Basin and Range

Hundreds of small valley-fill aquifers between the mountain ranges of the Western interior — each its own water budget, each with arsenic, fluoride, and uranium showing up in most of them

States

NV, UT, AZ, NM, OR, CA, ID

Type

Alluvial valley-fill basins recharged by mountain-block snowmelt; some carbonate-rock components

Status

Most basins in long-term decline; widespread natural arsenic, fluoride, uranium

The Basin and Range Province of the western United States is a geologic region of parallel north-south mountain ranges separated by alluvium-filled valleys, formed by Cenozoic crustal extension. Hydrogeologically, each basin holds its own discrete valley-fill aquifer — recharged from snowmelt and rainfall in the surrounding mountain ranges, drained either by evaporation in the valley playa or by underflow to adjacent basins. There are hundreds of these basins, most of them in Nevada (which is essentially nothing but Basin and Range geology), with significant extensions into western Utah, southern Arizona, southwestern New Mexico, southeastern Oregon, and parts of California.

For private well owners, the Basin and Range presents a different shape of problem than the more familiar major aquifer systems. Each basin is its own water budget — what your neighbor in the next valley over drinks has nothing to do with your water. The natural water-quality problems, however, are remarkably uniform across the province: arsenic, fluoride, and uranium are present at meaningful concentrations in most basins, derived from the volcanic, granitic, and metamorphic source rocks that make up the surrounding mountain blocks.

What it is, geologically

Basin-and-range structure: alternating horsts (uplifted mountain blocks) and grabens (down-dropped valleys), the result of Miocene-to-recent crustal extension. The valleys fill with alluvium eroded from the bordering ranges; the alluvial fill is the principal aquifer. Saturated thickness varies from tens to thousands of feet depending on basin geometry. Some basins are also underlain by carbonate rocks (the Basin and Range Carbonate Aquifer System) that act as a deeper, more transmissive secondary aquifer connecting some adjacent basins through subsurface flow.

Recharge happens primarily as mountain-block recharge: snowmelt and rainfall in the surrounding ranges either runs off to the basin floor and infiltrates, or moves through fractures in the bedrock and emerges as base flow to streams and aquifers. In a typical Nevada basin, total annual recharge is a small fraction of total precipitation — most of which evaporates or transpires before reaching the water table.

Major population centers and their basins

• Las Vegas Valley — primarily on Colorado River surface water, but also the underlying Las Vegas Valley aquifer. Long-term overdraft has caused subsidence (small fissures, but real).
• Phoenix and Tucson — Active Management Areas under Arizona's 1980 Groundwater Management Act; CAP (Central Arizona Project) surface water has partially substituted for groundwater pumping.
• Reno-Sparks — Truckee Meadows aquifer; water-level declines documented in eastern Reno suburbs.
• Salt Lake Valley — Wasatch Front basin-fill; long-term declines in some sub-basins.
• El Paso and Ciudad Juárez — Hueco Bolson aquifer (binational); severely overdrafted.
• Albuquerque — Middle Rio Grande Basin (part of the broader Rio Grande Aquifer System).

For rural private well owners — Nevada's vast empty middle, southern Arizona's small communities, southeastern Oregon's high desert ranching country — the relevant aquifer is whatever basin you happen to live in. There are no shortcuts.

Water quality: the universal triple

Three contaminants are pervasive across the Basin and Range, all from natural geologic sources:

Arsenic — derived from weathered volcanic rocks (rhyolites, ash flows, hot-spring deposits). Nearly every basin in the province has arsenic at detectable levels; many basins have substantial fractions of wells exceeding the 10 μg/L MCL. Particularly elevated in central Nevada, southwestern Arizona, and parts of southern Utah. See arsenic.

Fluoride — also from weathered volcanic and granitic rocks (fluorite, fluorapatite). Many basins have wells exceeding the 4 mg/L EPA primary MCL, and a much larger fraction exceed the 2 mg/L secondary MCL associated with dental fluorosis. New Mexico, Arizona, and parts of Texas have particularly hot zones. See fluoride.

Uranium — particularly elevated in basins adjacent to or downstream of the Colorado Plateau uranium province (UT, AZ, southwestern CO, NM). Co-occurs with arsenic in many wells. See uranium.

Plus the usual suspects: hardness (high in basins draining limestone-rich ranges), iron and manganese (particularly in the deep alluvial fill), radon in granitic-source basins. See also hardness.

The closed-basin concentration effect

Most Basin and Range basins are hydrologically closed — water enters from precipitation and leaves only through evaporation. Over geologic time, dissolved minerals concentrate in the aquifer because the water leaves but the dissolved load doesn't. This is why basin-fill aquifers tend to have higher TDS, higher fluoride, and higher arsenic than aquifers in flow-through regions. It is also why drought makes the chemistry worse: when total water budget shrinks, dissolved concentrations rise.

Known contaminant concerns

Communities on this aquifer