Well water figures: 5 numbers and what each measures

Key takeaways

• Check the sample date, location, unit, and method before comparing results.
• Use separate tests for specific contaminants.
• Ask for pump test details behind a yield figure.
• Keep lab reports and well records together.

Most of these numbers are not hard to obtain, and many are inexpensive to check. The harder part is knowing what a figure does and does not say. The goal is to make each number clear enough that you can decide what to check next, rather than reacting to one figure taken out of context.

Groundwater is more common than many people realize. A groundwater guide notes that forty-seven percent of the United States depends on groundwater for its basic drinking water supply. Groundwater reaches homes through community systems as well as private wells, so it helps to know which kind of supply you have before you read any number about it. For more on that distinction, read about private and public water supply standards.

Start with the question behind each number

A reading from a kitchen tap can differ from one taken at the wellhead, because pipes, filters, and water heaters can change what comes out. A test from last spring may not match today's water, especially after heavy rain or after nearby digging disturbs the ground.

Before you decide whether a figure is good or bad, find out where the sample came from, when it was taken, who measured it, and what unit the result uses. Most confusion about well water comes from skipping one of those questions.

1. Total dissolved solids (TDS)

Total dissolved solids, usually shortened to TDS, is the combined amount of material dissolved in water. Most of it is minerals such as calcium, magnesium, sodium, bicarbonate, chloride, and sulfate, which water picks up as it moves through rock and soil. Lab reports usually give TDS in mg/L. For water like this, one milligram in a liter is nearly the same as one part in a million, so the two are often used interchangeably.

TDS is a count of everything dissolved, not a list of what is there. Two samples can share a total and have very different chemistry. A low TDS also does not prove the water is safe. Bacteria, nitrate, arsenic, and lead may barely change the total, so each one needs its own test. For help with one of those results, see how to read a private well nitrate result.

Taste is one of the most practical reasons TDS matters. A groundwater guide notes that TDS levels above 1000 mg/L will usually yield poor-tasting water. The same guide says levels above 2000 mg/L are considered undrinkable due to taste, and levels more than 10,000 mg/L are defined as undrinkable. Use those figures as reference points rather than a verdict on your own water. If your result is anywhere near them, ask a certified lab to confirm it and to test the other substances that matter in your area.

The most reliable way to measure TDS is a lab, usually as part of a water panel. Handheld TDS pens are inexpensive and useful for tracking changes in one well over time, but they estimate the total rather than identify each substance.

Some labs measure TDS by evaporating a measured volume of water and weighing what remains. Others estimate it from electrical conductivity. The two approaches generally agree, but the conversion is an estimate, so compare trends rather than exact values when methods differ.

2. pH

pH measures how acidic or basic water is. Each whole step on the scale is a tenfold change, so water one step more acidic has ten times the acidity. Water takes on its pH from the rock and soil around the aquifer and from gases it has dissolved, which is why two wells a short distance apart can read differently.

pH is not a contaminant on its own, but it affects how water behaves in your home. Acidic water can dissolve metals from pipes and taps and can corrode plumbing over time. Water with a high pH can leave mineral deposits and can change how some treatment equipment performs. Neither effect is a reason to panic, but both are worth knowing when you read a number.

Public health agencies publish guidance on acceptable pH ranges for drinking water. Check the current guidance from your state health department before comparing your reading to a target.

pH is one of the few numbers you can measure on the spot. Field meters and test strips are common and cheap, but a lab gives a more reliable result. pH can also shift once water is exposed to air, because dissolved gases escape. A reading taken from a glass that has sat on the counter may differ from one taken at the tap or the source, so collect samples the same way each time.

3. Hardness

Hardness measures mainly the calcium and magnesium dissolved in water. It is usually reported in grains in a gallon or in mg/L, often expressed as calcium carbonate. Hardness describes how water behaves, not whether it is harmful to drink. Hard water leaves a film of mineral scale on sinks, showerheads, and kettles, reduces lather from soap, and can build up inside water heaters and appliances over time.

Hardness is often described in words such as "soft," "moderately hard," or "very hard." Those labels are not standardized, so one seller's "hard" may be another's "moderate." Check the unit before comparing two figures, because grains in a gallon and mg/L must be converted before they can be compared.

Hardness is typically measured on a lab report or with a water test kit. Water softener dealers often run their own tests, which can be a fair starting point, but ask for the actual reading and its unit, not just a recommendation. The reading helps you decide whether a softener is worth installing and how it should be set.

4. Well depth

Well depth is the distance from the ground surface to the bottom of the well, usually in feet. It appears on drilling logs and state well records, and it indicates roughly which layer of rock or sediment supplies the water. Depth describes where the water is drawn, not how good it is. A deep well can have excellent water or poor water, and so can a shallow one.

Depth also helps distinguish a drilled well from a dug well. Dug wells are typically wide and often shallow. A groundwater guide notes that abandoned dug wells do not typically lead to contamination risk for deep aquifers, but their wide diameter creates a physical safety hazard for construction equipment and a danger to people and animals who may fall in. For an old dug well, its width matters as much as its depth.

If you find an old dug well on a property, treat it as a hazard first. Keep people, children, and animals away from it, and bring in a licensed professional before deciding what to do. Do not assume it is harmless because it is old or looks shallow.

Depth figures usually come from the driller's log made when the well was built, or from state well records. Old records are often incomplete, so a missing record does not prove that no well exists. A depth figure on a listing may never have been verified, so ask where it came from. Use this guide to find old well records when tracing a listing's depth figure.

Depth is often paired with the static water level, which is how far down the water sits when the well is not being pumped. The gap between the two describes how much room the well has to draw water. Listings sometimes give depth and skip the water level, which leaves out half the picture.

5. Well yield

Yield is how much water a well can produce, usually in gallons a minute (gpm). It is measured during a pump test, when the well is pumped at a steady rate and the water level is tracked. Yield tells you how quickly a well can supply a household. It says nothing about quality, so a productive well can still produce water that tastes bad or fails a lab test.

A pump test has more to it than one number. The water level drops as water is pumped, and the level it recovers to afterward matters too. A short test can make a well look stronger than it will be over a full day of use. Ask whether a yield figure came from a timed pump test, how long the test ran, and what the water level did during and after pumping.

Yield figures come from a driller's report, a pump test by a licensed contractor, or sometimes the previous owner. Listing numbers are often informal. If someone quotes a high gpm figure with no test details, treat it as a claim rather than a measurement.

Yield also has to match how the water is used. A household running a dishwasher, a washing machine, and several showers at once places a very different demand on a well than one person using the tap now and then. Ask a driller or water professional to compare the measured yield with your expected use, and to explain how that changes if you add irrigation, a pool, or more people in the home.

Where the figures come from

Most of the numbers you will see come from four kinds of sources:

• Accredited labs. A lab report lists the test method, sample location, and date. It is the most dependable source for total dissolved solids, hardness, pH, and specific contaminants.
• Home test kits and meters. These are quick and useful for tracking change, but they usually estimate rather than measure each substance.
• Drilling and well records. Depth, casing details, and yield from driller logs reflect what was measured when the well was built or last tested.
• Trade groups and sellers. Treatment companies publish charts too. A water treatment trade association says it represents more than 2,500 member companies around the globe. That group formed in 1974 by merging two organizations founded in the 1940s, one for equipment dealers and one for manufacturers and suppliers. That history does not make the numbers wrong, but a chart from a trade group or seller may be designed to support a product. Check the underlying test before acting on it.

How to read a figure you found online

Run through a short checklist before acting on any number:

• Date. Water quality can change with the seasons, after heavy rain, or when nearby work disturbs the ground.
• Sample point. Was the water taken at the wellhead, a kitchen tap, or after a filter? Each describes something different.
• Units. Mg/L, parts in a million, and grains in a gallon need care when comparing; check which units require conversion.
• Method. Was the result from a lab using a named method, or from a meter or strip? Check whether a lab marked a result as below its reporting limit, since that means the substance was not found above that limit.
• Source. Who published the number, and do they sell something related to it?

One figure is a snapshot. Several results collected the same way over time tell you far more and make real change easier to spot.

What to do next

If a number worries you, or you have never tested your water, start with a certified lab. Ask for a panel that includes total dissolved solids, pH, and hardness, along with any contaminants your region is known for. Your state health department or county extension office can tell you which tests are worth paying for where you live.

Retest after any event that might affect the water, such as a flood, a repair to the well, or a change to the system.

Illustrative example: comparing well records

Imagine comparing two wells from online listings. The first gives a TDS figure with no date, a yield from a quick check, and a depth with no source. The second includes a lab report with a sample date and named method, plus a pump test summary that records water levels before and after pumping. The second is not automatically the better well, since its water may still taste poor or need treatment. But it gives enough detail to compare like with like. Illustrative TDS readings of 50 mg/L and 70 mg/L would still need that context.

Start your water report folder today

Keep every report in one folder, with the date, sample point, and lab name.

Questions about well water figures

What does one water figure tell me?

Every water figure answers one question about one sample, taken at one place and one time.

How should I use a handheld TDS pen?

Treat a pen reading as a trend, not a final answer.

Should I compare hardness labels or numbers?

A number is more useful than a label.