Determining impairment

Water quality criteria are the benchmarks against which data describing the actual condition of the water are compared. This methodology for the comparison differs quite a bit from state to state, but an example from Minnesota illustrates the basic idea. For conventional pollutants such as dissolved oxygen, pH and turbidity, a "percent exceedance" is calculated. It is calculated by dividing the number of times that samples from a particular water body don't meet the water quality criteria by the total number of samples. A minimum of 10 samples in 10 years is required.

• If < 10 % of the samples don't meet the criteria, the water is deemed to be "Fully Supporting" its designated uses.
• If 10 – 25 % of the samples don't meet the criteria, the water is deemed to be "Partially Supporting" its designated uses.
• If >25% of the samples don't meet the criteria, the water is deemed to be "Not Supporting" its designated uses.

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