Backbone Lake IA 01-MAQ-20

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

Assessment Cycle

2016

Release Status

Final

Data Collection Period

For purposes of developing stream/river water quality assessments for integrated reporting, three years of water quality data are typically used for both conventional pollutant parameters (e.g., indicator bacteria) and the less frequently monitored toxic parameters (e.g., toxic metals). Since the 2004 IR cycle, DNR has used a three-year data gathering period for streams, rivers, and beaches. For most assessments, the use of three years of data increases the number of samples upon which the decision on use support is based and helps address the problem of weather-related year-to-year fluctuations in water quality.

Due to the lower sampling frequency in Iowa’s ambient lake, biological, fish kill, and fish tissue monitoring programs, five years of data are used for developing IR assessments and for identifying Section 303(d) listings.

Data Type	Data Timeframe
Beach Sampling Data	CY 2012 - 2014
Stream/River Water Quality Data	CY 2012 - 2014
Lake Water Quality Data	CY 2010 - 2014
Biological Sampling Data	CY 2010 - 2014*
Fish Kill Data	CY 2010 - 2014
Fish Tissue Sampling Data	CY 2010 - 2014
Continuous Dissolved Oxygen Sampling Data	CY 2010 - 2014
*Biological sites sampled in both 2009 and 2013 were also assessed.

Overall IR Category: 5 - Water is impaired or threatened and a TMDL is needed.
Trophic: Unknown
Trend: Unknown

Created: 4/11/2016 9:33:39 AM
Updated: 1/19/2017 9:28:02 AM

Assessment conducted in accordance with Iowa's 2016 IR methodology

Use Support

Class A1

Recreation

Primary contact

Partially Supported

Impairments

Support Level

Partially Supported

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 III

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 IDNR/UHL beach monitoring from 2012 through 2014.

Assessment Explanation

Results of IDNR beach monitoring from 2012 through 2014 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 2012 (19 samples), 2013 (18 samples) and 2014 (16 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 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 2012 was below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric means from 2013 and 2014, however, were 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 78 E. coli orgs/100 ml in 2012, 257 E. coli orgs/100 ml in 2013 and 346 E. coli orgs/100 ml in 2014. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 37% in 2012, 50% in 2013 and 56% in 2014. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2012, 2013 and 2014. According to IDNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

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.

Monitoring and Methods

Assessment Key Dates

5/21/2012

Fixed Monitoring Start Date

8/25/2014

Fixed Monitoring End Date

Methods

None listed