Iowa DNR
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Water Quality Assessments

Impaired Waters List

Backbone Lake IA 01-MAQ-20

Backbone Lake Dam to S Fk Maquoketa R. (S16 T90N R6W Delaware Co.)

Assessment Cycle
2018
Release Status
Final
Release Period
2014 - 2016
Created
10/12/2018 2:04:18 PM
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Trophic
Unknown
Trend
Unknown
Last Updated
1/29/2019 10:34:25 AM
Assessments conducted in accordance with Iowa's Integrated Report methodology.
Use Support
Class A1
Recreation
Primary contact
Partially Supporting
Bacteria: Indicator Bacteria- E. coli
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
High
Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2004
Impairment Rationale
Geometric mean criterion exceeded
Data Source
Beach monitoring: Iowa DNR WQMA
TMDL Priority
Tier II
Class BWW1
Aquatic Life
Warm Water Type 1
Not Assessed
Class HH
Human Health
Not Assessed
General Use
General Use water
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Documentation
Assessment Summary

The Class A1 (primary contact recreation) uses remain assessed (monitored) as "partially 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 (1) IDNR/UHL beach monitoring from 2014 through 2016 and (2) IDNR TMDL monitoring from 2012.

Assessment Explanation

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.

Monitoring and Methods
Assessment Key Dates
5/9/2012 Fixed Monitoring Start Date
8/29/2016 Fixed Monitoring End Date
Methods
420 Water column surveys (e.g. fecal coliform)
222 Non-fixed-station monitoring (conventional during key seasons and flows)