Assessment Comments

Assessment is based on: (1) results of IDNR/UHL beach monitoring from 2006 through 2008 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 A1 (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 2006 through 2008 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 2006 through 2008 suggest that the Class A1 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 2006 (28 samples), 2007 (19 samples), and 2008 (22 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 Backbone Lake beach, the geometric means of 37 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 exceeded the Iowa water quality standard of 126 E.  coli orgs/100 ml: 11 of 24 geometric means violated in 2006, 9 of 15 geometric means violated in 2007, and 17 of 17 geometric means violated in 2008.    The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E.  coli orgs/100 ml) was 25% in 2006, 32% in 2007 and 64% in 2008.   According to IDNR’s assessment methodology and U.S.  EPA guidelines, these results suggest impairment (nonsupport) of the Class A1 (primary contact recreation) uses.  

Backbone Lake 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 65 samples collected from 2006 through 2008.   Only 1 of 65 samples (2%) violated the Class A1,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.