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.