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Regional Assessment of Groundwater Quality in Principal Volcanic Aquifers of the Western United States

The Western Volcanics study brings together three areas with similar aquifer properties: Columbia Plateau basaltic-rock aquifers, Snake River Plain basaltic-rock aquifers, and Hawaiian volcanic-rock aquifers. These are extensive provinces of layered igneous rocks (mostly basaltic lava flows) that host regional ground-water flow systems. Important sedimentary aquifers (basin-fill aquifers) overlie basalts of the Columbia Plateau and Snake River Plain. In Hawaii, sediments overlie volcanic-rock aquifers along the coastal perimeter of the islands, but the sedimentary aquifers are not used as drinking-water sources.

The volcanic-rock aquifers are highly susceptible to contamination because they are mostly unconfined and are overlain by thin or well-drained soils. All three areas are farmed intensively, were irrigated heavily for most of the 20th century, and have had agricultural fertilizers and pesticides applied. Numerous agricultural and industrial chemicals have been detected in groundwater, though most concentrations are below human-health guidelines.

A notable exception is nitrate, a nutrient for which elevated concentrations commonly are ascribed to fertilizer application, animal manure, or nitrogen-fixing plants (legumes) such as alfalfa. In ground-water samples collected by NAWQA (1991-2000), nitrate concentrations in 20 percent of Columbia Plateau wells and 3 percent of Snake River Plain wells were above the U.S. Environmental Protection Agency drinking-water standard of 10 milligrams per liter. No wells in Hawaii had nitrate concentrations above the standard.

The western volcanics study seeks to explain the causes of such similarities and differences through statistical analysis of soil properties, crop types, and other factors. Past trends in water quality also are under study, with an eye toward future forecasting through the use of ground-water modeling.

Topics selected for the assessment of water quality:

  • How does water quality in surficial aquifers compare to water quality in volcanic bedrock aquifers in the Western United States? (SIR  2012-5123)
  • Can we predict how nitrate concentrations in groundwater of the Snake River Plain Aquifer system will vary over time? (SIR 2012–5237)
  • Can we predict where atrazine and nitrate contamination may occur in the Columbia Plateau, Snake River Plain, and Oahu aquifers?

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  • The Western Volcanics study compares and contrasts groundwater quality in three areas with similar aquifer properties: Columbia Plateau basaltic-rock aquifers (Washington), Snake River Plain basaltic-rock aquifers (Idaho), and Hawaiian volcanic-rock aquifers (Oahu). These are extensive provinces of layered igneous rocks (mostly basaltic lava flows) that host regional ground-water flow systems. Important sedimentary aquifers (basin-fill aquifers) overlie basalts of the Columbia Plateau and Snake River Plain. Concentrations of naturally occurring constituents (radon, arsenic, manganese, and molybdenum) above human-health benchmarks were more common in the Columbia Plateau and the Snake River Plain aquifers than on Oahu. Concentrations of constituents related to human activities (the pesticide dieldrin, three soil fumigants (Ethylene dibromide (EDB), 1,2-Dibromo-3-chloropropane (DBCP), and 1,2-Dichloropropane (DCP)), and the solvent Trichloroethylene) were more commonly detected above human-health benchmarks in groundwater on Oahu than groundwater in the Columbia Plateau or the Snake River Plain aquifers. (SIR  2012-5123)

General Information

Informational handout about the principal volcanic aquifers of the Western United States (PDF format, 1Mb)

Detailed information on the hydrogeology of the principal volcanic aquifers of the Western United States can be accessed through the USGS Ground Water Atlas of the United States

For more information, contact:
Michael G. Rupert, Hydrologist

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