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.
Sample Stream A was designated as Tier II in 2000. The baseline benthic data for Tier II Sample Stream A is 5.00. The Tier I water quality score for biological water quality is represented by 3.00. Therefore, the available assimilative capacity is 2.00.
Shown as an equation:
Represented as a graph:
Due to natural variability, such as climatic factors, natural variability in annual populations, etc., it is unlikely that multiple samples from the same monitoring location over different years will yield identical IBI scores. Therefore, after taking into account natural variability, the assimilative capacity threshold establishes the minimum benthic IBI target score for new data. This threshold is the difference between the baseline score, and the available assimilative capacity reduced by 25% for natural variability (COMAR 26.08.02.04-1). For this example, the assimilative capacity threshold is 4.50.
It is important to note that streams with different Tier II baselines have different Tier II assimilative capacity thresholds. This means that a stream with a lower baseline IBI score, 4.00 for instance, would have a capacity threshold of 3.75 when factoring in natural variability.
Remaining assimilative capacity is the difference between recent MBSS sample data, and the Tier II AC threshold. If the evaluation yields a positive number, then some assimilative capacity remains. If the evaluation yields a zero or a negative number, then there is no assimilative capacity.
In 2003, MDE collects new data for Sample Stream A. The scenarios, below, show two possible calculation outcomes when determining remaining assimilative capacity for Sample Stream A.
In Scenario 1, the new, current, benthic IBI score is 4.75. In this scenario, the Tier II stream segment is still considered healthy because there is some assimilative capacity left.
In Scenario 2, the current benthic IBI score is 4.00. In this scenario, the Tier II stream segment is considered degraded because there is no assimilative capacity left.
*To date, all Tier II waters have been identified using MBSS data.
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This page was last updated 7/2017.
Please direct any questions or comments concerning Maryland's Antidegradation program to Angel Valdez at Angel.Valdez@maryland.gov or 410-537-3606.
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