treat their own well for nitrate and other pollutants. While nitrate does occur naturally in groundwater, concentrations greater than 3 mg/l generally indicate contamination (Madison and Brunett, 1985), and a more recent nationwide study found that concentrations over 1 mg/l nitrate indicate human activity (Dubrovsky et al. 2010). EPA’s maximum contaminant level (MCL) for nitrate set to protect against blue-baby syndrome is 10 mg/l. The data in this indicator show the total area and percent of state area predicted to have nitrate concentrations exceeding EPA’s MCL, or 10 mg/l in groundwater used for drinking. Also presented is the estimated percent of state populations served by self-supplied drinking water, 98% of which is from groundwater wells.
| Estimated area of state with groundwater nitrate concentrations >10 mg/l (mi2) | Estimated % of state with groundwater nitrate concentrations >10 mg/l | Estimated % of population with self-supplied drinking water in 2005 | Estimated % of population with self-supplied drinking water in 2015 | |
|---|---|---|---|---|
| Alabama | 5 | 0 | 11 | 11 |
| Alaska | No data | No data | 35 | 26 |
| Arizona | 216 | 0 | 4 | 3 |
| Arkansas | 0 | 0 | 7 | 5 |
| California | 2,201 | 1 | 7 | 4 |
| Colorado | 1,120 | 1 | 6 | 5 |
| Connecticut | 0 | 0 | 24 | 24 |
| Delaware | 189 | 10 | 10 | 19 |
| Florida | 19 | 0 | 10 | 12 |
| Georgia | 10 | 0 | 18 | 15 |
| Hawaii | No data | No data | 6 | 4 |
| Idaho | 247 | 0 | 30 | 24 |
| Illinois | 23 | 0 | 9 | 9 |
| Indiana | 10 | 0 | 26 | 25 |
| Iowa | 463 | 1 | 18 | 16 |
| Kansas | 8,880 | 11 | 5 | 5 |
| Kentucky | 189 | 1 | 17 | 10 |
| Louisiana | 0 | 0 | 12 | 11 |
| Maine | 0 | 0 | 44 | 50 |
| Maryland | 347 | 4 | 17 | 24 |
| Massachusetts | 0 | 0 | 8 | 9 |
| Michigan | 15 | 0 | 29 | 26 |
| Minnesota | 21 | 0 | 22 | 21 |
| Mississippi | 0 | 0 | 19 | 15 |
| Missouri | 17 | 0 | 15 | 14 |
| Montana | 97 | 0 | 32 | 29 |
| Nebraska | 1,622 | 2 | 18 | 9 |
| Nevada | 0 | 0 | 8 | 7 |
| New Hampshire | 0 | 0 | 42 | 37 |
| New Jersey | 54 | 1 | 11 | 11 |
| New Mexico | 734 | 1 | 20 | 14 |
| New York | 0 | 0 | 10 | 13 |
| North Carolina | 3 | 0 | 26 | 24 |
| North Dakota | 0 | 0 | 16 | 6 |
| Ohio | 0 | 0 | 17 | 16 |
| Oklahoma | 8,108 | 12 | 8 | 9 |
| Oregon | 24 | 0 | 19 | 16 |
| Pennsylvania | 772 | 2 | 20 | 27 |
| Rhode Island | 1 | 0 | 8 | 11 |
| South Carolina | 12 | 0 | 30 | 24 |
| South Dakota | 73 | 0 | 14 | 15 |
| Tennessee | 62 | 0 | 9 | 9 |
| Texas | 9,653 | 4 | 10 | 5 |
| Utah | 8 | 0 | 3 | 2 |
| Vermont | 0 | 0 | 30 | 39 |
| Virginia | 2 | 0 | 22 | 19 |
| Washington | 386 | 1 | 14 | 14 |
| West Virginia | 54 | 0 | 23 | 21 |
| Wisconsin | 386 | 1 | 30 | 28 |
| Wyoming | 85 | 0 | 17 | 20 |
Notes
- Values are rounded to the nearest whole number. Therefore, values < 0.5% = 0%.
- The previous iteration of this table (that had used the USGS GWAVA-DW model with data from 1991-2003) had a lower threshold concentration (5 mg/l), which may result in a higher number or percentage of state locations that meet that lower threshold compared to this updated table (10 mg/l).
Sources: Nitrate concentrations were predicted by a USGS study titled, Machine Learning Predictions of Nitrate in Groundwater Used for Drinking Supply in the Conterminous United States, which used calibration data collected from 1988-2018. Drinking water source data is from USGS' Estimated Use of Water in the United States in 2005 and 2015.
- Groundwater Data (data from 1988 to 2018) (xlsx)
- Previously Available Groundwater Data (data from 1991 - 2003) (xlsx)
Sources of data
- Kenny, J.F., Barber, N.L., Hutson, S.S., Linsey, K.S., Lovelace, J.K., and Maupin, M.A. 2009. Estimated use of water in the United States in 2005: U.S. Geological Survey Circular 1344.
- Dieter, C.A., Maupin, M.A., Caldwell, R.R., Harris, M.A., Ivahnenko, T.I., Lovelace, J.K., Barber, N.L., and Linsey, K.S., 2018, Estimated use of water in the United States in 2015: U.S. Geological Survey Circular 1441, pp. 65.
- Ransom, K.M., Nolan, B.T., Stackelberg, P.E., Belitz, K., Fram, M.S., Reddy, J.E., Johnson, T.D., and Rodriguez, O., 2021, Data for Machine Learning Predictions of Nitrate in Groundwater Used for Drinking Supply in the Conterminous United States: U.S. Geological Survey data release. Appendix A: Supplementary data.
Data source information
The data presented in this indicator are based on predicted groundwater nitrate concentrations generated by the U.S. Geological Survey (USGS) three-dimensional extreme gradient boosting (XGB) machine learning model which was developed to predict the distribution of nitrate in groundwater across the conterminous United States (CONUS). Nitrate was predicted at a 1-square-kilometer (km) resolution for two drinking water zones, each of variable depth, one for domestic supply and one for public supply. The model used measured nitrate concentrations from 12,082 wells, and included predictor variables representing well characteristics, hydrologic conditions, soil type, geology, land use, climate, and nitrogen inputs. Predictor variables derived from empirical or numerical process-based models were also included to integrate information on controlling processes and conditions. The estimated percent of the population with self-supplied drinking water (98% groundwater) are from USGS’ reports on Estimated Use of Water in the United States in 2005 and 2015.
Note - Any reference to trade names or commercial products is for informational purposes only and does not constitute any endorsement or recommendation for use. US EPA and its employees do not endorse any commercial products, services or enterprises.
What to consider when using these data
A threshold value of 10 mg/l was chosen because this value represents EPA’s maximum contaminant level set to protect against blue baby syndrome. Data for this indicator are presented on a state-wide scale; however, alternative aggregations of nitrate data are possible, including exploration of average nitrate concentrations within major aquifers. Because the distribution of private drinking water well users among major aquifers is unknown, reporting data by major aquifer does not appear to offer an advantage over reporting by state areas.
References and links to other data sources
- Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., Hamilton P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., Nolan, B.T. Puckett, L.J., Rupert, M.G., Short, T.M., Spahr, N.E., Sprague, L.A., and Wilber, W.G. 2010. The quality of our Nation’s waters—Nutrients in the Nation’s streams and groundwater, 1992–2004: U.S. Geological Survey Circular 1350.
- Madison, R.J. and Brunett, J.O. 1985. Overview of the occurrence of nitrate in ground water of the United States, in National Water Summary 1984-Hydrologic Events, Selected Water-Quality Trends, and Ground-Water Resources: U.S. Geological Survey Water-Supply Paper 2275, pp. 93-105.