Assessment Comments
Assessment is based on results of county beach monitoring from 2008 through 2010.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on results of the county beach monitoring program at Gabrielson Park beach at Gustafson Lake. The Class B(LW) aquatic life uses are "not assessed" due to a lack of information upon which to base an assessment. The fish consumption uses are also "not assessed" due to a lack of fish contaminant monitoring at this lake.
EXPLANATION: Results of IDNR city/county beach monitoring from 2010-2012 suggest that the Class A1 uses are “partially supported." Levels of indicator bacteria at Gabrielson Park beach were monitored once per week during the primary contact recreation seasons (May through September) of 2010 (12 samples), 2011 (13 samples), and 2012 (12 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 Gabrielson Park 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 109 E. coli orgs/100 ml in 2010, 108 E. coli orgs/100 ml in 2011, and 34 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 25% in 2010 and 17% in 2012. These are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. The percentage of samples excceding Iowa'a single-sample maximum criterion was 38% in 2011, and suggests "partial support" for the Class A1 uses at Gustafson Lake.
