Well Water in Lower Gila Basin, Arizona
Yuma / Maricopa County · Population Varies by community · Aquifer: Lower Gila River Basin Alluvial Aquifer
Hardness: Extremely Hard
The Lower Gila Basin contains some of the most challenging groundwater in Arizona — in many areas, it is not suitable for drinking without treatment. ADEQ data shows 88% of wells exceed TDS standards, 71% exceed chloride standards, and 57% exceed sulfate standards. This is water that has been concentrating salts in an arid basin for millennia. Ancient inland lakes that could not reach the sea left behind massive salt deposits that dissolve into groundwater. If you're on a well in the Lower Gila Basin, reverse osmosis is not optional — it's a necessity.
The Salinity Problem
The Lower Gila Basin's groundwater is among the saltiest in Arizona. ADEQ monitoring data tells the story:
| Parameter | % Exceeding Standard | EPA Standard |
|---|---|---|
| Total Dissolved Solids | 88% | 500 mg/L (secondary) |
| Chloride | 71% | 250 mg/L (secondary) |
| Sulfate | 57% | 250 mg/L (secondary) |
These aren't health-based standards — they're secondary standards for taste, odor, and aesthetic quality. But when TDS is measured in thousands of milligrams per liter, the water tastes terrible, corrodes plumbing, and is impractical for many household uses without treatment.
Why Is the Water So Salty?
The Lower Gila Basin's salinity has deep geological roots. In the geologic past, Arizona's groundwater basins included river drainage systems that could not reach the sea, generating large inland lakes. As water evaporated in the arid climate, salts concentrated. Over millions of years, massive salt deposits accumulated.
Today, groundwater flowing through these deposits dissolves the accumulated salts. Low precipitation means minimal fresh recharge to dilute the mineralized water. Irrigation practices compound the problem — applied water picks up additional salts as it percolates back to the aquifer.
Large deposits of evaporite salts are common across Arizona, but the Lower Gila Basin is among the most extreme examples.
Arsenic and Fluoride
On top of the salinity, the Lower Gila Basin's groundwater frequently contains elevated arsenic and fluoride — consistent with the statewide pattern in Basin and Range aquifers. The same geological conditions that produce high TDS also tend to produce high arsenic: volcanic and granitic source rocks, long groundwater residence times, and minimal dilution.
In western Arizona, the belt of basins along the western Basin and Range province is predicted to have some of the highest arsenic concentrations in the state.
Treatment Is Essential
If you're on a private well in the Lower Gila Basin, treatment is not optional. The baseline water quality in most of the basin is not suitable for drinking without reverse osmosis or equivalent treatment.
- Point-of-use RO at the kitchen sink ($300-$600) — minimum for drinking and cooking water. Removes TDS, arsenic, fluoride, and most dissolved contaminants.
- Whole-house RO ($4,500-$20,000+) — for extremely high TDS situations where even bathing and laundry water needs treatment
- Water softener — addresses hardness but does NOT remove TDS, arsenic, or fluoride
Test first to know what you're dealing with, then match treatment to the specific contaminants and concentrations. See our testing guide and resources page.
Every well is different. Two wells on the same street can produce completely different water. The data on this page reflects documented conditions in the Lower Gila Basin area, but the only way to know what's in your water is to test it.
Sources
- ADEQ — Lower Gila Basin Groundwater Quality Fact Sheet
- ADEQ — Groundwater Quality in Arizona: A 15-Year Overview
- University of Arizona Cooperative Extension — Arizona Guide to Water Quality and Uses (AZ1610)
- USGS — Arsenic in Southwest Basin-Fill Aquifers