Backbone Lake IA 01-MAQ-20

Assessment Comments

Assessment is based on: results of IDNR/UHL beach monitoring from 2010 through 2012.

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 "not assessed" due to a lack of data upon which to base an assessment.    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 IDNR/UHL beach monitoring from 2010 through 2012.    

EXPLANATION:  Results of IDNR beach monitoring from 2010 through 2012 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 2010 (21 samples), 2011 (16 samples), and 2012 (19 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 2012 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 Backbone Lake beach, the geometric means from 2010 and 2011 were above the Iowa water quality standard of 126 E.   coli orgs/100 ml.     The geometric mean was 195 in 2010 and 400 in 2010.    The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E.   coli orgs/100 ml) was also significantly greater than 10% in 2010 (63%), in 2011 (31%) and in 2012 (37%).   According to IDNR’s assessment methodology and U.S.   EPA guidelines, these results suggest that the Class A1 uses should be assessed as "not supported."

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 "not assessed" due to a lack of information upon which to base an assessment.    

The fish consumption uses of this lake are “not assessed” due to the lack of fish contaminant information upon which to base an assessment.