Brushy Creek Lake IA 04-UDM-1276
Webster Co. in S34 T88N R27W; 5 miles E. of Lehigh
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trophic
- Eutrophic
- Trend
- Stable
- Created
- 10/15/2018 7:52:07 AM
- Updated
- 6/6/2019 1:44:48 PM
The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to violations of the state's indicator bacteria criteria. The Class B(LW)(aquatic life) uses are assessed (evaluated) as “partially supported” due to invasive species. Fish consumption uses are assessed as "fully supporting." Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2012 through 2016 by Iowa State University (ISU), (2) results of IDNR/UHL beach monitoring from 2015 through 2016, (3) information from the IDNR Fisheries Bureau, and (4) IDNR RAFT fish contaminant monitoring.
Results of DNR beach monitoring from 2015 through 2016 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria at Brushy Creek Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2015 (20 samples) and 2016 (17 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 Brushy Creek Beach, the geometric means from 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean from 2015, however, was 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 128 E. coli orgs/100 ml in 2015 and 50 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 40% in 2015 and 12% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2015. 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 Brushy Creek Lake are assessed (monitored) as "fully supported" 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 49, 55, and 57 respectively for Brushy Creek Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Brushy Creek Lake in between the Mesotrophic and the Eutrophic categories. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and relatively low levels of phosphorus in the water column. The data show one violation of the Class A1 criterion for pH in 15 samples (7%). The level of inorganic suspended solids was low at Brushy Creek Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Brushy Creek Lake (1.6 mg/L) was ranked 9th among the 138 lakes by the ISU lake survey. Data from the 2012-2016 ISU lake survey suggest a small population of cyanobacteria exists at Brushy Creek Lake. These data show that cyanobacteria comprised 54% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (4.9 mg/L) was ranked 25th of the 138 lakes sampled. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the DNR Fisheries Bureau and results from the ISU lake surveys. Information from the DNR Fisheries Bureau indicate this lake has excessive growth of curly leaf pondweed and brittle naiad. Additionally, Zebra mussel veligers were recently discovered in the lake. While the lake shows good water quality now, the potential of poor water quality in the future is possible. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and one violation of the criterion for pH in 15 samples(7%). 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 Brushy Creek Lake.