Lake Orient IA 05-NOD-1396

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Lake Orient are assessed (monitored) as "not supported" due to poor water transparency, aesthetically objectionable conditions caused by algae blooms and due to frequent violations of the state criterion for pH based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 70, and 91 respectively for Lake Orient. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Orient in the Hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and extremely high levels of phosphorus in the water column. The data show 6 violations of the Class A1 criterion for pH in 15 samples (40%).

Note:A TMDL for siltation at Orient Lake was prepared by DNR and approved by EPA in 2001; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because not all Section 303(d) impairments identified for the 2016 assessment/listing cycle (algae, pH) are addressed by the TMDL, this waterbody remains in category 5a (impaired, TMDL needed).

The level of inorganic suspended solids was high at Lake Orient, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Lake Orient (9.7 mg/L) was ranked 119th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a large population of cyanobacteria exists at Lake Orient, which suggests the potential for an impairment due to nuisance aquatic life These data show that cyanobacteria comprised 98% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (239 mg/L) was ranked 137th of the 138 lakes sampled. This median is in the worst 25% of the 138 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa's narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2012-2016. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. Assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.

he occurrence of a fish kill at this lake in March 2004 is consistent with the assessment of the Class B(LW) uses as "partially supported." This kill occurred on or before March 2, 2004 and was attributed to natural causes (winterkill). An estimated 475 fish were killed with an estimated value of $7,125. Bluegill (75), crappie (150), and channel catfish (250) were observed killed. According to the investigation conducted by Adair County, the kill was caused by low oxygen conditions that developed during the winter. This lake has a maximum depth of eight feet and was approximately 3 feet low at the time of the investigation. This is the same assessment as that developed for the 2008 and 2010, 2012, 2014 and 2016 assessment/listing cycle. This impairment remains appropriate for Category 3b of Iowa's 2018 Integrated Report. The occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2009-2012) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the DNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated” and will be placed in IR subcategories 3b and will be added to the state list of waters in need of further investigation. Thus, this impairment will remain in Category 3b of Iowa’s 2018 Integrated Report.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "not supported" due to violations of the Class A1,B(LW) criterion for pH. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), one violation of the criterion for dissolved oxygen in 15 samples(7%), and 6 violations of the criterion for pH in 15 samples(40%). Based on DNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of Lake Orient.The pH violations at Orient Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.

The Class C (drinking water) uses are not assessed due to the lack of recent information upon which to base an assessment. The only parameter collected as part of the ISU lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU surveys from 2012-2016 show that nitrate levels are extremely low at this lake (maximum value = 5.8 mg/l; median = 0 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.