Willow Creek IA 02-IOW-6586

from mouth (T81N R9W Sec8 Benton Co.) to headwaters (T82N R9W Sec29 SW Benton Co.)

Open Data Collection Period Modal

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 2014 - 2016
Stream/River Water Quality Data	CY 2014 - 2016
Lake Water Quality Data	CY 2012 - 2016
Biological Sampling Data	CY 2012 - 2016*
Fish Kill Data	CY 2012 - 2016
Fish Tissue Sampling Data	CY 2012 - 2016
Continuous Dissolved Oxygen Sampling Data	CY 2012 - 2016
*Biological sites sampled in both 2011 and 2015 were also assessed.

Open Segment Map
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Cycle: 2018
Release Status: Final
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trend: Unknown
Created: 7/15/2019 7:27:55 AM
Updated: 7/15/2019 7:30:37 AM

Use Support

Class A1

Presumptive Use

Recreation - Primary contact

Not Supported

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

5p - Impairment occurs on a waterbody with a presumptive A1 or B(WW1) use.

Cause Magnitude

High

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2014

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Special project/study

TMDL Priority

Tier III

Class BWW1

Presumptive Use

Aquatic Life - Warm Water Type 1

Not Assessed

General Use

General Use water -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The presumptive Class A1 (primary contact recreation) uses remain assessed (monitored) as “not supported” due to violations of Iowa water quality criteria for indicator bacteria. The presumptive Class B(WW1) aquatic life uses remain "not assessed" due to the lack of sufficient water quality information upon which to base an assessment. The source of data for this assessment is the results of monitoring at STORET station 13480009 (Willow Creek at Benton-Iowa Road near T Ave, Site 7B) conducted as part of a Clean Water Act Section 319 water quality project from May 2013 through September 2014.

Assessment Explanation

The presumptive Class A1 uses are assessed (monitored) as “not supported" due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion. The geometric mean of indicator bacteria (E. coli) in the 2 samples collected at Site 7B during the recreation season of 2013 was 442 orgs/100 ml, and the geometric mean of the 11 samples collected during the recreation season of 2014 was 1,270 orgs/100 ml. Note: According to the data completeness guidelines in Iowa DNR’s assessment/listing methodology, at least seven independent samples per recreation season are needed to calculate a geometric mean appropriate for comparison to the Class A geometric mean criteria. Too few samples (2) were collected at the Site 7B during the 2013 recreation season to allow calculation of a useful geometric mean. During the 2014 recreation season, however, a sufficient number of samples (11) was collected at thismonitoring site.Twelve of the 13 samples (92%) exceeded the Class A1 single sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and DNR’s assessment/listing methodology, if a recreation season geometric mean is greater than the respective criterion (126 orgs/100 ml for Class A1, the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). In addition, the percentage of samples with levels of E. coli that exceeded Iowa single-sample maximum criteria indicates impairment of presumptive Class A1 uses at this monitoring site.

Monitoring and Methods

Assessment Key Dates

5/16/2013

Fixed Monitoring Start Date

9/29/2014

Fixed Monitoring End Date

Methods

420

Indicator bacteria monitoring