BLOG: Understanding Water Test Results

by Jennifer Wemhoff, Groundwater Foundation

We've preached for years the importance of regular testing of private well water - at least once a year or more often if an issue is suspected. (side note: use this handy tool to help find a certified testing lab)

Understanding the results can be tricky, especially when you see terms like ppm, mg/L, ppb, µ/L that you may not recognize.

• ppm = parts per million
• mg/L = milligrams per liter (one ppm = 1 mg/L)
• ppb = parts per billion
• µ/L = microgram, which = 1/1,000 of a milligram

Water tests generally list the concentration of most minerals in parts per million. Pesticides and other chemicals aren't usually found at concentrations as high as parts per million, so they're listed in parts per billion or micrograms per liter. Compounds other than minerals or pesticides may be listed in other forms of measurement - water hardness is noted as grains per gallon, the corrosion index says if water is corrosive or not corrosive, bacteria may be listed as positive or negative.

While the labels may make sense now, do you really know what 1 ppm means? Here are some easier ways to think about them:

Think of 1 ppm as:

• 1 inch in 16 miles
• 1 minute in 2 years
• 1 cent in $10,000
• 1 bad apple in 2,000 barrels

One part per billion compares with:

• 1 inch in 16,000 miles
• 1 second in 32 years
• 1 cent in $10,000,000
• 1 bad apple in 2 million barrels

While these comparisons emphasize how small the amounts being measured are, keep in mind that cumulative exposure over the long-term to even small amounts of a contaminant may be of concern. 

For more information on testing your well water or interpreting your test results, visit wellowner.org/water-quality/water-testing.