Assessment Comments
Assessment is based on: (1) results of IDNR/UHL beach monitoring from 2004 through 2006 and (2) results of monitoring for conventional parameters (i.e., dissolved oxygen, pH, and water temperature) conducted as part of the IDNR/UHL beach monitoring.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses remain assessed (monitored) as "not supported" due to high levels of indicator bacteria. The Class B(WW-1) (aquatic life) uses are assessed (evaluated) as “fully supporting.” Fish consumption uses were not assessed due to lack of fish tissue information upon which to base an assessment. The sources of data for this assessment include results of (1) IDNR/UHL beach monitoring from 2004 through 2006 and (2) results of monitoring for conventional parameters (i.e., dissolved oxygen, pH, and water temperature) conducted as part of the IDNR/UHL beach monitoring.
EXPLANATION: Results of IDNR beach monitoring from 2004 through 2006 suggest that the Class A uses are "not supported." Levels of indicator bacteria at Backbone Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2004 (22 samples), 2005 (23 samples), and 2006 (28 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 A (primary contact recreation) uses: (1) all thirty-day geometric means for 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 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A 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 A 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 Backbone Lake beach, the geometric means of 40 thirty-day periods during the summer recreation seasons of 2004, 2005 and 2006 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml: 18 of 18 geometric means violated in 2004, 11 of 19 geometric means violated in 2005, and 11 of 24 geometric means violated in 2006. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was significantly greater than 10% in the following recreation seasons: 2004: 59%, 2005: 39%, and 2006: 25%. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A (primary contact recreation) uses.
BackboneLake was sampled as part of IDNR’s Safe Lakes Program, which aims to identify sources of bacteria to selected beaches where bacteria levels have consistently violated the state water quality criteria. The Safe Lakes Program found that geese and bacteria in the sand at the beach were the likely sources of bacteria to the lake.
The Class B(WW-1) aquatic life uses of this lake are assessed (evaluated) as “fully supported” based on physical/chemical data collected as part of the IDNR/UHL beach monitoring program. These data show no violations of the Class B(WW-1) criterion for dissolved oxygen in the 70 samples collected from 2004 through 2006. Only 1 of 70 samples (1%) violated the Class A, B(WW-1) criterion for pH. Although these results indicate “full support” of the aquatic life uses of this lake, the parameter coverage is relatively poor, and the level of this assessment is therefore considered “evaluated” rather than “monitored.”
The fish consumption uses of this lake are “not assessed” due to the lack of fish contaminant information upon which to base an assessment.
