Interpreting the Units

On each report we interpret the units used to describe the concentrations found in the sample. Here is a little more detail on that topic.

For many of the coliform tests the word Negative is used to describe that coliform bacteria was not detected. This means that the sample was found to be clean from coliform bacteria. The word Positive is used to describe that coliform bacteria was detected in the sample. This coliform test only gives results as negative or positive.  A positive result should be followed up with investigation to determine why coliform bacteria was detected. It could be the result of a sampling issue (dirty faucet, screen on faucet, bottle cap placed on countertop, etc.) or it could mean that parts of the plumbing, filtration or well system need to be cleaned. Please consult your local health department or well driller to determine the method for examining or disinfecting the well system. The EPA regulatory standard for safe drinking water coliform is Negative (no coliforms present).

In the case of pool and spa samples and surface water samples the coliform bacteria, e.coli, or fecal coliform results are quantified and an actual number of units (defined as MPN or Most Probable Number per 100 ml of sample) is reported. In these types of samples there are regulatory levels to which the number is compared.

  • mg/L means milligrams per liter or parts per million (ppm) which is used as a quantification term for liquid samples
  • mg/kg means milligrams per kilogram or parts per million which is used as a quantification term for solid samples
  • ug/L means micrograms per liter or parts per billion (ppb) for liquids
  • ug/kg means micrograms per kilogram or parts per billion (ppb) for soils

To translate from ppm to ppb you multiply the number in ppm by 1000.

Likewise to translate from ppb to ppm you divide the number in ppb by 1000.

ND means “not detected,” the lab was not able to detect any amount of the parameter in the sample.

MDL stands for “method detection limit.” Each test has a low limit below which it cannot detect any amount of a substance. This limit is represented as the method detection limit.

MQL stands for “method quantitation limit.” Each test has a low limit below which the quantitation of the parameter is less certain and is considered an estimate. When a sample result is below the MQL but is still detected at a low level the result is reported, then it is marked with a “J” flag.

The H flag indicates that there was an exceedance of the holding time allowed from sampling collection to sample analysis.  For drinking water coliform analysis the sample must be analyzed within 30 hours of sample collection time.  For surface water e.coli and fecal coliform analysis the sample must be analyzed within 6 hours of sample collection. The holding time allowed for other tests is given in Test/Bottles Preservative table.