Private Well WaterA field guide to the water under your house
For the 43 million Americans on a private well

In a private well, the rock is the usual suspect.

Nobody tests a private well for you. When the U.S. Geological Survey did, the contaminants it most often found above health limits were mostly ones the ground put there. So this guide is organized the way groundwater is: by the rock it sits in.

Matches your address against the USGS Principal Aquifers map. Nothing is stored.

23%of about 2,100 private wells USGS sampled had at least one contaminant above a human-health benchmark.
All but nitrateof the contaminants most often over the line came mainly from natural sources: the rock and sediment around the well.
60% vs <1%Man-made chemicals were detected in 60% of wells, but above a health benchmark in fewer than 1%.
USGS Circular 1332 · wells sampled 1991–2004 · source
Plate 1Three ways inSchematic · not to scale
Cross-section of a private well A house, its septic field, a farm field and a shop sit on the land surface. Below are soil, dry sand, the water table, water-filled sand and gravel, a clay layer and fractured bedrock. The well casing runs through the sand into the bedrock, where water enters the open borehole through fractures. Arrows show contaminants reaching the water from the land above, dissolving out of the rock, and from the house plumbing. water table sand & gravel, full of water dry sand clay fractured bedrock casing ends open hole 1 1 2 3
  1. 1
    From the land aboveSeptic fields, farm fields, fuel and solvents soak down to the water table: Nitrates, PFAS, Bacteria, VOCs, Pesticides.
  2. 2
    From the rock itselfWater sitting in rock for years dissolves what the rock is made of: Arsenic, Radon, Iron & Manganese, Uranium, Hardness, Chromium-6, Fluoride, Radium, Hydrogen Sulfide, Sulfate.
  3. 3
    From your plumbingPicked up after the water leaves the well: Lead. Near the coast, heavy pumping can also pull in seawater.
A deep well cased through the sand is sealed off from the surface, which is why it is often cleaner of nitrate and bacteria — and also why it spends its life in contact with the rock. Which rock that is depends on where you live.
Plate 2What the country's wells are drilled into63 USGS principal aquifers
Ada-Vamoosa aquifer — SandstoneArbuckle-Simpson aquifer — Limestone & dolomiteBasin and Range basin-fill aquifers — Sand & gravelBasin and Range carbonate-rock aquifers — Limestone & dolomiteBiscayne aquifer — Limestone & dolomiteBlaine aquifer — Limestone & dolomiteCalifornia Coastal Basin aquifers — Sand & gravelCambrian-Ordovician aquifer system — SandstoneCastle Hayne aquifer — Limestone & dolomiteCentral Oklahoma aquifer — SandstoneCentral Valley aquifer system — Sand & gravelCoastal lowlands aquifer system — Semiconsolidated sandColorado Plateaus aquifers — SandstoneColumbia Plateau basaltic-rock aquifers — Crystalline & volcanicColumbia Plateau basin-fill aquifers — Sand & gravelDenver Basin aquifer system — SandstoneEarly Mesozoic basin aquifers — SandstoneEdwards-Trinity aquifer system — Sandstone & carbonateFloridan aquifer system — Limestone & dolomiteHawaiian Sedimentary deposit aquifers — Crystalline & volcanicHawaiian Volcanic-rock aquifers — Crystalline & volcanicHigh Plains aquifer — Sand & gravelJacobsville aquifer — SandstoneKingshill aquifer — Limestone & dolomiteLower Cretaceous aquifers — SandstoneLower Tertiary aquifers — SandstoneMarshall aquifer — SandstoneMississippi River Valley alluvial aquifer — Sand & gravelMississippi embayment aquifer system — Semiconsolidated sandMississippian aquifers — Sandstone & carbonateNew York and New England carbonate-rock aquifers — Limestone & dolomiteNew York sandstone aquifers — SandstoneNorthern Atlantic Coastal Plain aquifer system — Semiconsolidated sandNorthern Rocky Mountains Intermontane Basins aquifer system — Sand & gravelOrdovician aquifers — Limestone & dolomiteOzark Plateaus aquifer system — Limestone & dolomitePacific Northwest basaltic-rock aquifers — Crystalline & volcanicPacific Northwest basin-fill aquifers — Sand & gravelPaleozoic aquifers — Sandstone & carbonatePecos River Basin alluvial aquifer — Sand & gravelPennsylvanian aquifers — SandstonePiedmont and Blue Ridge carbonate-rock aquifers — Limestone & dolomitePiedmont and Blue Ridge crystalline-rock aquifers — Crystalline & volcanicPuerto Rico North Coast Limestone aquifer system — Limestone & dolomitePuerto Rico south coast aquifer — Sand & gravelPuget Sound aquifer system — Sand & gravelRio Grande aquifer system — Sand & gravelRoswell Basin aquifer system — Limestone & dolomiteRush Springs aquifer — SandstoneSeymour aquifer — Sand & gravelSilurian-Devonian aquifers — Limestone & dolomiteSnake River Plain basaltic-rock aquifers — Crystalline & volcanicSnake River Plain basin-fill aquifers — Sand & gravelSoutheastern Coastal Plain aquifer system — Semiconsolidated sandSouthern Nevada volcanic-rock aquifers — Crystalline & volcanicSurficial aquifer system — Sand & gravelTexas coastal uplands aquifer system — Semiconsolidated sandUpper Cretaceous aquifers — SandstoneUpper Tertiary aquifers — SandstoneUpper carbonate aquifer — Limestone & dolomiteValley and Ridge aquifers — Sandstone & carbonateValley and Ridge carbonate-rock aquifers — Sandstone & carbonateWillamette Lowland basin-fill aquifers — Sand & gravelHawaiiPuerto Rico (2×)
  • Sand & gravel
  • Semiconsolidated sand
  • Sandstone
  • Limestone & dolomite
  • Sandstone & carbonate
  • Crystalline & volcanic
USGS principal aquifers cover about 59% of the lower 48. The blank land isn't dry: wells there draw from local rock too small or patchy for the national map. Tap an aquifer to open its page.

Unconsolidated sand and gravel

Water fills the spaces between loose grains of sand and gravel, usually close to the surface.

Semiconsolidated sand

Sand layers, partly cemented, stacked between clay beds that slow water moving between them.

Sandstone

Sand grains cemented into rock; water moves through the pore space that is left and through cracks.

Carbonate rock

Limestone and dolomite that water slowly dissolves, opening cracks into channels and caves.

Sandstone and carbonate rock

Layers of sandstone and limestone stacked together; water moves through pores in one and channels in the other.

Igneous and metamorphic rock

Granite, gneiss, schist or lava rock: almost no pore space, so water lives only in fractures (or between lava flows).

Most of what matters, you can't taste

The contaminants that announce themselves — stains, smells, scale — are mostly nuisances. Most of the ones linked to cancer, kidney damage or sick infants have no color, taste or smell. A clear, good-tasting glass of well water tells you very little.

You'd never notice

ArsenicFrom the rockCan't see, smell or taste itRadonFrom the rockCan't see, smell or taste itNitratesFrom the land aboveCan't see, smell or taste itPFAS (Forever Chemicals)From the land aboveCan't see, smell or taste itUraniumFrom the rockCan't see, smell or taste itBacteria (Coliform & E. coli)From the land aboveCan't see, smell or taste itLeadFrom your plumbingCan't see, smell or taste itChromium-6 (Hexavalent Chromium)From the rockCan't see, smell or taste itFluorideFrom the rockCan't see, smell or taste itPesticidesFrom the land aboveCan't see, smell or taste itRadiumFrom the rockCan't see, smell or taste it

You might notice

Iron & ManganeseFrom the rockOrange or black stains, metallic tasteHardnessFrom the rockScale on fixtures, soap that won't latherSaltwater IntrusionFrom the seaSalty taste once chloride is highVOCs (Petroleum & Solvents)From the land aboveA fuel or solvent smell, only at high levelsHydrogen SulfideFrom the rockRotten-egg smellSulfateFrom the rockBitter taste at high levels

About this guide

Each aquifer page covers how the water sits in that rock, what it tends to carry, and what to test for. Each contaminant page covers health effects, testing and treatment. Sources are listed on every page; aquifer outlines come from the USGS Principal Aquifers map and PFAS estimates from the USGS national PFAS model. More about the project →