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Saturday May 18, 2013 09:45:11 PM
|Dept. of Administration / Office of Geographic and Demographic Analysis|
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Indicator : Nitrate in ground water
Rationale: Ground water is a vital source of drinking water for more than 75 percent of Minnesotans and 98 percent of the state's nearly 1,000 community water systems.
About this indicator: Less than half a percent of public water supply systems in Minnesota had average nitrate-nitrogen concentrations above the drinking water standard during a two-year period from 1999 to 2000. The U.S. Environmental Protection Agency has established a public drinking water standard for nitrate-nitrogen (nitrate) concentrations at 10 parts per million (ppm). Nitrate in water poses a health threat, especially to infants, and is used nationally as an indicator of overall water quality. Many experts consider nitrate levels above one part per million to be a sign of human influence on water quality. Sources of nitrate include fertilizer, crop residue, manure, septic systems and deposits from the atmosphere.
Twenty-eight public water supply systems, or less than half a percent of Minnesota's 8,121 public water supply systems, had average nitrate-nitrogen concentrations above the drinking water standard in 1999-2000. Public water supply systems include both community and non-community systems. Community systems supply drinking water for more than 15 connections or 25 residents year-round. Non-community water systems, such as for schools, factories, hospitals, restaurants, and the like serve at least 15 connections used by people other than year-round residents for 60 days a year or 25 or more people for at least 60 days a year.
Nearly one-fourth of Minnesota's population relies on private wells as a primary source of drinking water. Local water quality databases, developed by counties such as Nicollet, Brown, and Cottonwood in the southwestern part of the state, indicate that the statewide database for public water supply wells does not accurately reflect nitrate-nitrogen concentrations for private water wells. This is because wells installed by the homeowner (such as sand points), those installed before the state well code went into effect in 1974, and hand-dug wells with large diameters are not included in statewide databases. According to data from these counties, about 5 percent of the private wells in Nicollet County, 12 percent of the private wells in Brown County, and 27 percent of the private wells in Cottonwood County have average nitrate-nitrogen concentrations above the drinking water standard of 10 parts per million. However, if wells of questionable construction (i.e., wells less than 50 feet in depth and with diameters 10 inches or greater) are excluded, the percentage of unacceptable nitrate-nitrogen concentrations drops to 3 percent for Nicollet and Brown counties and to 11 percent for Cottonwood County.
Things to think about: The data from Nicollet, Brown and Cottonwood counties suggest the value of localized water quality tracking. Data from the three counties also provides evidence that Minnesota's well construction code is effective, since wells that do not meet code requirements are responsible for most of the problems.
Nitrate contamination is more frequent in wells that are poorly constructed or sited. In addition, some areas are more susceptible to nitrate contamination because geologic conditions, such as sandy soil, allow nitrate to seep into the ground more easily or the chemical makeup of the groundwater does not break down nitrate.
Technical notes: This indicator was calculated for public water supply systems where the average nitrate-nitrogen concentrations for all the wells for each system were fairly consistent (within one standard deviation). Results for 440 public water supply systems were excluded because the results were not as consistent between sampling events or because the system used various wells with diverse water quality. Eight of the 28 public water supply systems with average nitrate results above the drinking water standard are no longer active. None of these 28 public water supply systems are community systems.
Other related indicators:
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