R18-11-108.03. Narrative Nutrient Criteria for Lakes and Reservoirs

A. The narrative nutrient criteria in this Section apply to those lakes and reservoirs categorized in Appendix B. B. The narrative water quality standard for nutrients at R18-11- 108(A)(6) is met when, based on a minimum of two lake sample events conducted during the peak season based on lake productivity, the results show an average chlorophyll-a value below the applicable threshold for designated use and lake and reservoir category in subsection (D). 1. The mean chlorophyll-a concentration is less than the lower value in the target range chlorophyll-a for the lake and reservoir category, or 2. The mean chlorophyll-a concentration is within the target range for the lake and reservoir category and: Secchi Total Phosphorus Designated a. The mean blue green algae count is at or below 20,000 per milliliter, and b. The blue green algae count is less than 50 percent of the total algae count, and c. There is no evidence of nutrient-related impairments such as: i. An exceedance of dissolved oxygen or pH standards; ii. A fish kill coincident with a dissolved oxygen or pH exceedance; iii. A fish kill or other aquatic organism mortality coincident with algal toxicity; iv. Secchi depth is less than the lower value prescribed for the lake and reservoir category; v. A nuisance algal bloom is present in the limnetic portion of the lake or reservoir; or vi. The concentration of total phosphorous, total nitrogen, or total Kjehldal nitrogen (TKN) is greater than the upper value in the range prescribed for the lake and reservoir category; or 3. For a shallow lake. In addition to meeting the mean chlorophyll-a concentrations in subsections (B)(1) or (2), submerged aquatic vegetation covers 50 percent or less of the lake bottom and there is less than a 5 mg/L swing in diel- dissolved oxygen concentration measured within the photic zone. C. The following threshold ranges apply during the peak season for lake productivity: 1. Warm water lakes peak season, April – October; 2. Cold water lakes peak season, May – September. D. The following table lists the numeric targets for lakes and reservoirs. NUMERIC TARGETS FOR LAKES AND RESERVOIRS Total Kjehldal Nitrogen Blue- Green Algae (% of total Blue- Green Algae (per ml) Dissolved Oxygen Total Nitrogen pH (SU) FBC and Lake Category Chl-a (µg/L) Depth Use (m) (µg/L) (TKN) (mg/L) (mg/L) count) (mg/L) Deep 10-15 1.5-2.5 70-90 1.2-1.4 1.0-1.1 20,000 6.5-9.0 Shallow 10-15 1.5-2.0 70-90 1.2-1.4 1.0-1.1 Igneous 20-30 0.5-1.0 100-125 1.5-1.7 1.2-1.4 Sedimentary 20-30 1.5-2.0 100-125 1.5-1.7 1.2-1.4 Urban 20-30 0.5-1.0 100-125 1.5-1.7 1.2-1.4 A&Wc All 5-15 1.5-2.0 50-90 1.0-1.4 0.7-1.1 <50 7 (top m) PBC 6.5-9.0 25-40 0.8-1.0 115-140 1.6-1.8 1.3-1.6 6 (top m) Urban 30-50 0.7-1.0 125-160 1.7-1.9 1.4-1.7 A&Wedw All 30-50 0.7-1.0 125-160 1.7-1.9 1.4-1.7 6.5-9.0 DWS All 10-20 0.5-1.5 70-100 1.2-1.5 1.0-1.2 20,000 5.0-9.0 A&Ww All (except urban lakes)