How to Read Your Well Driller's Log

Somewhere in your house records — or in your state agency's files — sits the most useful document your well will ever have: the driller's log. It records how deep the well goes, what the driller hit on the way down, how much water it makes, and how it was sealed. Most homeowners never read it. Then something goes wrong and the first thing the service tech asks is, "Do you have the well log?"

This guide walks through every section of a typical log in plain English: what each entry means, what numbers are good, and the five things worth checking on yours. It's also called a well completion report or well construction report — same document, different states' naming.

What you're holding

A driller's log is the written record of how your well was built: depths, materials, water levels, and test results. States take it seriously. Nebraska's well-log form, for example, requires the driller to record the well's legal location, every geologic material encountered and its depth, the constructed and excavated depths and diameters, gravel-pack depth and particle size, grout depth and thickness, casing length/diameter/wall thickness/material, screen details (length, slot size, material), static and pumping water levels with the test rate and duration, the well yield, and the contractor's signature and license number.

In Delaware, the contractor has to file a Well Completion Report with the state environmental agency within 30 days of finishing the well — and the owner should get a copy for future reference. In Michigan, the drilling contractor is required to give you a copy of the well log and keep one themselves. Rules vary by state, but the pattern is the same everywhere: the log exists, it belongs with your house records, and a copy usually lives with the state agency too.

If you're buying a home, ask the seller or the local health department for the well record. One caveat: records for older wells may not exist. In Virginia, for instance, private-well records weren't required before 1990 — so a 1970s well may simply have no paperwork.

Static water level — the first number to find

The static water level is the distance from the ground surface down to the water surface in the well when the well is not being pumped — the level the water settles to with nothing acting on it. On the log it may appear as "static water level," "pre-pumping static level," or "SWL."

This number is the reference point for everything else. A shallow static level means water sits close to the surface; a deep one means the pump has farther to lift. When a driller talks about how a well "performs," they're usually talking about how far this level drops under pumping — which is the next section.

One detail worth checking: what the depth is measured from. Some logs measure from the ground surface, others from the top of the casing. If you're ever comparing a new measurement to the log, make sure you're using the same reference point.

Pumping water level and drawdown — the two-number test

When the driller finished the well, they pumped it and measured where the water level settled while pumping. That's the pumping water level — always deeper than the static level.

The difference between the two is drawdown:

Drawdown = pumping water level − static water level

If the static level was 50 feet and the pumping level was 75 feet, the drawdown is 25 feet. That's the single most informative number on the log about how the well behaves under load: small drawdown means the aquifer feeds the well easily; large drawdown means the well is working hard for every gallon.

A related piece of arithmetic worth doing yourself: total depth minus static water level tells you how many feet of water column stand in the well. A 300-foot well with a static level at 60 feet holds 240 feet of water — and in a 6-inch well, each foot of water column stores about 1.5 gallons (about 1 gallon per foot in a 5-inch well).

Yield — what the gpm number actually proves

The log's "test rate" is the rate, in gallons per minute, at which water was pumped out of the well during the driller's test. A good log also shows how long the test ran and what the drawdown was at the end. Older forms may call it a bailing or pumping test; newer ones say "well test." Either way, it's one number: gpm, plus the conditions.

What counts as adequate for a home? The benchmarks that matter:

• 3 to 5 gallons per minute is adequate for most domestic water-supply systems.
• New Hampshire's well board frames it as volume over time: a minimum domestic supply of 600 gallons within a two-hour period — the equivalent of 5 gpm for two hours. (An industry alternative: 4 gpm for four hours.)
• If you have an FHA loan, the lender's rule applies: 5 gpm continuous for at least 4 hours on new construction, 3 gpm on existing homes — and a lower-yielding well can still qualify with enough pressurized storage (720 gallons over 4 hours for an existing dwelling).

Available supply is a function of both recovery rate and storage in the well bore, which is why the per-foot storage figures above matter: a low-yield well with a deep water column can still serve a house fine, because the bore itself is a reservoir.

Two honest caveats. First, test duration and minimum yield are set by state or local code and by lenders — they vary, so don't treat any single number as universal. (As one example of how specific this gets, one New York town code requires a 4-hour test with a minimum sustained yield of 5 gpm.) Second, the number on the log is the yield on test day, under the driller's conditions — useful, but not a promise about every August.

Total depth and casing — the hardware

Total depth is the full drilled depth, surface to bottom of the borehole. Everything else on the log is measured against it.

Casing is the pipe that lines the well — usually steel or PVC. It does three jobs: it keeps the hole from caving in, it keeps surface water out of the groundwater, and (with grout, below) it keeps water confined to its zone so aquifers don't mix. The log records the casing's length, inside diameter, wall thickness, and material. Which material the driller used mostly follows local geology — hard-rock regions tend to be "steel states," for example.

Two things to check on your log:

1. How deep the casing goes. In wells drilled through loose sand and gravel, casing typically runs the full depth with a screen at the bottom. In hard-rock wells, the casing runs into the top of the rock and is sealed there — and no screen is needed below it.
2. The stick-up. The casing and cap should extend at least 12 inches above the ground surface (more near flood zones). If yours doesn't, that's a cheap fix with real contamination-prevention value.

Screen or open hole?

Below the solid casing, the well finishes one of two ways — and the log tells you which:

• Screen: a slotted section of pipe that lets water in while blocking sand and soil. Required in unconsolidated materials like sand or gravel. The log records the screen's length, slot size, and material, plus any sand/gravel filter pack around it.
• Open hole: in solid bedrock that won't cave, the well is finished open below the casing — no screen, with water entering through fractures in the rock.

This is also why two neighbors' logs can look completely different. In bedrock, water lives in fractures, cracks, and solution cavities, so yields vary wildly over short distances — your neighbor's 20 gpm says nothing about yours. Sand-and-gravel aquifers are more consistent.

The check that matters: the finish should match the geology. Sand/gravel zones should show a screen; a bedrock well is normally open hole.

The formation log — reading the geology column

This is the part of the log that looks like a core sample in text form: a list of every material the driller went through, with the depth at which each layer ended. Depths are cumulative from the surface — "clay, 0–28 ft; sand, 28–64 ft; limestone, 64–210 ft" and so on. Entries usually name the material and sometimes the color or texture, often in the driller's own shorthand.

Read it with the right expectations: these are field descriptions, not lab geology. Drillers aren't required to produce geologically precise descriptions, and professionals are advised to treat lithology notes as qualitative. USGS scientists routinely have to convert driller descriptions into assumed values for their models — which tells you exactly how approximate the raw notes are. Use the formation log for the big picture (where the water-bearing zones are, how deep the rock starts), not for fine distinctions.

One genuinely useful reading trick: compare the static water level to the depth of the water-bearing zone. If the log shows water-bearing rock at, say, 167–182 feet but the static level sits at 11 feet, the well taps a confined (artesian) aquifer — water rises in the well far above the aquifer itself, in proportion to the pressure down there. That's a sign of a well-protected supply, and it explains static levels that look "too shallow" for the depth of the well.

Grout and the annular seal — the part that keeps water clean

The log should record the depth and thickness of grout or other sealing material. This is the annular seal: grout pumped into the space between the outside of the casing and the borehole wall. Its job is to stop surface water and shallow contaminants from traveling down along the outside of the casing into the aquifer — and to stop water from migrating between aquifers along the well. The National Ground Water Association calls grouting "an effective and necessary measure to protect public health and the quality of our groundwater supplies" and holds that it should be part of every state's well construction code.

Typical materials are neat cement or bentonite grout, placed from the bottom of the sealed interval upward. How deep the seal must go is state-specific and varies widely — anywhere from 10 feet to more than 100 feet depending on conditions. Washington, for example, requires a minimum 18-foot surface seal.

The red flag is simple: a log with no grout or seal entry at all, or a seal far shallower than your state's minimum, deserves a follow-up with the driller or the state agency. An unsealed annulus is a direct contamination pathway — surface water running down the outside of your casing, straight into the aquifer.

Pump setting depth — where the intake sits

The log may note the depth the pump (or its intake) is set at. If the driller didn't install the pump, this may not be on the log at all — the pump installer is a different trade on many jobs.

The rules of placement, wherever they're written down:

• The water level must always stay above the pump intake. If pumping draws the level below the intake, the pump sucks air.
• The intake sits above the screen (or above the open-hole interval), not at the very bottom of the well. Wisconsin's code, for example, requires the intake at least 5 feet above the bottom of the casing or the top of the screen in unconsolidated formations — and pump-test procedures call for setting the test pump immediately above the screen.

Why this matters to you: compare the pump depth against the static level, the pumping level, and the total depth. That comparison is your margin. If the pumping level on test day came within a few feet of the intake, the well has little headroom for a dry year or a bigger household — worth knowing before you add that irrigation zone.

The five-minute homeowner checklist

Pull out your log (or request it from the state agency) and check these five:

1. Yield vs. need. Is the test yield in the 3–5 gpm range, or does well-bore storage make up the difference? Below that, plan around conservation or storage — and know your lender's minimum if you're buying.
2. Water-level margins. Static level vs. pump setting vs. total depth: how many feet of water stand above the intake, and how close did the pumping level get to it on test day?
3. Seal present. Does the log show grout depth and thickness consistent with your state's minimum? No entry is the red flag.
4. Finish matches geology. Sand/gravel should show a screen; bedrock is normally open hole.
5. Completeness. Missing casing, screen, grout, or test data makes every future service call harder. A complete log is part of what a good driller delivers — and it's worth chasing down from the state files if yours is thin.

Then keep the log with your house records — with your water-test results, maintenance receipts, and treatment-system manuals — and hand it to the next owner when you sell. It's the one document that makes every future well conversation shorter.