Groundwater Contamination on the Navajo Nation


Arsenic occurrence in groundwater is prevalent throughout the United States. Elevated arsenic levels are common for aquifers located in semi-arid conditions, including portions of the Four Corners. In Arizona arsenic is the most common naturally occurring contaminant that is found throughout the state (Cory & Rahman, 2009; Uhlman, Rock, & Artiola, 2009). There are more than 100,000 private domestic wells, many in rural areas that are not regulated by the Safe Drinking Water Act. As a result, water quality for many of these wells is unknown and may be impaired. The Arizona Department of Environmental Quality regularly conducts groundwater measurements in basins throughout the state testing for contaminants such as arsenic. Between 1995 and 2009, twenty percent of collected samples exceeded the arsenic MCL (10 parts per billion). The ADEQ has completed groundwater basin studies for 29 of 51 basins in Arizona (Uhlman, 2008), though, to date, no basin in the Four Corners area has been studied in detail.


Arsenic Sources

Caption: Arsenic cycling including natural and anthropogenic sources (Jones, 2007).

Arsenic drinking water contamination is derived from natural sources including lake beds with fine material such as clay, aquifers with rocks of volcanic origin, geothermal environments or areas with gold and uranium deposits (Spencer, 2002). One documented example of arsenic groundwater contamination near the NN is in the Verde Valley in central Arizona. Arsenic concentrations at Montezuma Well were measured to exceed 500 parts per billion, which is well above the current MCL (10 parts per billion). Researchers measured arsenic in groundwater in the area and determined that levels were elevated due to groundwater interaction with the Supai and Verde Formations. They determined that groundwater contact with the Supai and Verde limestone Formations caused elevated arsenic levels because arsenopyrite, an arsenic bearing mineral, precipitated in the formation. The Supai Formation is common in the Four Corners of northeast Arizona and is a component of the C aquifer used by NN residents for drinking water.


Four Corners Arsenic Contamination

Elevated arsenic levels are common for aquifers located in semi-arid conditions, including portions of the Navajo Nation, which includes portions of Arizona, New Mexico and Utah. An ongoing water sampling campaign is assessing water quality of unregulated groundwater wells used by people without municipal water access. Of the 239 unregulated water wells tested for arsenic to date, 71% of the water sources had detectable arsenic levels and 25% had arsenic levels exceeding the safe drinking water limit.
Caption:Illustration of unregulated water source testing on NN. Data sources: EPA (2007) and USGS (2001-2011)


According to electronically available 2010 consumer confidence reports, for 90 Navajo Tribal Utility Authority (NTUA) operated water systems, 17 systems reported measurable levels of arsenic in drinking water. Of these 17 systems, eight reported arsenic levels that exceed the safe drinking limit (Maximum Contaminant Level) of 10 parts per billion.


Arsenic Health Impacts

Long term arsenic exposure through drinking water can lead to:


  • Skin discoloration
  • Stomach pain
  • Nausea
  • Partial paralysis
  • Blindness
  • Cancer


The US Environmental Protection Agency and World Health Organization classify arsenic as a known human carcinogen.



Congratulations! You have just learned the basics of arsenic groundwater contamination and drinking water supply for the Four Corners area. Please use the webGIS application to investigate arsenic contaminated groundwater on the Navajo Nation.