Water contaminants are measured using a variety of units depending on what fits best. Some units can readily convert between one another (like PPM, PPB, and PPT), others are entirely unrelated.

The best way to quantify chemical contaminants in water is through their concentrations.

Concentrations describe the mass amount of a particular contaminant per a volume amount of water and are represented (fittingly) by concentration units.

Concentration units make up the majority of units on water testing reports and describe contaminants such as:

The first unit we will explore is actually a pseudo-unit! Part per notation is one of the most common concentration units used in water testing. This notation describes how many “parts” of a contaminant exist per the number of “parts” of water, like a ratio. For example, you could have 40 parts of elemental magnesium in 1 million parts of water, or 40 parts per million.

One reason why parts per notation can be so confusing is because it is not actually a unit of measurement–like mg/L or μg/L. Parts per notation is actually a pseudo-unit because it has no predefined dimension (like length, volume, or mass).

The most common parts-per units are parts per million (PPM), parts per billion (PPB), and parts per trillion (PPT). These parts are either mass (e.g. mg of contaminant per gram of water) or volume based (e.g., mg of contaminant per liter of water). In water testing, it is almost always safe to assume parts-per notation represents a mass per volume value.

The parts per million (PPM) unit equals the number of parts of a contaminant per million parts of water volume. PPM is seen very regularly on water test reports.
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If the scale of PPM is difficult to conceptualize, try imagining: one minute in the span of two years equals around 1 PPM.

When volume-based, 1 PPM equals 1 mg/L. PPM can also easily convert to parts per billion (PPB) and parts per trillion (PPT), other important units that quantify contaminants in water.

PPB and PPT notation represents the number of parts of a contaminant per billion parts water and trillion parts water, respectively. If you ever find yourself looking at a “grams-per-liter” notation, just remember 1 µg/L equals 1 PPB and 1 ng/L equals 1 PPT.

To put PPB and PPT into perspective:

Arsenic and lead concentrations are almost always written using PPB. For example, EPA’s Maximum Contaminant Level (MCL) for arsenic is 10 PPB (or 10 µg/L).

PPT is used for contaminants detected at extremely low levels. PPT is seen far less on water testing reports than PPM or PPB. This is either because instrumentation that tests water samples cannot detect levels that low or the water contaminant exists at much higher concentrations such that PPM and PPB are more appropriate units of measurement.

Although they are frequently and effectively used in water testing reports, parts-per notation cannot quantify all water contaminants. For example, biological contaminants require a totally different way to describe their presence. Such biological contaminants include:

“Colony formation units” per volume of water unit (represented as CFU/mL or CFU/100 mL depending on the dilution of the sample) frequently quantifies a range of biological contaminants.

A “colony formation unit” per milliliter (CFU/mL) represents the number of living cells in one milliliter of sample. CFU/mL is used specifically for measuring biological contaminants like molds, fungus, and some bacteria.

To measure CFUs, cells from a sample of water are first cultured, or grown in controlled environments suitable for the specific cells. Then, the resulting cell colonies, or clusters of cells that come from the same parent cell, are counted per volume of sample added.

When the biological contaminant in question is considered harmful at any concentration, a CFU/mL measurement is unnecessary. Instead, a presence/absence test is all that’s needed.

A presence/absence (P/A) test indicates whether or not a specific biological contaminant exists in your water sample. There are no units associated with a P/A test because it does not quantify the concentration of bacteria.

Many regulatory bodies like the EPA use P/A tests because they are generally more accurate, low cost, and fast.

P/A tests are used for Total Coliform and E. coli because their presence (at any concentration) could indicate the presence of other harmful pathogens. The US EPA Maximum Contaminant Level (MCL) for both Total Coliform and E. coli is zero, meaning any presence exceeds the legal limit and is considered a concern.

Radiological units are used for measuring contaminants such as:

One of the most common radiological units seen on water testing reports, picocuries per liter (pCi/L) describes the radioactivity emitted per volume of water.

One “picocurie” equals to a trillionth of a “curie” (named after scientists Marie and Pierre Curie who were well-known for studying radiation). A curie is defined as the amount of radioactivity emitted by 1 gram of radium.
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In the United States, the standard unit for radioactivity is pCi/L. Wherever the metric system is used (i.e. virtually everywhere else), the becquerels per cubic meter (Bq/m3) is the accepted unit of measurement. One pCi/L is equivalent to 37 Bq/m3.

Some physical water parameters like pH, Sodium Absorption Ratio (SAR), and Langelier Saturation Index (LSI) don’t need units. Instead, these values are understood as a point on a scale.

For example, pH describes the acidity or basicity of water on a logarithmic scale–0 meaning extremely acidic, 14 meaning extremely basic, and 7 meaning neutral. So if your water’s pH is 8, then it is considered slightly basic with respect to this scale.