EPA guidance defines an exceedance in water quality standards as an inconsistency between ambient conditions and the combined three elements of numeric water quality criteria: magnitude, duration and frequency. A single sample exceeding a numeric criterion is defined as a digression
and an average exceeding a criterion an excursion. EPA guidance also states that digressions and excursions alone are not impairment. To constitute impairment, the frequency of digressions or excursions must be greater than the frequency criterion. Given the variable nature of ambient waters,
most monitoring programs do not generate enough data to assess compliance with bacteria water quality standards with statistical confidence. Even fewer programs generate enough data to meaningfully assess the frequency of digressions or excursions. Small data sets can magnify the impact of
bad, biased or wet weather influenced samples and create the potential for 303(d) impaired water listing of a water body when comprehensive daily monitoring would show that the water body is, in fact, in compliance with bacteria standards. Consistent with EPA guidance and under peer review
by the Water Environment Research Foundation (WERF), comprehensive wet and dry weather bacteria monitoring and representative water quality modeling were performed over a nine-year period to support TMDL Implementation Planning for the Middle Chattahoochee River at Columbus, GA. The calibrated
model indicates that when considering bacteria loads in daily mass flux over the entire period, the frequency of maximum value digressions and geometric mean excursions are less than 1%. This is in contrast to compliance analysis using limited, State scheduled, grab sampling data which suggests
that over the same period the water body exceeds the State's 10% frequency criterion.
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