Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2010-2012(2) results of the statewide survey of Iowa lakes conducted from 2009 through 2012 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) IDNR RAFT fish contaminant monitoring.
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. The Class B(LW)(aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses are assessed as "fully supporting." Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2010-2012 (2) results of the statewide survey of Iowa lakes conducted from 2009 through 2012 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) IDNR RAFT fish contaminant monitoring.
EXPLANATION: Results of IDNR beach monitoring from 2010-2012 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 2010 (16 samples), 2011 (15 samples), and 2012 (15 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 2014 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 2010-2012 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 84 E. coli orgs/100 ml in 2010, 69 E. coli orgs/100 ml in 2011, and 18 E. coli orgs/100 ml in 2012. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 31% in 2010, 33% in 2011 and 18% in 2012. The number of samples exceeding the single-sample maximum criterion in 2010 and 2011 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 15 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 56, 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 extremely 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 1.6 mg/L, which was ranked 4th 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 (18.0 mg/L) was ranked 66th of all 134 monitored lakes and that cyanobacteria accounted for 88% 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 (15 samples), dissolved oxygen (15 samples), or pH (15 samples).
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Brushy Creek Lake in 2013. The composite samples of fillets from largemouth bass and catfish had low levels of contaminants. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.232 ppm. Levels of primary contaminants in the composite sample of catfish fillets were as follows: total PCBs: <0.6 ppm; and technical chlordane: <0.2 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the YEAR RAFT sampling conducted at this Brushy Creek Lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
