Why Arizona Well Water Is the Way It Is

From Basin and Range alluvial aquifers to volcanic arsenic sources to ancient salt deposits — the geology beneath your well determines what's in your water.

The Basin and Range Province

Most of Arizona lies within the Basin and Range province — a geological landscape defined by uplifted mountain ranges separated by down-dropped basins. These basins are filled with hundreds to thousands of feet of alluvial deposits: unconsolidated gravels, sands, silts, clays, and evaporites eroded from the surrounding mountains over millions of years.

These basin-fill deposits are Arizona's primary aquifers. Vast amounts of groundwater have accumulated within them over thousands of years. More than 80% of Arizona residents rely on groundwater for drinking water — and most of that groundwater is in basin-fill aquifers.

Why Basin-Fill Aquifers Concentrate Contaminants

The Basin and Range creates a perfect system for mineral accumulation in groundwater:

• Source rocks — the mountain ranges contain volcanic, granitic, and sedimentary formations rich in arsenic, fluoride, uranium, and other minerals
• Weathering and transport — erosion breaks down source rocks and deposits mineral-laden sediment in the basins
• Long residence times — groundwater moves slowly through basin fill, spending centuries or millennia in contact with mineral-bearing sediments
• Minimal dilution — low precipitation (3-15 inches/year in most basins) means little fresh water enters to dilute the accumulating minerals
• Evaporative concentration — in closed or poorly drained basins, evaporation concentrates dissolved minerals

Arizona's Three Geological Provinces

Arizona has three distinct geological provinces, each with different implications for groundwater:

1. Basin and Range (Southern and Western Arizona)

Covers roughly the southern two-thirds of the state. Broad alluvial basins separated by mountain ranges. This is where most of Arizona's population lives and where most groundwater is pumped. Primary concerns: arsenic, fluoride, TDS, hardness.

2. Transition Zone (Central Arizona)

A rugged belt running diagonally from northwest to southeast, including the Mogollon Rim, Prescott, Payson, and the Verde Valley. Complex geology with Precambrian basement rocks, Paleozoic sedimentary sequences, and volcanic deposits. Primary concerns: arsenic, fluoride, mining contamination, limited supply.

3. Colorado Plateau (Northern Arizona)

Flat-lying to gently dipping sedimentary rocks, including the massive C Aquifer (Coconino Sandstone). More predictable geology than the other provinces. Primary concerns: generally better water quality, but arsenic and fluoride still occur; supply limitations in some areas.

Arsenic: Arizona's Defining Groundwater Contaminant

Arsenic occurs naturally in Arizona groundwater at rates far above the national average. The sources are geological:

• Volcanic rocks — the numerous volcanic fields across Arizona (Superstition, San Francisco, Mingus Mountain, and many others) contain arsenic-bearing minerals
• Granitic rocks — Precambrian basement rocks throughout the Transition Zone
• Sedimentary deposits — arsenic adsorbed onto iron oxides and clay minerals in basin fill

The USGS found that arsenic concentrations in Southwest basin-fill aquifers exceeded the MCL more than twice as frequently as nationwide. See our arsenic guide for the complete story.

Ancient Salt Deposits

Arizona's groundwater basins have a salty history. In the geologic past, some basins contained river drainage systems that could not reach the sea, generating large inland lakes. As water evaporated, salts concentrated. Over millions of years, massive evaporite deposits accumulated — primarily halite (sodium chloride), gypsum (calcium sulfate), and other mineral salts.

These deposits are common across the state. Where groundwater contacts them, TDS skyrockets. The Lower Gila Basin is the extreme example: 88% of wells exceed TDS standards, 71% exceed chloride standards. But elevated salinity affects many Arizona basins.

Land Subsidence and Earth Fissures

When groundwater is pumped faster than it recharges, the aquifer compacts. Clay layers that once held water are squeezed, permanently losing their ability to store water. The ground surface drops — this is land subsidence.

Arizona has some of the most dramatic groundwater-related subsidence in the nation:

• The Willcox Basin — up to 12 feet of subsidence since the 1950s, with current rates of 6 inches/year in some areas. 50 miles of earth fissures.
• The Gila Bend Basin and Ranegras Plain Basin — among the three most seriously jeopardized basins in the state
• The Phoenix metropolitan area — historical subsidence from agricultural pumping, now largely stabilized in AMA areas

Earth fissures — cracks in the ground caused by differential subsidence — damage infrastructure, compromise well casings, and create pathways for surface contaminants to reach groundwater. The Arizona Geological Survey maps fissures statewide.

Arizona's Groundwater Management System

Arizona's 1980 Groundwater Management Act created a framework unique among western states:

Active Management Areas (AMAs)

Seven designated regions where groundwater is actively managed:

• Phoenix AMA — the state's largest, covering most of the metro area
• Tucson AMA
• Prescott AMA
• Pinal AMA
• Santa Cruz AMA
• Douglas AMA
• Willcox AMA — designated December 2024, the newest

Within AMAs: conservation requirements, pumping limits, metering/reporting for non-exempt wells, and Assured Water Supply rules for new development.

Outside AMAs

Most of Arizona's land area is outside any AMA. In these areas:

• Groundwater is subject only to "reasonable and beneficial use"
• No pumping restrictions on domestic wells
• ADWR cannot regulate the impact of one landowner's pumping on a neighbor's
• Well registration/notice of intent is required, but the regulatory framework is minimal

Exempt Wells

Most private domestic wells in Arizona are exempt wells — pumping 35 GPM or less, used for household purposes. "Exempt" means exemption from metering, reporting, and certain conservation rules. Within AMAs, exempt wells for non-residential purposes are limited to 10 acre-feet per year. Outside AMAs, there are essentially no restrictions on exempt well usage.