Why New Mexico Well Water Is the Way It Is

The geology that makes NM water beautiful and dangerous. Understanding what's beneath your well helps you understand what's in your water.

The Albuquerque Basin

Most of central New Mexico's population lives and draws water from the Albuquerque Basin — one of the largest and deepest basins of the Rio Grande Rift.

The basin is a half-graben — it tilts, sloping east toward the Sandia and Manzano mountain fronts. It's filled with alluvium (river and stream deposits) and volcanic rocks, thousands of feet deep. This fill material is where your water comes from.

The Santa Fe Group Aquifer

The Santa Fe Group aquifer is the principal water source for the entire Albuquerque Basin. Virtually every municipal well and most private wells in the metro area draw from it.

The Santa Fe Group consists of poorly cemented sands and gravels deposited over millions of years as the rift basin subsided and filled with sediment. The deposits are 2,400 to 14,000 feet thick, though only the upper ~2,000 feet are used for water supply. Below that, the water is too mineralized to be useful.

This is important for understanding water quality: the deeper you go, the older the water and the more minerals it has dissolved from the surrounding rock. Depth is one of the strongest predictors of what will be in your well water.

Volcanic Features

The Albuquerque Basin is not just sediment. It's punctuated by volcanic features that profoundly affect water quality:

• A dozen or more small volcanoes dot the basin from Bernalillo to Belen
• West of the Rio Grande, basin sediments are interbedded with basalt flows — layers of lava sandwiched between sand and gravel
• The Jemez Mountains to the northwest form a massive volcanic complex that is the primary source of arsenic in the basin's groundwater

The Jemez volcanic rocks can contain several hundred parts per million of arsenic. As groundwater flows through these formations, it dissolves arsenic and carries it into the basin aquifer. This is why arsenic concentrations tend to be highest on the north and west sides of the basin, closer to the Jemez source.

How Depth Affects Your Water Quality

One of the most important things to understand about NM well water is the relationship between well depth and water quality. It's not a simple "deeper is better" story.

Shallow Wells (less than ~100 feet)

• More vulnerable to surface contamination: bacteria, nitrates from septic systems, agricultural chemicals
• More seasonal variation — water quality changes with rainfall, irrigation, and nearby land use
• Generally lower arsenic and uranium (less contact time with deep rock)
• More affected by drought and pumping — shallow wells are first to go dry

Deep Wells (more than ~100 feet)

• Generally cleaner for bacteria — natural filtration through hundreds of feet of sediment
• Higher arsenic, uranium, and fluoride — the water has been in contact with rock for much longer
• More stable — less affected by surface conditions and seasonal changes
• Harder — more dissolved minerals overall

The Deep-Water Rise Problem

Here's a mechanism that catches many well owners by surprise: when a well pump is idle (overnight, during vacations, in winter when irrigation stops), deep, high-mineral water rises through the wellbore. This can contaminate intermediate zones that normally have better water quality.

This is the same mechanism behind the seasonal arsenic cycle — winter concentrations can exceed summer concentrations because the pump has been idle and deep arsenic-rich water has risen.

The Rift Zone: Where the Deep Water Comes Up

The Rio Grande Rift is an active continental rift — the earth's crust is pulling apart here, and has been for about 30 million years. This process created the Rio Grande valley and the deep basins that hold our aquifers.

Critically, the rifting also created faults — fractures in the rock that extend thousands of feet deep. These faults act as conduits, allowing deep, ancient, highly mineralized water to rise toward the surface. Where this deep water mixes with shallower groundwater, you get elevated arsenic, uranium, radon, and other contaminants.

This is why water quality can change so dramatically over short distances — a fault a quarter mile away can affect one well and not the next.

Groundwater Depletion

Across central New Mexico, we are pumping water out of our aquifers faster than nature replaces it. The rates vary dramatically by location:

Basin / Area	Decline Rate	Severity
Albuquerque Basin (overall)	-0.33 ft/year (2019-2023)	Moderate — stable but not recovering
East Mountains / Sandia Basin	-1.8 ft/year	Serious — wells going dry
Estancia Basin (center)	>2 ft/year	Severe — "hotspot for drying"

Depletion matters for water quality, not just quantity. As aquifer levels drop:

• Contaminant concentrations increase — less water diluting the same amount of dissolved minerals
• Wells must be drilled deeper — accessing older, more mineralized water
• The aquifer structure changes — clay layers compact irreversibly, permanently reducing storage capacity