The Tier II assimilative capacity (AC) analysis is a measure for determining when Tier II stream water quality is diminished or degraded beyond natural changes in condition.
Assimilative capacity is defined in regulation as the difference between the Tier II water quality at the time the stream segment was designated as Tier II (e.g. Tier II baseline), and the water quality criterion (Code of Maryland Regulations (COMAR) 26.08.02.04-1). for Tier II waters designated using Maryland Biological Stream Survey (MBSS) data*, the Tier II baseline water quality is represented by the baseline index of biotic integrity (IBI) scores used to initially designate a stream as Tier II (i.e. a fish and benthic score of 4.00 or greater). the water quality criterion is represented by the Tier I IBI score of 3.00. Available AC is the difference between the Tier II baseline score and 3.00.
To protect high water quality Tier II streams, MDE must identify a numeric limit that may be lower than the Tier II baseline, but does not use all of a stream’s available assimilative capacity. To accomplish this, MDE must calculate an assimilative capacity threshold which takes into account natural variability. When combined with recent MBSS data, this threshold helps determine how much AC is remaining in designated Tier II streams.
MDE first calculates the available AC for each Tier II stream. This number may be different for each stream because baseline scores for each Tier II stream are different. Next, based on the available AC, MDE calculates the Tier II threshold. Again, the threshold value differs from stream to stream depending upon Tier II baseline scores, and available AC. Finally, MDE compares recent MBSS data to the threshold value. If the recent data IBI score is higher than the threshold, the Tier II stream segment is healthy, and there is some AC remaining. If the recent IBI data score is equal to, or lower than the threshold, the Tier II stream segment is diminished (or degraded), and there is no remaining AC.
The following simplified AC analysis illustrates how MDE analyzes recent MBSS data collected on a Tier II stream segment to determine the current AC status of the water body. In the example provided below, only benthic data is evaluated. Where available, and in most situations, both the benthic and fish data are evaluated.