Our Methodology

How we verify labs

We check every lab in two layers, and a lab must pass both.

The registry layer. State programs certify the labs that test drinking water in each state. In Michigan, EGLE's Laboratory Certification Program certifies labs for the analysis of drinking water, and in Pennsylvania the Department of Environmental Protection accredits environmental labs that test drinking water. Ohio EPA publishes a combined list of labs certified to analyze public drinking water. We read certifications from these state lists, not from lab marketing. For every list we record where we got it, the date the document carries (Ohio's list is dated by month; Michigan's microbiological list carries no revision date, so we rely on our own fetch date), the date we fetched it, and a fingerprint of the file (a SHA-256 hash), so we can tell when a list changes.

The homeowner layer. A certified lab does not always take samples from homeowners. We check that on the lab's own website and record how strong the evidence is: whether we fetched the lab's page and saw it ourselves, only have a search snippet, or the lab told us by phone. Each of these facts gets a check-by date when we verify it; once that date passes without a new check, the fact counts as unknown, and the lab is not shown.

Where prices come from

Every price we show comes from the lab's own website or from our contract with the lab, and each one is stored with its source and the date we checked it. When a price passes its check-by date without being confirmed again, we hide it rather than show a number that may be out of date. A lab that does not publish a price stays in your list as "price on request".

How we choose labs for your plan

We first apply hard rules, in this order. A lab that fails one is left out, and we record why.

1. The lab is active.
2. The lab accepts home samples, and our evidence for that is still fresh.
3. The lab is certified by your state to test drinking water, and for each test that has a state certification of its own: for bacteria, the microbiological certification; for chemicals, the certification group that covers it.
4. The lab can reach you: it accepts mail-in samples from your state, or it is close enough to your ZIP code to take your sample locally, by drop-off or local collection.
5. Bacteria samples have a short holding time. Federal rules for public water systems allow 30 hours^[5] from collection to the start of the test, and labs work to that window. So for a bacteria test we keep mail-in labs to your state or a neighboring state, unless the lab publishes its own shipping window for bacteria samples, which then takes priority.
6. The lab's tests cover everything in your plan, either as single tests or as a panel. We pick the lowest-cost combination.

The labs that remain are ranked three ways: best value (the lowest total price), easiest (mail-in with shipping included, then lowest price) and fastest (the shortest stated turnaround, then lowest price). A lab with no published price is never shown as best value.

WTA's margin and whether a lab is a partner are never inputs. The ranking code has no access to what we earn.

Which benchmarks we use

We compare results against published benchmarks, and each one carries its status as of the date we last checked it. From most to least binding:

• Federal maximum contaminant level (MCL). EPA's National Primary Drinking Water Regulations are legally enforceable standards that apply to public water systems. Arsenic's MCL, for example, is 0.010 mg/L^[6].
• Action level. For lead and copper the rule sets an action level that triggers treatment steps instead of an MCL. Lead's action level is 0.015 mg/L^[6], and a lower level of 0.010 mg/L (effective Nov 1, 2027)^[8] is set for the future; we show both.
• Secondary standard (SMCL). EPA sets non-mandatory standards for 15^[7] contaminants that affect taste, odor or appearance, and does not enforce them.
• Maximum contaminant level goal (MCLG). EPA defines this as the level below which there is no known or expected risk to health. It is a non-enforceable public health goal, and for some contaminants, such as lead, it is 0 mg/L^[6].
• Health advisories and state limits. Where our benchmark data holds a health advisory or a state's own limit, we show it next to the federal values, with its source.

Private wells are not covered by these federal rules, so for well water they serve as reference points, not legal requirements. Every value lives in one versioned data set, listed at the end of this page.

How we read your results

When you check your water test results, each value gets a plain-words band: not detected, below the guideline, approaching it, above a health guideline, or above the legal limit.

Only a legal limit that is in force today can put a result "above the legal limit". A future, proposed or litigated limit can only put it "above a health guideline". "Approaching" means at least half of the lowest in-force legal limit or health advisory for that contaminant; we don't use an MCLG of zero or a secondary standard for this, because either would flag almost any detection.

If the lab's reporting limit is higher than the benchmark and the result is not detected, we tell you the test can't show whether your water is below that benchmark. For bacteria, total coliform present is a caution: disinfect and retest. E. coli present is urgent: don't drink the water until a retest is clear.

If you upload a lab report, software only reads the values, and you confirm every row before anything is interpreted. The bands and the advice come from our versioned rules, never from that software.

Lab profiles and state pages

Lab profiles in the certified lab directory are data pages built only from facts verified under this methodology, so this methodology is their byline, not a named reviewer. A fact that is missing its source or check date is not shown. State pages, such as water testing in your state, carry written guidance as well as lab facts, so each one is reviewed as its own page.

What our lab boxes promise

Every lab box lists four claims, explained below.

• Labs certified by your state for this test. A lab appears only when it meets the certification rule under "How we choose labs for your plan". A box for several tests says Labs certified by your state for the tests you need. A box in which no test has a state certification of its own, such as hardness, says Labs certified by your state to test drinking water.
• The lab is named before you order. You always see which lab will run your test before you pay, whether you order through us or from the lab directly.
• Prices include shipping when the lab publishes it. We mark shipping as included only when the lab's own price says so; otherwise it may be extra.
• Results explained against EPA and state benchmarks. Your results are compared with the benchmarks listed above, including a state's own limit where our data holds one.

Data versions

Our test plans, benchmarks and result explanations come from versioned data sets. When one changes, its version and date change here.