Assessment Comments

Assessment is based on (1) results of IDNR beach monitoring for indicator bacteria during summers of 2002, 2003, and 2004 and (2) results of chemcial water quality monitoring conducted as part of the IDNR beach monitoring program.

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 source of data for this assessment is the IDNR beach monitoring program from 2002 through 2004.   The Class B(WW) aquatic life uses are assessed (evaluated) as “fully supporting”.   Fish consumption uses were not assessed due to lack of water quality information upon which to base an assessment.   The sources of data for this assessment include results of (1) IDNR/UHL beach monitoring from 2002 through 2004 and (2) results 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 at Backbone Lake from 2002 through 2004 suggest that the Class A uses are "not supported."  Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (31 samples), 2003 (29 samples), and 2004 (23 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 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 3-35of U.S.  EPA 1997b).  

At Backbone Lake beach, the geometric means of 33 thirty-day periods during the summer recreation seasons of 2002, 2003 and 2004 exceeded the Iowa water quality standard of 126 E.  coli orgs/100 ml.   8 of 27 geometric means violated in 2002, 6 of 25 geometric means violated in 2003, and 19 of 19 geometric means violated in 2004.   Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E.  coli orgs/100 ml) was greater than 10% in all three recreation seasons:  2002:  32%; 2003:  24%; 2004:  61%);  According to IDNR’s assessment methodology and U.S.  EPA guidelines, these results suggest impairment (nonsupport) of the Class A (primary contact recreation) uses.

The Class B(WW) aquatic life uses of this lake are assessed (evaluated) as "fully supported " based on information on conventional parameters collected as part of the IDNR/UHL beach monitoring program.   These data show no violations of Iowa water quality criteria for dissolved oxygen in the 82 samples collected over the summer seasons of 2002 through 2004.   Only 1 of 82 samples (less than 2%) violated the water quality criterion for pH:  the sample collected on August 17, 2004 contained a pH of 9.3 units, thus exceeding the state criterion of 9.0 units.   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 thus relatively low (i.e., “evaluated” vs.  “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.