pH and TDS in well water: how to test them, read the numbers and know what to test next

The short answer: EPA's guideline range for pH is 6.5 to 8.5, and its guideline for total dissolved solids (TDS) is 500 mg/L. Neither is a health limit, and EPA does not enforce either one. They matter because of what they point to. A low pH is the classic sign of water that can pull copper and lead out of your plumbing. A rising TDS says something in the water changed, and a high one points to a follow-up test (chloride, sodium, sulfate or hardness) that depends on where you live and what you notice.

What neither number can do is tell you the water is safe. A TDS of 200 says nothing about bacteria, nitrate, arsenic or lead. That's why EPA's yearly advice for private wells lists four tests, not two: "Test your private well annually for total coliform bacteria, nitrates, total dissolved solids, and pH levels" (EPA, Protect Your Home's Water, updated February 26, 2026).

What pH and TDS actually measure

NDSU Extension's guide for well owners puts it plainly: "The pH of water is a measure of acidity or alkalinity. The pH is a logarithmic scale based on a measure of the free hydrogen ions in the water. The scale runs from 0 to 14, where 7 is considered neutral, 0 to 7 is acidic and 7 to 14 is alkaline" (NDSU WQ1341, revised February 2022). Logarithmic means the steps are not even, so a one-point drop is a bigger change in the water than it looks on paper.

The U.S. Geological Survey calls pH "an important control on the solubility of many metals" in its national study of private wells (USGS Circular 1332, 2009).

TDS is a total, not a substance. NDSU's meter guide calls it the water's dissolved minerals, "commonly referred to as total dissolved solids (TDS) or sometimes as total dissolved salts" (NDSU WQ1923, revised January 2025). USGS lists what goes into it: "major ions, such as calcium, sodium, bicarbonate, sulfate, and chloride; nutrients, such as nitrogen and phosphorus; and trace elements, such as selenium, chromium, and arsenic." Most comes from "the dissolution of soil and aquifer materials," but it "can be augmented by the effects of human activities, such as irrigation, waste disposal, and road salting." In the lab, NDSU explains, the standard method is "evaporating all the water from a 0.1-liter sample and weighing the residual minerals." A home meter does something different, covered below.

EPA's numbers, word for word

EPA's secondary standards are "non-mandatory water quality standards for 15 contaminants." EPA "does not enforce these," sets them "as guidelines to assist public water systems," and says they "are not considered to present a risk to human health at the SMCL" (EPA, Secondary Drinking Water Standards, updated May 26, 2026). Federal drinking water rules apply to public water systems, not private wells, so for a well owner these are reference points. The rows that matter here, with EPA's "noticeable effects" copied exactly:

Not every agency draws the same line. NDSU's quick-reference table lists pH "6.0 to 9.5" and TDS "Less than 1,500 mg/l," and its text says "The EPA recommends that water containing more than 500 mg/l of dissolved solids not be used if other less mineralized supplies are available. However, water containing more than 500 mg/l of TDS is not dangerous to drink." Penn State Extension uses EPA's numbers: "Water with a pH lower than 6.5 or greater than 8.5 can cause corrosion of lead and copper from household plumbing or bad tastes," and TDS "should be below 500 milligrams per liter (mg/L)" (Penn State, Testing Your Drinking Water). This guide uses EPA's figures, the federal reference values.

Testing frequency varies too. EPA says annually. Penn State says pH and TDS "every three years" and compares them to "a doctor taking your temperature." NDSU lists "Bacteria, pH, nitrate and total dissolved solids" under "Each year" for existing wells.

How often well water falls outside the range

In the USGS survey of about 2,100 private wells, sampled before any treatment, "pH in 80 percent of the samples ranged from 6.1 to 7.6, with a median of 7.3." Water from "about 14 percent of the wells was more acidic than recommended, and water from about 2 percent of wells was more alkaline than recommended." When a well misses the pH range, it's usually on the acidic side.

Geography explains much of it. USGS found that "Low pH values occurred in the East," mostly in "crystalline-rock and coastal plain sand aquifers, which do not weather easily and have relatively little capacity to neutralize acid precipitation." TDS ran the other way: above 500 mg/L "in 15 percent of the domestic wells," most often "in the West, Southwest, and south-central United States." In North Dakota, NDSU writes, "very few water supplies" are under 500 mg/L, and many households use water "up to 2,000 mg/l and greater."

Well type matters too. A Virginia study of 2,146 samples from private systems found 26 percent of households had a pH outside 6.5 to 8.5, and 89 percent of those readings were below 6.5 (Pieper et al., Journal of Water and Health, 2015). Shallow sources were the most acidic:

Low pH: why acidic well water is a plumbing problem first

The acid itself isn't the main worry. What it dissolves on the way to your glass is. EPA explains that "the corrosion of iron and copper, may stain household fixtures and impart objectionable metallic taste and red or blue-green color to the water supply." Penn State's list of symptoms: "Bluish-green stains in sinks," "Metallic taste to water, especially first water in the morning," and "Small leaks in plumbing system."

Penn State's symptom table ties those signs to "Corrosive water generally caused by low pH and low total dissolved solids." That second half surprises people: a very low TDS is not a gold star. EPA says lead can enter drinking water "especially where the water has high acidity or low mineral content that corrodes pipes and fixtures" (EPA, Basic Information about Lead in Drinking Water, updated June 18, 2026). USGS saw the pairing in the field: its low-pH aquifers "also had low concentrations of dissolved solids."

The lead connection

Penn State's lead guide: "acidic water that has a pH less than 7 and that is low in calcium carbonate is more corrosive than water that has a pH higher than 7 and that is high in calcium carbonate" (Penn State, Lead in Drinking Water). Vermont's Health Department lists "Low pH (acidic) and low alkalinity levels in your water" among the things that make lead more likely, alongside a shallow system such as a dug or bored well or spring (Vermont Department of Health, Lead in Drinking Water, updated March 17, 2025).

The Virginia data agree. Lead "concentrations were negatively correlated with pH values," and homeowners who reported obvious signs of corrosion, "blue-green staining on plumbing fixtures" or a metallic taste were "1.7–2.8 times more likely to have elevated lead concentrations." Still, stains are not a lead test. Penn State says they "are not reliable indicators of the risk of lead in water," and "You cannot see, smell, or taste lead in water."

So treat a low pH as a reason to order lead and copper tests, sampled the way lead needs: a first-draw sample after water has sat in the pipes, plus a flush sample. Vermont: "A first draw sample will tell you if there is lead in your plumbing inside your home. A flush sample will help you find out if the lead is coming from the pipes outside your home." Our lead guide walks through both, and the flush result decides whether a neutralizer is even an option. EPA's trigger table agrees: for "Corrosion of pipes, plumbing," test "Corrosion, pH, lead."

High pH: scale, taste and a few side questions

High pH is less common in wells (about 2 percent in the USGS survey). EPA's effects above 8.5 are "slippery feel; soda taste; deposits," and it lists pH among the standards "related to scale and sediments." Scale, in EPA's words, "is a mineral deposit which builds up on the insides of hot water pipes, boilers, and heat exchangers."

A high reading isn't automatically fine. Penn State's warning covers both ends: water "greater than 8.5 can cause corrosion of lead and copper." NDSU notes that alkalinity "generally is associated with high pH values, hardness and excessive dissolved solids," that such water "may have a distinctly flat, unpleasant taste," and lists treatment as "ion exchange via the addition of a tank media or reverse osmosis."

One geology footnote: USGS describes "arsenic release from iron oxides, which is favored by low dissolved oxygen concentrations and high pH." That doesn't mean a high-pH well has arsenic. Our suggestion is simply not to skip the arsenic test because the pH looked unremarkable.

TDS: what the number includes and what it can't tell you

A TDS result can't say which minerals make up the total. It is useful in three ways:

• Against EPA's 500 mg/L guideline, set, USGS says, "to avoid unpleasant taste or color or greater costs because of mineral or sediment deposits in plumbing."
• Against your last result. Penn State, writing about TDS tests in general, says the value "should not change much from one test to the next. Increases in the TDS of water could indicate pollution has occurred, warranting further, more detailed testing."
• As a pointer to the next test, in the table below.

It can't detect the things that matter at tiny concentrations. CDC lists TDS and total coliforms as separate yearly tests (CDC, Guidelines for Testing Well Water, July 1, 2024), and a measure of dissolved minerals can't count living bacteria. EPA's nitrate limit for public systems is 10 mg/L as nitrogen, 2 percent of a 500 mg/L reading. The arsenic limit is 0.010 mg/L. The lead action level is 0.015 mg/L, dropping to 0.010 mg/L from November 1, 2027 (EPA, National Primary Drinking Water Regulations, updated August 31, 2026). Lead at 0.015 mg/L is three thousandths of one percent of 500 mg/L. Our test kits vs. lab guide covers this in more depth.

When a high TDS calls for a specific follow-up test

EPA's private-well table and Penn State's tables match situations to tests:

NDSU adds that "An increase in the normal chloride content of water may indicate possible pollution from human sewage, animal manure or industrial wastes." If your TDS jumped and nothing fits, ask your county or state health department what is common in your area. The which-tests checklist builds a list from EPA and CDC triggers, and the hard water guide covers hardness.

How to test: lab, strips or a meter

For a result you will act on, use a certified lab. EPA's advice for a well that may be near contamination is to "have your water tested by a certified laboratory." NDSU's "routine water analysis" for new wells already bundles pH and TDS with sodium, nitrates, calcium, iron and hardness. Its sampling tip: a "raw" sample is preferred, so bypass softeners, reverse osmosis units and iron filters, and "A second sample taken after the water has passed through the treatment equipment will help you determine if your equipment is functioning properly."

pH strips and liquid kits

The best evidence we found is a 2024 Virginia Tech study of low-cost kits (Kriss and Edwards, ACS ES&T Water, 2024). In the lab, "liquid and color tile strip pH tests are relatively accurate (R2 > 0.99)." In a home over about 1.5 years, "the liquid test (R2 = 0.978) was more accurate in the field than the color tile strip (R2 = 0.623). Therefore, the liquid test is recommended for residential use." Funders included the Copper Development Association and EPA. It's one study, and the field-phase home was on a municipal utility, not a private well, so read it as a signal, not a ranking.

TDS meters: what the display is really doing

A pocket TDS meter doesn't weigh anything. NDSU: "The EC is a proxy measurement to determine the TDS in water," and "The meter calculates an estimate of TDS by multiplying the EC reading with a conversion factor." EC is electrical conductivity, and the right factor depends on the water:

In practice: NDSU's table puts water at EC 1,000 µS/cm at about 650 mg/L (factor 0.65). A meter using a factor of 0.50 would show 500 for the same water, right on EPA's line instead of above it. That's our arithmetic, not a test of any product. NDSU adds that "EC meters must be calibrated regularly," checked against a calibration solution "prior to sampling," and recalibrated "each time batteries are changed or after the meter has been dropped on a hard surface." The publication is written mainly for livestock water.

A meter is fine for spotting change. If last year's lab report said 320 mg/L and the meter now shows a big jump, send a sample to a lab, in line with Penn State's point about rising TDS.

Reading your results: a decision table

Use this with a lab report, top to bottom. Each action is credited to the source that says it.

The results checker compares each number on your report with EPA's benchmarks.

Worked example 1: pH 6.1, TDS 90 mg/L, blue stains in the tub

Illustrative numbers, not a real report. Low pH plus stains puts you in row 3, and the low TDS matches row 11, Penn State's pairing of low pH and low dissolved solids. Order lead and copper with first-draw and flush samples. Say the first draw shows lead and the flush doesn't: by Vermont's logic the lead is in the plumbing inside the house, so you move to row 5, where a neutralizer is a candidate once alkalinity is known; the lead guide covers what to do about the first-draw result meanwhile. Lead in the flush sample would put you in row 4 instead.

Worked example 2: pH 7.4, TDS 780 mg/L, near a salted highway

The pH is fine. TDS is over 500 and the setting matches EPA's salted-road row, so row 7: add chloride and sodium. Chloride over 250 mg/L is EPA's "salty taste" line, and Penn State calls chloride "very difficult to remove from water." If anyone in the house is on a low-sodium diet, share the sodium result with their doctor.

Worked example 3: pH 8.8, TDS 420 mg/L, white crust on faucets

TDS is under 500, but pH is above 8.5, so row 6. Ask the lab for alkalinity and hardness, and keep lead and copper on the list if the plumbing is metal.

Fixing low pH: neutralizers and soda-ash feed

Penn State describes two whole-house approaches (Penn State, Corrosive Water Problems, updated May 29, 2025). Acid-neutralizing filters "are intended to increase pH and add calcium, thereby decreasing corrosivity. They consist of a tank filled with calcium carbonate (limestone) chips, marble chips, magnesium oxide, or other alkaline material," usually "installed after the pressure tank." The trade-offs: "this process will increase the hardness of the water, but this is necessary for proper corrosion control," "the resistance of the neutralizing material may lower water pressure," and the tank "must be routinely refilled," at a rate that "can range from weeks to months." Penn State's lead guide adds that the added hardness "produces a thin scale inside the pipes that reduces lead corrosion," and that the units "normally cost over $1000."

Chemical feed means "injecting a sodium hydroxide or soda ash solution using a chemical feed pump before the pressure tank." Penn State says it "does not increase water hardness" and avoids the pressure drop, but needs "significant maintenance." Soda ash "is preferred over sodium hydroxide because it is safer to handle." Vermont names "marble, calcium carbonate, soda ash or lime" as the most common neutralizing media (Vermont Department of Health, How to Treat Your Drinking Water, updated September 21, 2026).

Penn State also lists replacing metal plumbing with approved plastic pipe, and point-of-use distillation, reverse osmosis or activated alumina for copper and lead at the tap. Vermont notes that anion exchange units "lower the pH, which can make the treated water more corrosive," and that neutralization "is usually required after installing a POET reverse osmosis or anion exchange system." The treatment-by-result guide sets these beside other technologies.

Lowering TDS: reverse osmosis and distillation

High TDS is mostly a taste and scale problem, and the fix depends on what it's made of. If it's mostly hardness, see the hard water guide. For the total itself, EPA (writing for public systems) says "distillation, reverse osmosis, and electrodialysis are effective for removal of chloride, total dissolved solids, and other inorganic substances," but calls them "fairly expensive." NDSU's advice for North Dakota homes: "Treatment for household use is reverse osmosis."

Both usually treat one tap. Vermont describes reverse osmosis as "installed as a separate tap by the kitchen sink," with efficiency "only about 25%, which means that for every 1 gallon of treated water, there will be 3 to 4 gallons wasted." Hard water "can cause scale on RO membranes," and hardness generally "should not be higher than 120 to 170 mg/L"; look for NSF/ANSI Standard 58 certification. Distillers "boil water and then condense the steam," leaving the contaminant behind "along with hardness and other minerals," use "a lot of energy," and should carry NSF/ANSI Standard 62 certification.

For salty water, Penn State recommends "Reverse osmosis or distillation systems at individual taps. As chloride is very difficult to remove from water, consider developing new source of water."

Where to go from here

If all you have is a strip or meter reading, get a lab baseline with pH, TDS, bacteria and nitrate, plus lead and copper if the pH is low or you see blue-green stains. The lab finder lists labs from official state certification lists, and the smell, taste and stains guide matches symptoms to tests. Where a result touches health (a baby on formula, pregnancy, a low-sodium diet), ask your doctor or health department what it means for you.

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