Development of Numeric Nutrient Criteria

EPA is requiring the TNRCC to develop numeric nutrient criteria. At this point, EPA's guidance uses a percentile as the criterion. Note that the median is the 50th percentile.Therefore, EPA wants to regulate nutrients and chlorophyll like TNRCC regulates solids. The theory behind this type of regulation is "what exists is good and any increase is bad." Or the corollary, "it is too hard to determine what level causes an adverse response, so let's guess at that question and be super precise at applying regulations to support the level that is a guess." TAA should note that nutrients are more difficult to treat than solids.

EPA has already adopted a process for developing nutrient criteria.  The first step is to calculate the median value at each site based on 10 years of sampling data. The criterion is then established as the 25%ile of all the median values in the eco-region. (There are five or six eco-regions in the state.) Alternatively, if only unimpacted sites are used, the criterion is the 75%ile. Click here for Information on EPA's process.

TNRCC's Draft Plan for Developing Numeric Nutrient Criteria (PDF)

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