Columbia Plateau Basalts

The Columbia Plateau is the basaltic counterpart to the Snake River Plain — built from layer after layer of Miocene flood basalts (the Columbia River Basalt Group) covering most of Eastern Washington, northeastern Oregon, and parts of Idaho. Together these two basalt aquifers underlie a substantial share of the Pacific Northwest's irrigated agriculture and rural population. The Columbia Plateau version supplies water to Yakima, Pasco, Kennewick, Walla Walla, and dozens of smaller cities, and to tens of thousands of private wells in the irrigated valleys around them.

The Plateau's water is generally productive and chemically straightforward. Where it gets ugly is downwind of large dairies and intensive row-crop operations: the Lower Yakima Valley in particular has been the subject of EPA enforcement action since 2013 for nitrate contamination so severe and so widespread that infants in the affected counties were warned away from local well water. The story repeats at smaller scale across the irrigated Plateau.

What it is, geologically

The Columbia River Basalt Group is one of the largest flood-basalt provinces on Earth — repeated eruptions between roughly 17 and 6 million years ago laid down individual flows hundreds of feet thick across an area now exceeding 60,000 square miles. Each flow has the characteristic basalt structure: dense flow interior, more permeable rubbly top and bottom (interflow zones), and vertical cooling joints (columnar basalt). Water moves through the aquifer almost entirely through the interflow zones and joints — not through the dense flow rock itself.

This makes the aquifer's productivity highly anisotropic: vertical flow is severely restricted (water doesn't easily move from one flow to the next without a connecting structure), while horizontal flow within an interflow zone can be substantial. A single flow can act like a confined aquifer; a stack of flows behaves like a stack of confined aquifers, each somewhat isolated from the others.

For private well owners, this means well productivity depends heavily on which interflow zone(s) the well intersects. Two neighbors can have very different yields. Deepening a marginal well by 200 feet may or may not help, depending on whether the deeper formation has a productive interflow zone within reach.

The Lower Yakima Valley nitrate disaster

The Lower Yakima Valley — Sunnyside, Granger, Toppenish, Mabton, Wapato — has documented nitrate concentrations in private wells that are among the highest sustained in the US. The 2010 EPA dairy-impact study found about 20% of private wells exceeded the 10 mg/L MCL; some clusters showed exceedance rates above 40%. The 2013 EPA Compliance Order (the Yakima Valley CAFO consent decree) required several large dairies to take specific actions to reduce nitrate loading to groundwater. Implementation has been contested, slow, and partial. As of the early 2020s, nitrate remains widespread.

The mechanism is straightforward: dairy lagoons and field-applied manure deliver large nitrogen loads to permeable surface soils overlying the shallow basalt aquifer. The interflow zones transmit the contaminated recharge laterally; downgradient wells receive it. The valley's rapid water-table response to recharge events makes the contamination acute as well as chronic.

This is the highest-priority US private-well region for nitrate testing outside the corn belt. If you're on a Lower Yakima Valley well, see nitrates — and check the EPA Region 10 enforcement documents for your specific township.

Hanford and the technetium plume

The Hanford Nuclear Reservation — Eastern Washington, where the US made plutonium for the Manhattan Project and Cold War — sits on the Columbia Plateau basalt and has a documented groundwater contamination plume of technetium-99, tritium, carbon tetrachloride, chromium, and nitrate. The plume is mostly contained on the reservation; ongoing DOE remediation is in its eighth decade with decades remaining. For most private well owners in the region, Hanford is not a direct contamination concern — but if you're on a well downgradient of the southern Hanford boundary or near the Columbia River below the site, the Department of Energy's annual Hanford Site Environmental Report is the relevant reference.

Other water quality concerns

• Arsenic — present in pockets, derived from volcanic source rocks. The Snake River Plain patterns extend into the Columbia Plateau in southeastern Washington. See arsenic.
• Bacteria — like all fractured/structured aquifers, surface contamination can reach wells quickly through vertical fractures. Annual coliform testing is standard. See bacteria.
• Pesticides — atrazine, simazine, and others detected in monitoring wells; usually below MCL but worth knowing if you're in agricultural areas.
• Iron and manganese — moderate; common in deeper interflow zones with reducing conditions. See iron and manganese.

PFAS risk modeling

The USGS 2024 national PFAS-in-groundwater predictive model estimates a 12.5% mean probability of detecting any PFAS compound at typical private-well drinking-water depth across the Columbia Plateau Basalts. Roughly 2.2% of the aquifer's area falls in the model's >50% probability band — land where a freshly drilled domestic well is predicted more likely than not to show some detectable PFAS. That is below the national CONUS aquifer median of 19%, and places the Columbia Plateau Basalts in the lower quartile of CONUS Principal Aquifers.

This is a 1 km-resolution statistical prediction, not a measurement of any specific well. What PFAS is and how it gets into groundwater → · How to test a private well for PFAS → · Full state-and-aquifer breakdown with methodology →