Gustafson Lake IA 06-LSR-1625

Assessment Comments

Assessment is based on results of county beach monitoring from 2006 through 2008.

Basis for Assessment

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "fully 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 2006 through 2008 suggest that the Class A1 uses are "fully supported."  Levels of indicator bacteria at Gabrielson Park beach were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (15 samples), 2007 (14 samples), and 2008 (13 samples) as part of the IDNR beach monitoring program.    According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E.  coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses.   If a 5-sample, 30-day 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”.   This assessment approach is based on U.S.  EPA guidelines (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).      

At Gabrielson Park beach, the geometric means of all 28 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 were below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E.   coli orgs/100 ml) was 7% in 2006, 0% in 2007 and 23% in 2008.   According to IDNR’s assessment methodology and U.S.  EPA guidelines, these results suggest full support of the Class A1 (primary contact recreation) uses.