How To Interpret Water Testing Units

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. 

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.

“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.

What Is a CFU/mL?

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.

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. 

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.

Why Are Some Water Testing Results Unit-less?

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.

What Are the Takeaways?

Getting your water properly tested with a lab testing kit should include a wide variety of water contaminants. For example, Tap Score’s Extended Well Water Test analyzes a wide range of factors including, pH, alkalinity, metals (like arsenic and lead and nickel), bacteria, VOCs and radioactive particles. Such a test will inherently include a wide variety of units. All these units and their implicit meaning might seem difficult to understand at first, but hopefully your Report and this Tips for Taps article make it more clear.

If you have any additional questions or need more explanation on some units that are tripping you up, don’t hesitate to reach out to our expert water chemists, scientists, and engineers at hello@gosimplelab.com.