How to read a water well log

To read a water well log, work through it in order: identify the well (location, owner, drill date, driller), then the depth fields (total depth and casing depth), the construction details (casing, screen, grout), the materials encountered at each depth, the static water level, and the yield measured at completion. A log is a dated, site-specific record of how a well was built and what it produced that day, not a promise about the well today.

The fields on a typical well log

Field names and layouts vary by state, driller and era.

Total depth vs. casing depth

Many logs distinguish total depth, how far the hole was drilled, from casing depth, how far the steel or plastic casing extends. Casing, together with grout, keeps out dirt, surface water and lower-quality groundwater, and some states set minimum casing lengths. EPA: Learn About Private Water Wells When you compare wells, compare the same fields; mixing total and casing depth is a common mistake. See well casing and completion. [EPA-LEARN source note]

How do you read the static water level on a well log?

The static water level is how far below a reference point water stood in the finished well when it was not being pumped. Check which reference the driller used, usually the ground surface or the top of the casing. The difference between total depth and static level tells you roughly how much water stood in the hole, which matters for storage and pump setting. Water levels change with seasons, drought and pumping, so treat this number as a dated snapshot. How deep a residential well should be explains why depth and water level are different questions.

How do you read the yield on a well log?

Yield is usually reported in gallons per minute (GPM), measured during a test when the well was completed. Check how the test was done and how long it ran if the log says. A short test may overstate what the well can sustain. For what a meaningful test shows, including drawdown and recovery, see well pump tests, and compare the number with household needs in how many GPM a well should produce.

Reading the materials column

The materials (or lithology) column lists what the drill passed through, such as clay, sand, gravel, shale, limestone or granite, with depths. Notes like "water" or "water-bearing" mark where water entered. This column explains why a well was cased to a certain depth and which formation supplies the water. Groundwater conditions vary with local geology. USGS: Groundwater Wells In fractured rock, water may come only from cracks, so a neighbor's log may not predict yours. Penn State Extension: Water Well Location by Fracture Trace Mapping [USGS source note] [PSU-FRACTURE source note]

What a well log does not tell you

A log is not a water-quality certificate. Private well water quality is the owner's responsibility and is not federally regulated the way public water is. EPA: Private Wells Test the water through a certified lab; see private well water testing. A log also doesn't show what has happened since: repairs, a replaced pump, a falling water table or a damaged casing. [EPA source note]

Comparing an old log with today

If you own the well, compare the original yield and water level with a current test. A large drop can signal clogging, a failing pump or a falling water table; Penn State Extension says a yield decrease of 25 percent or more generally indicates rehabilitation is in order. Penn State Extension: Water Well Maintenance and Rehabilitation See how long water wells last. [PSU-REHAB source note]

When in doubt, ask locally

If a field is unclear or a single number is driving a decision, show the log to the driller who filed it, the state well program, or a qualified local professional. Public lookups like the Texas well-report lookup explain how records were created, but only a site-aware professional can judge whether a reading still applies. Need a log first? See how to find water well logs. [TWDB-WR source note]