Most people with a private well know roughly how deep it is and nothing else about where the water comes from. The formation underneath the Gulf Coast has a name, a published description, and a documented history — and one property of it explains a lot of what shows up on local water tests.
Named in 1916, from outcrops in Mobile County
The Citronelle Formation is a Pliocene unit of sand, gravel and clay lenses that caps most of the coastal plain from Florida to Texas. It was described and named by G. C. Matson in USGS Professional Paper 98-L in 1916, and the outcrop he measured it against — its type locality, the reference exposure the whole formation is defined by — is around Citronelle, in northern Mobile County, along the old Mobile & Ohio Railroad for three or four miles.
The name stuck across five states. The reference section is a twenty-minute drive from a lot of the wells it supplies.
What it looks like underground
From the Geological Survey of Alabama's mapping of Mobile County:
- It is thin in the north of the county and thickens seaward, reaching an estimated 200 feet in the subsurface beneath Dauphin Island.
- Its largest northern outlier is near Turnerville.
- Around Semmes and Bates Field it outcrops as relatively thick sandy clay, 10 to 20 feet.
- Composition is reddish-brown weathered quartz sand, gravel of chert and quartz pebbles, and mottled lenses of clay — with limonite pebbles and limonite-cemented sand in weathered exposures.
Wells yielding a million gallons a day or more tap sands in this formation and the Miocene beds below it.
The sentence that matters
The USGS description of the unit contains one line that explains more about local well water than any other:
Much of the Citronelle Formation is highly permeable. It forms the Sand and Gravel Aquifer.
Permeability is why the water is here and easy to reach. It is also why what happens at the surface reaches the water.
That single property drives three things you will see on a local test:
Low pH. The formation is quartz sand and gravel with very little carbonate to buffer against. Water moving through it does not pick up much alkalinity, so mildly acidic water is normal rather than unusual here. Acidic water is hard on copper plumbing and fixtures, which is why pH correction is a routine service on Gulf Coast wells rather than an exotic one.
Low hardness. Same reason. There is not much calcium and magnesium to dissolve. USGS sampling across these two counties finds median hardness low by national standards — which surprises people who assume well water means hard water.
Surface vulnerability. A permeable formation with a shallow water table transmits what lands on it. That is the honest argument for testing a shallow well more often than a deep one, and for caring about setbacks, casing condition and what happens within a few hundred feet of the wellhead.
What this means practically
Roughly 800,000 Alabamians — one in five — drink from a private supply, according to the Alabama Department of Public Health, and no one tests it for them. ADPH says it plainly: if you get your drinking water from your own well, you alone are responsible for assuring that it is safe.
Knowing the formation does not substitute for that. It tells you what to expect; it cannot tell you what your well is doing. Two wells a few hundred feet apart in the same sand can read differently depending on depth, casing and what is upslope.
What it does give you is a reason to look at the right things. On a Citronelle well, pH and the possibility of surface influence are the questions worth asking first — not the ones a national article about hard water would put in front of you.
More on private wells in coastal Alabama, including who regulates them and where we stop. If you want a number instead of a formation, the in-home test is free and takes about half an hour.
Sources: Matson & Berry, USGS Professional Paper 98-L (1916); USGS Geolex unit 1073; Geological Survey of Alabama Special Map 093, "Geology of Mobile County" (1971); USGS State Geologic Map Compilation unit description; Alabama Department of Public Health, Well Water program. Verified 19 August 2026.