Prairie Rose Lake IA 05-NSH-1462

Results of DNR beach monitoring from 2014 through 2016 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria at Prairie Rose Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2014 (16 samples), 2015 (19 samples) and 2016 (16 samples), as part of the DNR beach monitoring program. According to DNR’s assessment methodology two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) the geometric mean of the samples from each recreation season of the three-year assessment period are less than the state's geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state's single-sample maximum value of 235 E. coli orgs/100 ml. If a sampling season geometric mean exceeds the state criterion of 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa's single-sample maximum value of 235 E. coli orgs/100 ml, the Class A1 uses should be assessed as “partially supported.” This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).

At Prairie Rose Beach, the geometric mean from 2015 was below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric means from 2014 and 2016, however, were greater than the Iowa water quality standard of 126 E. coli orgs/100ml and therefore suggests impairment of the Class A1 uses. The geometric mean was 215 E. coli orgs/100 ml in 2014, 54 E. coli orgs/100 ml in 2015 and 343 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 50% in 2014, 21% in 2015 and 56% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2014 and 2016. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Prairie Rose Lake are assessed (monitored) as "partially supported" due to aesthetically objectionable conditions caused by algae blooms based on information from the ISU lake survey and DNR TMDL monitoring. Using the median values from these surveys from 2013-2016 (approximately 12 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 58, 69, and 68 respectively for Prairie Rose Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Prairie Rose Lake in the Eutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and high levels of phosphorus in the water column. The data show 2 violations of the Class A1 criterion for pH in 11 samples (18%).Although the index value for Secchi is below the impairment trigger of 65 for this assessment cycle, Prairie Rose Lake was listed as partially supporting its Class A1 uses due to aesthetically objectionable conditions. Based on DNR's methodology, the median TSI value for Secchi must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 5). Therefore, Prairie Rose Lake will remain listed as "not supported" for the 2018 assessment/listing cycle.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2013-2016 show there were no violations of the criterion for ammonia in 11 samples(0%), one violation of the criterion for dissolved oxygen in 12 samples(8%), and 2 violations of the criterion for pH in 11 samples(18%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Prairie Rose Lake.

Note: Prairie Rose Lake was drawn down in July of 2011 and refilled in September of 2012 for restoration purposes. As a result, no samples were collected during that time. While the lake was drawn down, Stabilization of the eroding shoreline and removal of 60,000 cubic yards of sediment was completed and undesirable fish species (e.g. common carp)were removed from the lake and watershed. The lake was restocked starting in April of 2013. In lake dredging started in mid-July. Approximately 185,000 CY of sediment were to be removed from Prairie Rose Lake. The project was completed in 2015. This assessment does not include post-restoration data.