How to Read Your Well Water Test Results

Your lab report is back, and it's a page of abbreviations, decimal points, and numbers sitting next to other numbers. Reading it well comes down to three questions per line: what is this thing, does my number beat the limit, and is that limit about health or just about how the water tastes and stains? Answer those and you know what to fix, what to ignore, and what's genuinely urgent — which, once you know which is which, is almost nothing.

If you'd rather type your numbers in and get a plain-English readback, our well water test interpreter walks your exact results against the same EPA limits this guide covers.

The one rule before you read a single number

The EPA's drinking-water standards — the limits you'll see below — apply to public water systems. They do not apply to private wells. Nobody regulates your well; you do. Private well owners are responsible for their own testing, and the public-system standards are the accepted yardstick to measure against — health departments and labs all use them as the reference. When this guide says "limit," read it as "the federal guideline for drinking water."

MCL vs. SMCL: the two kinds of "limit" — learn this once, use it everywhere

The limits on your report come in two flavors, and mixing them up is the most common mistake in reading a lab report:

• An MCL (Maximum Contaminant Level) is an enforceable, health-based standard. It's set for contaminants that can make people sick — bacteria, nitrate, arsenic. Over the MCL means a health risk, full stop.
• An SMCL (Secondary Maximum Contaminant Level) is a non-enforceable guideline for aesthetics — taste, odor, color, staining, scale. Over an SMCL means the water may taste metallic, stain your fixtures, or shorten your water heater's life. It does not mean the water will make you sick.
• Two middle categories you'll meet: an action level (used for lead and copper) is a trigger — when enough samples exceed it, corrective action is required — rather than a hard MCL. A health advisory (used for manganese and sodium) is EPA's non-enforceable guidance on where health effects start showing up.

Rule of thumb: MCLs and action levels get a decision. SMCLs get a fix if they bother you.

The units on your lab report

• mg/L (milligrams per liter) = ppm (parts per million). For water these are interchangeable. Labs report most minerals in mg/L.
• µg/L (micrograms per liter) = ppb (parts per billion). 1 mg/L = 1,000 µg/L. Trace contaminants like arsenic and lead are usually reported in µg/L or ppb — always convert before comparing to a limit stated in mg/L.
• pH units are a unitless 0–14 scale. Hardness may come in mg/L as calcium carbonate (CaCO₃) or grains per gallon (gpg) — see the hardness section for the conversion.
• Non-detect (ND) or "< [value]" means the lab didn't find the contaminant above its detection limit — not that the water contains literally zero of it. The "<" number is the smallest amount the lab's method can reliably see. Penn State's example report shows arsenic at "< 0.003 mg/L" as a passing result, because 0.003 is below the 0.010 standard. Compare the detection limit to the standard, not to zero.

The nitrate trap: "as N" vs. "as NO₃" — read this before you panic

This is the single most misunderstood line on a well-water lab report, so it gets its own section. The EPA's nitrate limit — 10 mg/L — is expressed "as nitrogen" (as N). It counts only the nitrogen atoms in the nitrate molecule. Some labs report nitrate "as NO₃" — the whole molecule, nitrogen plus oxygen — and that number is about 4.43 times bigger for the same water.

The math: 10 mg/L as N equals about 45 mg/L as NO₃ (10 × 4.43 = 44.3, commonly rounded to 45). So if your lab reports "nitrate: 30 mg/L as NO₃," that's about 6.8 mg/L as N — under the limit. Compare the 30 directly against the 10 and you'd panic over passing water.

The same trap applies to nitrite: the MCL of 1 mg/L as N equals about 3.28 mg/L as NO₂. Before comparing any nitrate or nitrite result to its limit, check which basis your lab used — the report should say "as N," "as NO₃," or "nitrate-nitrogen."

What matters most vs. what's just noise

Every parameter on a typical report is in the reference table below, but they don't all deserve equal worry. Our read of the standards (this part is opinion, built on the numbers in the table): the health-based parameters are total coliform/E. coli, nitrate and nitrite, arsenic, and lead — these carry federal health standards and real consequences. Everything else — iron, hardness, TDS, pH, chloride, sulfate, sodium, zinc — is aesthetic or nuisance: worth fixing for comfort and your appliances, but not a health emergency. Two in-betweeners: manganese carries a health advisory above the range where staining starts, and copper has both a health action level and an aesthetic standard.

Fix the health list first. Fix the aesthetics list when you're annoyed enough to pay for it.

The parameter-by-parameter reference

For each parameter: what it is in plain English, the EPA limit, and what to do when your number is over. Treatment notes are categories, not product picks. The "NSF/ANSI" numbers are certification standards a product can carry — but certification is per-model and per-claim, so "NSF certified" alone doesn't tell you what a filter reduces. Check the product's specific listing.

Parameter	What it means (plain English)	EPA limit	What to do if it's high
Total coliform	A broad bacteria family used as an indicator — its presence means there's a pathway for disease-causing microbes to reach your water. Mostly harmless itself; it does not by itself prove the water will make you sick.	Effectively zero (none present)	Retest first — samples are easy to contaminate. If confirmed: inspect the wellhead and casing for cracks or openings, shock-chlorinate the well, retest. Recurring positives need continuous disinfection (UV, NSF/ANSI 55 Class A, or chlorination), not another shock.
E. coli	A specific fecal bacterium. Unlike most coliforms it doesn't grow in the environment — its presence means human or animal waste reached the water.	Zero — none present	Act now (see red flags below). Don't drink without boiling; use bottled water; contact your local health department; find and fix the pathway; disinfect; retest.
Nitrate (as N)	A nitrogen compound entering groundwater mainly from fertilizer runoff, septic systems, and manure. Tasteless, odorless, colorless.	MCL 10 mg/L as N (the 10 mg/L limit also applies to total nitrate + nitrite combined)	Ion exchange (nitrate-selective), reverse osmosis (NSF/ANSI 58), distillation (NSF/ANSI 62), or electrodialysis. Never boil — boiling concentrates nitrate as water evaporates.
Nitrite (as N)	Nitrate's chemical cousin, formed when nitrate breaks down; its presence can signal contamination by nitrate, ammonia, or organic waste.	MCL 1 mg/L as N	Same treatment categories as nitrate.
Arsenic	A naturally occurring toxic element. Odorless, colorless, tasteless — only a lab test finds it. EPA classifies inorganic arsenic as a human carcinogen, and its health goal for arsenic is zero.	MCL 0.010 mg/L (10 ppb)	Use bottled water until fixed. Treatment: reverse osmosis at the tap (NSF/ANSI 58; Penn State notes RO removes over 95% of arsenic under ideal conditions), adsorptive media (iron-oxide/hydroxide or activated alumina), anion exchange (removes arsenate, As(V), only — not arsenite, As(III)), or distillation (NSF/ANSI 62). Retest to verify the treatment works.
Lead	A toxic metal that almost never comes from the well itself — it leaches from lead pipes, lead solder, or brass fixtures, especially when water is corrosive (low pH).	Action level 0.010 mg/L (10 ppb) under the 2024 Lead and Copper Rule Improvements (older documents cite 0.015; compliance with the new level is effective November 2027). EPA's position: no safe level of lead exposure.	Find the plumbing source first — correct corrosive water with pH neutralization, since lead usually comes from pipes, not the aquifer. Then: point-of-use filters certified to NSF/ANSI 53 for lead, reverse osmosis (NSF/ANSI 58), or replace lead-containing plumbing. Ask your lab how to collect the sample — lead testing is sensitive to sampling technique.
pH	How acidic (below 7) or alkaline (above 7) the water is, on a 0–14 scale.	SMCL 6.5–8.5	Below the range (acidic): an acid-neutralizing filter (calcite) or a soda ash feed. Above the range: acid feed is the commercial approach — talk to a water treatment professional. The indirect health angle: acidic water is corrosive and can leach lead and copper from your plumbing.
Iron	A naturally occurring metal dissolved from rock and soil; common in wells.	SMCL 0.3 mg/L	Above 0.3: rusty color, sediment, metallic taste, reddish/orange staining of laundry and porcelain. Treatment: oxidation (aeration, chlorine, ozone, potassium permanganate) followed by filtration; oxidizing filters (manganese greensand, birm); a softener can handle low dissolved iron (generally under 5 mg/L, with pH above 6.7).
Manganese	A naturally occurring mineral, often found in groundwater alongside iron; an essential nutrient in small amounts.	SMCL 0.05 mg/L; lifetime health advisory 0.3 mg/L (non-enforceable; EPA recommends it for infants under 6 months even for short-term exposure)	Black-to-brown color and black staining start around the SMCL. Above the advisory level over many years, manganese can harm the nervous system. Same treatment family as iron: oxidation (chlorine, permanganate, ozone, aeration) plus filtration; manganese greensand or birm oxidizing filters; a softener for low levels.
Hardness	Dissolved calcium and magnesium — the minerals behind soap scum and scale.	No federal standard (primary or secondary)	USGS classification in mg/L as CaCO₃: soft 0–60, moderately hard 61–120, hard 121–180, very hard over 180 (in grains per gallon: soft under 3.5, moderate 3.5–7.0, hard 7.0–10.5, very hard over 10.5). Treatment: ion-exchange softener (NSF/ANSI 44). Note: softening adds sodium to the water, which matters on a sodium-restricted diet.
TDS	Total dissolved solids — the total weight of all dissolved minerals, salts, and metals. A screening indicator: a sudden jump can signal something new entered the well.	SMCL 500 mg/L	Above 500: hardness, deposits, colored water, staining, salty taste. Treatment: reverse osmosis (NSF/ANSI 58) or distillation (NSF/ANSI 62).
Sodium	A naturally occurring element; also added by water softeners and road salt. A small fraction of most people's dietary intake — food dominates — but it matters for sodium-restricted diets, infants, and people over 50.	No federal standard; EPA advisory (non-enforceable): 20 mg/L guidance for physician-prescribed sodium-restricted diets, 30–60 mg/L taste-based recommendation	Salty taste above roughly 30–60 mg/L. Treatment: reverse osmosis (NSF/ANSI 58) or distillation (NSF/ANSI 62). Note: conventional softeners add sodium — a potassium-chloride-regenerated softener is the workaround for sodium-restricted households.
Chloride	A salt component; elevated levels often point to road salt, septic influence, or seawater intrusion near coasts.	SMCL 250 mg/L	Salty taste above 250 mg/L; high chloride can also corrode pipes. Treatment: reverse osmosis (NSF/ANSI 58) or distillation (NSF/ANSI 62).
Sulfate	A naturally occurring mineral compound in groundwater.	SMCL 250 mg/L	Salty taste above 250 mg/L. Treatment: reverse osmosis (NSF/ANSI 58), distillation (NSF/ANSI 62), or anion exchange.
Fluoride	A naturally occurring mineral (public systems sometimes add it for dental health; wells have whatever the geology gives).	MCL 4.0 mg/L; SMCL 2.0 mg/L (tooth discoloration); the U.S. Public Health Service recommends 0.7 mg/L as optimal for cavity prevention in fluoridated community systems	Above 4.0 long-term: bone disease (pain and tenderness); children can develop mottled teeth (dental fluorosis) — the 2.0 secondary standard exists to prevent the discoloration. Treatment: reverse osmosis (NSF/ANSI 58) or distillation (NSF/ANSI 62).
Copper	A metal that enters water mainly by corroding copper household plumbing (also from natural deposits).	Action level 1.3 mg/L; SMCL 1.0 mg/L (metallic taste, blue-green staining)	Above the action level short-term: gastrointestinal distress; long-term: liver or kidney damage. Address corrosion first (pH neutralization reduces copper leaching), then point-of-use filters certified to NSF/ANSI 53 for copper or reverse osmosis.
Zinc	A naturally occurring metal; in wells it often comes from galvanized pipes or natural deposits.	SMCL 5 mg/L	Metallic taste above 5 mg/L. Treatment: reverse osmosis (NSF/ANSI 58); often the real fix is replacing corroded galvanized plumbing rather than treating the water.

Hardness units: gpg vs. mg/L

Labs report hardness two ways: mg/L as calcium carbonate (CaCO₃) or grains per gallon (gpg). The conversion: 1 gpg = 17.1 mg/L as CaCO₃. Multiply gpg by 17.1 to get mg/L; divide mg/L by 17.1 to get gpg. Example: 10 gpg × 17.1 = 171 mg/L — hard, bordering very hard. Softener sizing and salt settings usually work in gpg, so convert before you shop or compare numbers.

When to retest: the master schedule

One test is a snapshot. Groundwater changes — seasonally, with land use, with equipment age. The CDC baseline: test every year for total coliform bacteria, nitrates, total dissolved solids, and pH, using a state-certified lab, and ask your health department what else matters locally. North Carolina's tiered schedule is a good model of "and then what": coliform yearly; heavy metals, nitrates/nitrites, lead, and copper every two years; pesticides and VOCs every five years (yearly if a specific pesticide is used nearby) — plus a yearly wellhead check for cracks and openings.

When	What to test
Every year	Total coliform, nitrates, TDS, pH (CDC baseline); inspect the wellhead for cracks and openings
Every 2 years	Heavy metals, nitrates/nitrites, lead, copper
Every 3–5 years	Arsenic, copper, fluoride, hardness, iron, lead, manganese, uranium
Every 5 years	Pesticides, VOCs

And always retest on these triggers:

• After installing any treatment — confirm it actually works.
• After well repairs or replacing any part of the system.
• After flooding near the well.
• If taste, color, or odor changes.
• If a pregnant person or infant moves into the home — test nitrates.
• Before buying a home with a well.
• After a positive bacteria result: retest first, and after shock chlorination, retest to confirm the bacteria are gone before resuming normal use — ask your lab when to sample, after the chlorine has fully flushed from the system.

Red flags: act now, not eventually

Most results are "fix when convenient." These are the exceptions:

• E. coli present. Fecal contamination is confirmed. Don't drink the water or use it for food prep without boiling; use bottled water; contact your local health department; find and fix the pathway; disinfect the well; retest.
• Total coliform present (no E. coli). Retest immediately — samples contaminate easily. If confirmed, inspect the wellhead and casing and shock-chlorinate. If positives keep recurring, you need continuous disinfection, not another shock.
• Nitrate over 10 mg/L as N (or nitrite over 1 mg/L as N) with an infant under 6 months or a pregnant person in the home. Do not use the water for drinking or baby formula — use an alternate source. Do not boil it; boiling concentrates nitrate.
• Arsenic over 0.010 mg/L. Inorganic arsenic is a human carcinogen with no risk-free level per EPA. Use bottled water until treatment (reverse osmosis or adsorptive media) is installed and verified by retest.
• Lead at or above 0.010 mg/L. EPA's position: no safe level of lead exposure. Find the plumbing source, correct corrosive water, and filter (NSF/ANSI 53 for lead) or replace the plumbing — lead usually comes from your pipes, not your aquifer.
• A sudden jump in a previously stable parameter. TDS or nitrate climbing year over year can signal a new contamination source — a failing septic system, fertilizer application, road salt, or casing failure. Retest and investigate before buying treatment.

When this guide won't help

Honest limits, because a lab report is only the starting point:

• It's a snapshot. The report describes your water on the day it was sampled. It says nothing about next month, after the next heavy rain, or after new land use goes in upgradient.
• It doesn't design treatment. This guide names treatment categories (softener, reverse osmosis, oxidation plus filtration). Sizing, sequencing, and whether you need point-of-entry or point-of-use equipment depends on your water chemistry, flow rate, and household — that's a water treatment professional's job, ideally one who starts from your lab report rather than a sales script.
• It's not medical advice. If a result is over a health-based limit and someone in the household is pregnant, nursing, an infant, elderly, or immunocompromised, talk to your doctor or your local health department — not just this page.
• It doesn't cover everything a lab can test for. Uranium, pesticides, VOCs, and geology-specific contaminants aren't in this guide. Your state health department can tell you what else to test for in your area — the CDC says to ask them exactly that.
• One parameter doesn't tell the whole story. Corrosive low-pH water can leach lead and copper from your plumbing; high TDS can mask individual contaminants. Read the report as a system, not a checklist.

Putting your report to work

Once you've walked your numbers through the table above, run them through the well water test interpreter — it checks each result against the EPA limits and flags the health-based exceedances, the aesthetic ones, and the ones that just need watching. Print your lab report, mark it up, and keep it with your well records: next year's test only means something if you can compare it to this one.