On December 14, 2018, the U.S. Environmental Protection Agency (EPA) gave approval to Missouri’s revised lake nutrient water quality criteria applicable to lakes ten acres or larger. The Missouri Clean Water Commission adopted the revised criteria on January 4, 2018. Pursuant to a federal consent decree with the Missouri Coalition for the Environment, EPA was required to approve Missouri’s criteria or promulgate federal lake nutrient criteria by December 15, 2018.
Missouri’s lake nutrient criteria include response impairment threshold values for lakes in three ecoregions: plains, Ozark border, and Ozark highlands. EPA found that Missouri’s three ecoregional response impairment threshold values for chlorophyll-A were reasonable. Specifically, EPA’s value of 30 ug/L for the plains was “reasonable with respect to protecting a wide variety of biota and maintaining a healthy sport fish population.”
Missouri’s criteria also include nutrient screening threshold values for phosphorus, nitrogen, and chlorophyll-A. Values that exceed the screening thresholds undergo further assessment by reviewing five response assessment end points. These five endpoints include a review of fish kills, dissolved oxygen and pH excursions, elevated cyanobacteria counts, shifts in aquatic diversity and elevated turbidity that suppresses algal growth. EPA found that these assessment end points are “based on sound science and protective of the aquatic life use.”
Although Missouri’s criteria do not specifically refer to drinking water or recreational designated uses, EPA considers the criteria to be protective of these uses through implementation of Missouri’s narrative water quality criteria. EPA found that MDNR’s existing narrative criteria sufficiently address the types of harm excess nutrients may present to lakes designated for drinking water supply and recreational uses.
In conclusion, EPA approved Missouri’s lake nutrient criteria in their entirety. EPA found that the criteria are “based on sound scientific rationale and are protective of the designated uses for which they are developed to protect and therefore address EPA’s 2011 disapproval of Missouri’s previous lake nutrient criteria.”
EPA’s approval letter and Decision Document can be found here.
