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Most water-test numbers show how much of something is in the water or describe a water-quality property such as acidity, hardness, or cloudiness. A higher number is not always worse, and a low number does not always mean the water is safe.
The key is to read four things together: what was tested, the result, the unit, and the reference limit. For rainwater, well water, or other private supplies, a single test or test strip should not be treated as proof that the water is safe to drink.
Start With the Units
Water reports use several different units. Mixing them up can make a result look much larger or smaller than it really is.
| Unit | What it means | Often used for |
|---|---|---|
| mg/L | milligrams per liter | nitrate, TDS, hardness, iron |
| ppm | parts per million | many dissolved chemicals |
| µg/L | micrograms per liter | lead, arsenic and other trace contaminants |
| ppb | parts per billion | trace contaminants |
| CFU/100 mL | colony-forming units per 100 mL | bacteria |
| MPN/100 mL | most probable number per 100 mL | bacteria |
| NTU | nephelometric turbidity units | cloudiness |
| µS/cm | microsiemens per centimeter | electrical conductivity |
| pH | unitless scale | acidity or alkalinity |
For dilute water, 1 mg/L is approximately 1 ppm, and 1 µg/L is approximately 1 ppb.
There are 1,000 micrograms in one milligram. That means:
- 0.010 mg/L = 10 µg/L
- 0.015 mg/L = 15 µg/L
- 1 mg/L = 1,000 µg/L
This conversion matters when comparing a laboratory result with a drinking-water limit given in different units.
What Common Water-Test Numbers Mean
pH
pH tells you whether the water is acidic, neutral, or basic.
A pH of:
- 7 is neutral
- below 7 is acidic
- above 7 is basic, also called alkaline
The pH scale is logarithmic. A change of one whole pH unit represents a tenfold change in hydrogen-ion concentration, so the difference between pH 6 and pH 7 is more significant than it may look.
EPA's secondary drinking-water guidance lists a pH range of 6.5 to 8.5. Secondary standards mainly address issues such as taste, staining, corrosion, and treatment problems rather than serving as health limits.
Low-pH water can be more corrosive. That matters in a rainwater system because corrosive water may pick up metals from plumbing, fittings, pumps, gutters, or storage components.
A pH number alone does not tell you whether the water is safe to drink.
Total Dissolved Solids
Total dissolved solids, usually shortened to TDS, estimates the amount of dissolved material in the water.
That material can include:
- calcium
- magnesium
- sodium
- chloride
- sulfate
- other minerals and salts
A TDS reading of 150 mg/L means there are roughly 150 milligrams of dissolved material in each liter of water.
It does not tell you what those materials are.
For example, two water samples could both read 150 mg/L TDS while having very different chemical compositions.
EPA lists 500 mg/L TDS as a secondary drinking-water standard. Higher levels may contribute to taste, staining, deposits, or scale.
A very low TDS reading is not proof that water is clean. Bacteria and some harmful chemicals can be present even when TDS is low.
Electrical Conductivity
Electrical conductivity, or EC, measures how easily electricity moves through the water.
Dissolved salts and minerals make water more conductive, so conductivity usually rises as dissolved mineral content rises.
It may appear as:
- µS/cm
- mS/cm
Conductivity is especially useful for comparing water over time. If tank water normally reads 80 µS/cm and suddenly reads much higher, something may have changed.
But conductivity does not identify the contaminant. A laboratory test may be needed to find the cause.
Hardness
Hardness mainly measures dissolved calcium and magnesium.
It is commonly reported as mg/L as CaCO3, meaning the result is expressed as an equivalent amount of calcium carbonate.
Hard water can cause:
- scale on fixtures
- deposits inside heaters
- buildup in irrigation equipment
- increased soap use
Soft rainwater often has relatively little calcium and magnesium because rain has not passed through mineral-rich soil or rock.
That does not automatically make rainwater better. Very soft water can sometimes be more corrosive, depending on its pH, alkalinity, and other chemistry.
Alkalinity
Alkalinity tells you how well the water can resist changes in pH.
It is often reported as:
mg/L as CaCO3
Low-alkalinity water can have a pH that changes fairly easily. Rainwater commonly starts with relatively low mineral content, so alkalinity can matter when evaluating corrosion or planning treatment.
Do not confuse alkalinity with high pH. They are related, but they are not the same measurement.
Turbidity
Turbidity measures how cloudy the water is from suspended particles.
It is usually reported in NTU.
Particles may include:
- soil
- roof dust
- organic material
- fine sediment
- microorganisms
- rust
High turbidity can also interfere with some treatment methods. For example, suspended material can reduce the effectiveness of disinfection processes if the system is not properly designed and maintained.
Clear-looking water can still contain harmful germs or chemicals, so low turbidity should not be treated as a safety test.
Nitrate and Nitrite
Nitrate and nitrite are forms of nitrogen.
They can enter water from sources such as fertilizers, sewage, animal waste, or other contamination.
As of August 2026, EPA's maximum contaminant levels for public drinking-water systems are:
| Contaminant | EPA MCL |
|---|---|
| Nitrate measured as nitrogen | 10 mg/L |
| Nitrite measured as nitrogen | 1 mg/L |
Pay close attention to how nitrate is reported.
A laboratory may report:
- nitrate as nitrogen, often written NO3-N
- nitrate as nitrate, often written NO3
Those numbers are not directly interchangeable. About 10 mg/L nitrate as nitrogen equals 44.3 mg/L nitrate expressed as nitrate.
Use the comparison value specified for the exact form shown on your report.
Iron
Iron may cause:
- orange or reddish staining
- metallic taste
- sediment
- discoloration
EPA's secondary guideline is 0.3 mg/L.
Iron is often more of an appearance, staining, and equipment issue than a direct indicator of whether the whole water supply is safe.
Manganese
Manganese can cause dark staining, discoloration, and taste problems.
EPA's secondary guideline is 0.05 mg/L.
As with other individual readings, the manganese result tells you only about manganese. It does not clear the water of other contaminants.
Arsenic
Arsenic is very different from hardness or TDS because it is a health-related contaminant.
EPA's maximum contaminant level for arsenic in public drinking water is:
0.010 mg/L, which is the same as 10 µg/L or 10 ppb.
For example:
0.012 mg/L = 12 µg/L
That is above the EPA public drinking-water MCL.
Arsenic cannot be judged by taste, smell, or appearance.
Lead
Lead results are commonly reported in µg/L or ppb.
Lead numbers need especially careful interpretation.
EPA's health goal for lead in drinking water is zero. EPA also currently uses a 15 ppb action level as part of its regulatory treatment requirements for public water systems. That 15 ppb figure is not a line below which an individual household sample should simply be called safe.
Lead can come from plumbing, solder, fittings, fixtures, or other materials that contact the water.
Acidic or corrosive rainwater may increase the importance of checking plumbing materials and water chemistry.
Chlorine
Chlorine tests may show:
- free chlorine
- total chlorine
These are not the same measurement.
Free chlorine measures chlorine that remains available for disinfection. Total chlorine generally includes free chlorine plus combined forms of chlorine.
A chlorine reading only tells you about chlorine under the conditions of that test. It does not establish that every organism has been removed or that chemical contaminants are absent.
Disinfection also depends on factors such as water clarity, disinfectant concentration, contact time, pH, temperature, and system design.
Total Coliform
Total coliform bacteria are commonly used as water-quality indicators.
They are widespread in soil, plants, surface water, and animal or human digestive systems. Most total coliform bacteria are not themselves the main health concern. Their presence can indicate that contamination has found a way into the water supply.
CDC recommends testing private wells at least annually for total coliform bacteria, nitrate, TDS, and pH.
A bacteria result may appear as:
- absent
- present
- positive
- negative
- CFU/100 mL
- MPN/100 mL
The sampling method matters greatly. A contaminated bottle, faucet, hand, hose, or sample cap can affect the result.
E. coli
A positive E. coli result is more concerning than a general TDS, hardness, or pH reading.
Reviewing EPA guidance for TDS in drinking water helps compare safety testing and treatment aligned with the planned application.
CDC says detection of fecal coliforms or E. coli likely means fecal contamination has entered the water source.
For water intended for drinking, cooking, or brushing teeth, do not treat a positive E. coli result as something that can be ignored because the other numbers look good. Use another safe water source and get guidance from your health department or another qualified water professional while the contamination source and treatment are addressed.
What Do ND, <, and > Mean?
Laboratory reports often contain symbols instead of simple numbers.
ND
ND normally means not detected.
It does not necessarily mean the amount is exactly zero.
It usually means the laboratory did not detect the substance above the method's detection or reporting limit.
For example:
Lead: ND, reporting limit 1 µg/L
means the laboratory did not find lead at or above its stated reporting limit.
Less Than
A result such as:
<1 µg/L
means the contaminant was below the laboratory's reporting level of 1 µg/L.
It does not necessarily mean zero.
Greater Than
A result such as:
>500 mg/L
usually means the concentration exceeded the test's measurement range.
A different test range or laboratory method may be needed to get the actual value.
Do Not Compare Numbers Until the Units Match
Suppose a report gives arsenic as:
0.008 mg/L
and you find a reference value of:
10 µg/L
Those numbers may look very different.
Convert them first:
0.008 mg/L × 1,000 = 8 µg/L
Now they can be compared correctly.
This is one of the most common mistakes when reading water reports.
A Water-Test Limit Can Mean Different Things
Not every number listed beside a test result represents the same kind of limit.
Maximum Contaminant Level
An MCL, or maximum contaminant level, is an enforceable limit that EPA applies to regulated public drinking-water systems.
EPA's federal drinking-water rules do not automatically regulate a homeowner's private well or private rainwater system in the same way.
MCLs can still provide important health context, but private-system owners should also follow current state and local health guidance.
Secondary Standard
A secondary drinking-water standard usually deals with issues such as:
- taste
- odor
- staining
- corrosion
- scale
- appearance
For example, EPA lists secondary values of 500 mg/L for TDS and a pH range of 6.5 to 8.5.
Being outside a secondary range is not the same as exceeding a health-based contaminant limit.
Action Level
An action level can trigger specific regulatory steps.
It should not automatically be interpreted as a personal "safe below, dangerous above" line.
Lead is a good example. EPA specifically explains that its lead action level is used to evaluate corrosion-control performance in public water systems rather than define a safe concentration in an individual home.
TDS Is Not a Drinking-Water Safety Test
Small handheld TDS meters are useful, but their reading is often misunderstood.
A meter showing:
35 ppm
does not mean the water is 35 ppm contaminated.
It means the meter estimates the amount of electrically conductive dissolved material.
The reading cannot tell you whether the water contains:
- E. coli
- viruses
- parasites
- lead
- arsenic
- pesticides
- many other specific contaminants
Likewise, a reading of 300 ppm does not automatically mean the water is unsafe.
A TDS meter is useful for monitoring changes and some treatment processes. It is not a substitute for contaminant-specific testing.
Home Test Strips Give Different Information From a Laboratory
Test strips are useful for quick checks of parameters they are designed to measure.
They may help you monitor:
- pH
- hardness
- alkalinity
- chlorine
- nitrate
- some metals
But a strip usually has a limited measurement range and depends on matching colors by eye.
Results can also be affected by:
- expired strips
- incorrect dipping time
- incorrect reading time
- storage conditions
- unusual water color
- lighting
- temperature
- interfering chemicals
A strip reading should be treated as a screening result unless the test's documentation establishes otherwise.
For drinking-water decisions involving private wells or rainwater, CDC recommends appropriate testing for germs and chemicals, and advises use of qualified testing resources. For wells, CDC specifically recommends a state-certified laboratory.
Rainwater Test Numbers Need Extra Context
Rainwater can look clear and still contain germs or chemicals.
Contamination can enter the system from:
- airborne dust and smoke
- bird or animal droppings
- roofing materials
- gutters
- flashing
- pipes
- pumps
- storage tanks
- accumulated sediment
CDC states that rainwater is not necessarily safe to drink without appropriate removal of germs and chemicals. It recommends regular testing when collected rainwater is used for drinking, cooking, or bathing.
That means a rainwater test should be interpreted as part of the whole system.
The Sample Location Matters
Water sampled directly from a storage tank may produce different results from water taken after:
- sediment filtration
- carbon treatment
- disinfection
- household plumbing
If you are checking whether treatment works, it can be useful to know what is entering the treatment system and what is leaving it.
For drinking-water evaluation, use the sampling procedure specified by the laboratory. Improvised sampling can make microbiological and some metals results difficult to interpret.
One Good Number Does Not Clear the Water
Suppose a rainwater sample shows:
- pH: 7.1
- TDS: 55 mg/L
- hardness: low
Those numbers tell you useful things about the water chemistry.
They do not tell you whether the water contains E. coli, lead, arsenic, pesticides, or another contaminant that was never tested.
Water testing can only answer the questions included in the test panel.
How to Read a Water-Test Report Step by Step
When you get a test result, use this order:
- Find the parameter name. Determine exactly what was tested.
- Check the result. Look for a number, ND, positive, negative, or another laboratory notation.
- Check the unit. Do not compare mg/L directly with µg/L.
- Check how the contaminant is expressed. Nitrate as nitrogen is different from nitrate expressed as nitrate.
- Identify the type of reference value. It may be an MCL, secondary guideline, action level, irrigation target, or laboratory reference.
- Consider how the water will be used. Garden irrigation and drinking water have very different requirements.
- Confirm important results. For drinking-water concerns, use an appropriate certified laboratory and follow its sampling instructions.
- Fix the cause, not just the number. A treatment device may not solve contamination coming from an unsuitable roof, dirty tank, damaged plumbing, poor maintenance, or repeated entry of animal waste.
If harmful germs or chemicals are found in water intended for drinking, CDC advises using another safe water source while the problem is evaluated and treated. No single treatment method removes every possible contaminant.
Frequently Asked Questions
Is a lower number always better on a water test?
No. It depends on what is being measured. Lower lead or arsenic is desirable, but very low hardness, alkalinity, or pH can create other system concerns. Always interpret the number according to the specific parameter.
What does ppm mean on a water test?
PPM means parts per million. In dilute water, 1 ppm is approximately equal to 1 mg/L. It is commonly used for dissolved minerals and chemicals.
What does ppb mean?
PPB means parts per billion. In dilute water, 1 ppb is approximately 1 µg/L. It is often used for contaminants present at very low concentrations, such as lead or arsenic.
Does a low TDS number mean rainwater is safe to drink?
No. TDS estimates dissolved mineral content. It cannot establish that harmful bacteria, viruses, metals, or other chemicals are absent. Rainwater intended for drinking requires suitable collection, treatment, contaminant-specific testing, ongoing maintenance, and compliance with applicable local requirements.
What does ND mean on a laboratory water report?
ND normally means not detected above the laboratory's detection or reporting limit. It should not automatically be interpreted as an absolute concentration of zero.
What does a positive coliform result mean?
Total coliform bacteria can indicate that contamination is entering the water system. A positive E. coli or fecal-coliform result is more concerning because it can indicate fecal contamination. Water intended for drinking should be evaluated using appropriate laboratory and public-health guidance.
Can I compare my rainwater results with EPA drinking-water limits?
EPA drinking-water standards can provide useful reference points, but federal public-water regulations do not apply to most private rainwater systems in the same way they apply to regulated public water systems. For drinking-water use, also follow current state and local health requirements and laboratory guidance.
