US aquifer map & measured water hardness
Your water's hardness is set underground, in the aquifer your well or utility draws from — and different aquifers give up wildly different amounts of mineral.
Here are the major US aquifers with the measured hardness of each, from monitoring data — not a textbook guess — plus which rock makes water hard and how to find the aquifer under you.
The United States draws groundwater from major aquifer systems including the Floridan, the High Plains (Ogallala), and the Edwards-Trinity. Measured hardness differs sharply by geology: Alluvial aquifers — a broad USGS lithologic category — run a median of 370 mg/L, while Mississippi embayment aquifer system runs 20. Sediment-rich aquifers top the measured ranking, crystalline and coastal-plain sands the soft end.
Basics
What is an aquifer?
An aquifer is a body of rock or sediment saturated with groundwater that yields usable amounts to wells and springs. Unconfined aquifers sit near the surface and recharge directly from rainfall; confined aquifers lie beneath a low-permeability layer, under pressure, recharging where they reach the surface.
The rock sets the chemistry: water sits in contact with the aquifer material for years, dissolving whatever the rock gives up. That is why the same rain becomes hard in one aquifer and soft in another.
Rankings
Aquifers ranked by measured hardness
| Aquifer | Median | Band | Samples | sites | states | GW% |
|---|---|---|---|---|---|---|
| Alluvial aquifers | 370 | Very hard | 18,846 | 7,776 | 20 | 99 |
| Intermediate aquifer system | 370 | Very hard | 1,432 | 653 | 1 | 100 |
| Silurian-Devonian aquifers | 340 | Very hard | 1,988 | 1,118 | 8 | 99 |
| California Coastal Basin aquifers | 310 | Very hard | 19,423 | 5,003 | 1 | 100 |
| Sand and gravel aquifers (glaciated regions) | 300 | Very hard | 36,962 | 20,416 | 22 | 100 |
| Edwards-Trinity aquifer system | 290 | Very hard | 7,402 | 1,446 | 3 | 91 |
| Cambrian-Ordovician aquifer system | 290 | Very hard | 4,180 | 1,879 | 6 | 99 |
| Paleozoic aquifers | 276 | Very hard | 1,534 | 904 | 3 | 90 |
| Lower Cretaceous aquifers | 270 | Very hard | 5,662 | 3,701 | 7 | 100 |
| Piedmont and Blue Ridge carbonate-rock aquifers | 270 | Very hard | 822 | 524 | 3 | 96 |
| Biscayne aquifer | 267 | Very hard | 1,301 | 483 | 1 | 100 |
| Mississippi River Valley alluvial aquifer | 260 | Very hard | 4,010 | 2,701 | 6 | 100 |
| Floridan aquifer system | 220 | Very hard | 14,938 | 3,501 | 4 | 99 |
| Colorado Plateaus aquifers | 220 | Very hard | 5,247 | 3,113 | 5 | 85 |
| Ozark Plateaus aquifer system | 218 | Very hard | 4,158 | 2,317 | 4 | 74 |
| High Plains (Ogallala) aquifer | 218 | Very hard | 7,974 | 4,577 | 8 | 100 |
| Snake River Plain basaltic-rock aquifers | 212.5 | Very hard | 770 | 194 | 1 | 100 |
| Castle Hayne aquifer | 210 | Very hard | 838 | 632 | 1 | 100 |
| Central Valley aquifer system | 190 | Very hard | 11,387 | 6,043 | 1 | 100 |
| Rio Grande aquifer system | 176 | Hard | 5,278 | 3,012 | 3 | 96 |
| Pennsylvanian aquifers | 170 | Hard | 6,471 | 3,800 | 10 | 85 |
| Lower Tertiary aquifers | 170 | Hard | 3,930 | 2,484 | 4 | 98 |
| Central Oklahoma aquifer | 165 | Hard | 1,621 | 839 | 1 | 100 |
| Valley and Ridge aquifers | 160 | Hard | 3,478 | 2,625 | 9 | 86 |
| Early Mesozoic basin aquifers | 160 | Hard | 2,020 | 1,503 | 8 | 100 |
| Mississippian aquifers | 153.5 | Hard | 1,848 | 988 | 11 | 82 |
| Basin and Range basin-fill aquifers | 140 | Hard | 19,304 | 4,780 | 7 | 97 |
| Upper Cretaceous aquifers | 133 | Hard | 4,401 | 3,681 | 4 | 100 |
| Denver Basin aquifer system | 120 | Hard | 554 | 397 | 1 | 100 |
| Surficial aquifer system | 118 | Moderately hard | 3,413 | 1,605 | 4 | 100 |
| Pacific Northwest basin-fill aquifers | 91.6 | Moderately hard | 635 | 298 | 3 | 99 |
| Texas coastal uplands aquifer system | 79 | Moderately hard | 1,978 | 1,305 | 1 | 100 |
| New York and New England crystalline-rock aquifers | 70 | Moderately hard | 1,123 | 889 | 7 | 99 |
| Piedmont and Blue Ridge crystalline-rock aquifers | 55 | Soft | 4,933 | 3,068 | 12 | 96 |
| Coastal lowlands aquifer system | 47 | Soft | 17,686 | 10,191 | 5 | 100 |
| Northern Atlantic Coastal Plain aquifer system | 31 | Soft | 40,483 | 10,275 | 8 | 100 |
| Southeastern Coastal Plain aquifer system | 24.4 | Soft | 5,413 | 4,047 | 5 | 99 |
| Mississippi embayment aquifer system | 20 | Soft | 7,449 | 5,151 | 7 | 100 |
| Other / local aquifers | — | — | 20,971 | 11,451 | — | — |
Geology
Why geology sets hardness
Rainwater is mildly acidic and dissolves calcium carbonate from limestone and dolomite, so carbonate aquifers tend toward hardness; crystalline rock (granite, schist) and clean quartz sands give up little, so they stay soft.
The measured ranking, though, is topped not by the named carbonate systems but by broad sediment aquifers — alluvial deposits and glaciated sand-and-gravel. Those sediments carry minerals eroded from many source rocks, and groundwater lingers in them; that is the measured observation, and we keep the chemistry general rather than tell an unsourced story about any one basin.
Floridan
Floridan aquifer system
The Floridan aquifer system spans about 100,000 square miles beneath all of Florida and parts of Alabama, Georgia, and South Carolina, and is a sequence of carbonate rocks — principally limestone with some dolomite. It supplies drinking water for roughly 10 million people, and is divided into the Upper and Lower Floridan aquifers. (Source: USGS.)
Measured: 220 mg/L as CaCO₃ (Very hard).
Ogallala
Ogallala / High Plains aquifer
The High Plains (Ogallala) aquifer underlies about 174,000 square miles across parts of eight states from South Dakota to Texas, and is predominantly unconsolidated sand and gravel eroded from the Rocky Mountains. (Source: USGS.)
Measured: 218 mg/L as CaCO₃ (Very hard).
Edwards
Edwards & Edwards-Trinity
Two units share this name. The famous Edwards (Balcones fault zone) aquifer serving San Antonio is a distinct USGS unit at the edge of the broader Edwards-Trinity aquifer system — and it is the Edwards-Trinity system that our monitoring attribution reports.
The Edwards (Balcones) is a karst limestone aquifer in south-central Texas and the primary water source for the San Antonio region; the broader Edwards-Trinity aquifer system underlies the Edwards Plateau of west-central Texas. (Sources: USGS, Edwards Aquifer Authority.)
Measured — Edwards-Trinity aquifer system: 290 mg/L as CaCO₃ (Very hard). (No separate median is quoted for the Balcones Edwards unit.)
Wells
What aquifer is my well in?
We can't resolve that from a ZIP, and we won't pretend to. Three honest routes: your well driller's log, your state's well-record database, or the USGS NWIS site record by location. Once you know it, the table above gives the measured context — and see well water hardness.
Data
Data & downloads
- Aquifer hardness CSV — the gated aquifers with median, n, sites, states, GW share (CC BY 4.0).
- Part of the statistics downloads family.
Questions
Frequently asked
- What are the major aquifers in the US?
- The US draws groundwater from dozens of principal aquifers — the Floridan, High Plains (Ogallala), Edwards-Trinity, Central Valley, Coastal Plain systems, and more. The table below ranks the measured ones by hardness.
- Which aquifer has the hardest measured water?
- Among aquifers meeting our data gate, Alluvial aquifers — a broad USGS lithologic category, not one place — show the highest measured median at 370 mg/L as CaCO₃.
- What aquifer is my well in?
- We can't resolve that from a ZIP. Three honest routes: your well driller's log, your state's well-record database, or the USGS NWIS site by location. Then use the table here for context.
- Why do limestone aquifers produce hard water?
- Rainwater is mildly acidic and dissolves calcium carbonate from limestone as it moves through the rock, so carbonate aquifers tend toward hardness. See what water hardness is for the chemistry.
- Is the Floridan aquifer hard water?
- Yes — the Floridan aquifer system's measured median is 220 mg/L as CaCO₃ (very hard), from our monitoring join.
By Marko Visic, MPharm, BSc Physics. Source: USGS/EPA measured hardness joined to USGS aquifer attribution; of stations reporting aquifer attribution — 38.0% of hardness-measured sites. Data last verified . Methodology.
Cite this page: “US Aquifer Map & Measured Water Hardness.” Hard Water Map. Retrieved 2026-07-29, https://hardwatermap.com/us-aquifer-hardness-map/