Colorado private wells

Your well water starts with the rock under your house.

No agency tests a private well for you; that's up to the owner. What's in yours depends mostly on what it's drilled into: cracked granite, buried sandstone, river gravel or old valley fill.

256,361household wells on Colorado's permit records with a location and a depth, mapped here. Each square is coloured by how deep the wells in it were drilled.

Depth drilledunder 100 ft100–200200–400400–700700 ft +

How to test your well

Colorado Division of Water Resources well permits: residential wells recorded as built, with a total depth, in 0.04° squares. Circles mark the community guides below; tap one to open it.

Four kinds of ground

The same well, drilled in four places

Drawn to one depth scale from the state's well permit records: the typical depth household wells reach, where the water stood when they were drilled, and (dashed) how deep the deeper quarter of wells go. Then what CDPHE's lab records found when owners tested.

Foothills granite

Evergreen, Conifer, Bailey, Woodland Park

Water moves through cracks in solid rock. Yields are small, and the rock can release uranium and radon into the water.

Typical well 300 ftWater at 100 ft when drilled24,892 wells on recordTests: 32% over the uranium limit (250 tests)

Denver Basin bedrock

Black Forest, Castle Rock, Parker, Elbert County

Stacked sandstone aquifers holding mostly old water, much of it thousands of years old. Pumping takes out far more than rain adds back, and water levels are falling in parts of the basin.

Typical well 334 ftWater at 150 ft when drilled59,409 wells on recordTests: 10% over the bacteria limit (738 tests)

River-valley gravel

South Platte, Arkansas and Colorado river valleys

Shallow sand and gravel beside rivers, refilled from the surface, so whatever is on the surface (fertilizer, septic, manure) can reach it.

Typical well 100 ftWater at 35 ft when drilled4,636 wells on recordTests: 17% over the nitrate limit (1,799 tests)

San Luis Valley basin fill

Alamosa, Monte Vista, Center, Saguache

Deep layers of sand, clay and volcanic rock under a high-desert valley. Deeper wells that reach the confined aquifer are where arsenic turns up; shallow wells are where uranium does.

Typical well 120 ftWater at 36 ft when drilled15,481 wells on recordTests: 13% over the nitrate limit (54 tests)

Drawn to one depth scale from Colorado Division of Water Resources well permits (household wells recorded as built). Typical = median; the dashed outline reaches the depth the deeper quarter of wells go past. Lab results are CDPHE's private well dataset (2011–2020) for the counties around each kind of ground; people usually test because they have a reason to, so the rates describe tested wells, not every well.

No one tests your well for you.

Bacteria and nitrate every year, metals and uranium every few years, more in the foothills. What it costs, where to send it and what the numbers mean.

The Colorado testing guide

Community guides

Find your area

Each guide maps the area's wells, shows how deep they've been drilled decade by decade, what the county's lab tests found, and the local geology behind it. The column on each card is that area's ground with its typical well drawn in, all to one scale (full height = 700 ft).