Backbone Lake IA 01-MAQ-20

Results of DNR beach monitoring from 2014 through 2016 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria at Backbone Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2014 (16 samples), 2015 (17 samples) and 2016 (15 samples), as part of the DNR beach monitoring program. According to DNR’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 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or 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).

At Backbone Beach, the geometric mean from 2014, 2015 and 2016 wereall greater than the Iowa water quality standard of 126 E. coli orgs/100ml and therefore suggests impairment of the Class A1 uses. The geometric mean was 346 E. coli orgs/100 ml in 2014, 438 E. coli orgs/100 ml in 2015 and 662 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 56% in 2014, 65% in 2015 and 73% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2014, 2015 and 2016. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Backbone Lake are assessed (evaluated) as "fully supported" based on information from DNR TMDL monitoring. Using the median values from these surveys from 2012-2012 (approximately 5 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 58, and 67 respectively for Backbone Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Backbone Lake in the Eutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 5 samples.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as "fully supported." Results of theDNR TMDL monitorinfrom 2012-2012 show there were no violations of the criterion for ammonia in 5 samples(0%), no violations of the criterion for dissolved oxygen in 5 samples(0%), and no violations of the criterion for pH in 5 samples(0%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/evaluated) of the Class B(LW) uses of Backbone Lake.

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