Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2008, 2009, and 2010 (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to violations of the state's indicator bacteria criteria. This is a new impairment for this lake. The Class B(LW)(aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses remain “not assessed.” Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2008, 2009, and 2010 (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
EXPLANATION: Results of IDNR beach monitoring from 2008 through 2010 suggest that the Class A1 uses are “partially supported." Levels of indicator bacteria at Brushy Creek Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2008 (27 samples), 2009 (26 samples), and 2010 (16 samples) as part of the IDNR beach monitoring program. According to IDNR’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 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, 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).
NOTE: Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed. Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season. Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake. Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches. Thus, for the 2012 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion. This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.
At Brushy Creek Lake beach, the geometric means from 2008, 2009, and 2010 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 28 E. coli orgs/100 ml in 2008, 32 E. coli orgs/100 ml in 2009, and 84 E. coli orgs/100 ml in 2010. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 7% in 2008, 15% in 2009 and 31% in 2010. The number of samples exceeding the single-sample maximum criterion in 2010 were significantly greater than 10% of the samples and therefore suggest impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “partial support” of the Class A1 uses.
Results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program, however, show “full support” of the Class A1 uses of Brushy Creek Lake. Using the median values from these surveys from 2006 through 2010 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 55, 63, and 62 respectively for Brushy Creek Lake. According to Carlson (1977) the Secchi depth value places Brushy Creek Lake in the eutrophic category, while the chlorophyll a and total phosphorus values place Brushy Creek Lake in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately high levels of phosphorus in the water column.
The level of inorganic suspended solids is also low at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Brushy Creek Lake was 2.0 mg/L, which was the 8th lowest of the 134 monitored lakes.
A small population of cyanobacteria exists at Brushy Creek Lake, which does not cause impairment at this lake. Data from the ISU and UHL lake surveys show that the median cyanobacteria wet mass at Brushy Creek Lake (10.3 mg/L) was the 25th lowest of all 134 monitored lakes and that cyanobacteria only account for 63% of the total phytoplankton wet mass.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. The ISU and UHL lake survey results show good chemical water quality at Brushy Creek Lake. During 2006-2010 there were no violations of the Class B(LW) criterion for ammonia (22 samples), dissolved oxygen (23 samples), or pH (23 samples).
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
