Willow Creek IA 02-CED-6593

from mouth (T99N R18W Sec26) to headwaters (T99N R18W Sec19) Mitchell 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 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.

Open Segment Map
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Cycle: 2016
Release Status: Final
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trend: Unknown
Created: 8/12/2016 10:40:55 AM
Updated: 9/29/2016 9:07:38 AM

Use Support

Class A1

Presumptive Use

Recreation - Primary contact

Partially Supported

Bacteria: Indicator Bacteria- E. coli

Support Level

Partially Supported

Impairment Code

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

Cause Magnitude

Slight

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

Fully Supported

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 are assessed (monitored) as “not supported” (IR 5p) due to violations of Iowa water quality criteria for indicator bacteria. The presumptive Class B(WW1) aquatic life uses are assessed as “fully supporting (IR 2a) based on results of chemical/physical monitoring. The source of data for this assessment is the results of monitoring at STORET station 15660005 (Willow Creek at Echo Ave.) conducted as part of a Clean Water Act Section 319 water quality project from May 2012 through November 2013.

Assessment Explanation

The presumptive Class A1 uses are assessed (monitored) as “not supported" (IR 5p) 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 13 samples collected at station 15660005 during the recreation seasons of 2012 and 2013 were as follows: the geometric mean during the 2012 recreation season was 409 orgs/100 ml, and the geometric mean during the 2013 recreation season was 112 orgs/100 ml. The 2012 geometric mean exceeds the Class A1 geometric mean criterion of 126 orgs/100 ml. Five of the 13 samples (38%) 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 IDNR’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). Thus, the Class A1 uses are assessed as impaired (“partially supporting”). In addition, the percentage of samples with levels of E. coli that exceeded Iowa single-sample maximum criteria is significantly greater than 10% and thus also indicates impairment of presumptive Class A1 uses at this monitoring site.

Results of chemical/physical monitoring at station 15660005 during the 2012-2013 period suggest "full support" of the presumptive Class B(WW1) aquatic life uses. None of the 15 samples analyzed violated the Class B(WW1) criteria for dissolved oxygen, pH, or ammonia, and none of the seven samples analyzed for chloride violated the respective Class B(WW1) criteria. One of the 15 samples (7%) violated the Class B(WW1) criterion for temperature of 32C. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as pH and dissolved oxygen suggests impairment of aquatic life uses. Based on IDNR’s assessment methodology, however, these results suggest that the frequency of violations is not significantly greater than 10 percent; thus, these results do not suggest impairment of the Class B(WW1) aquatic life uses.

Monitoring and Methods

Assessment Key Dates

5/22/2012

Fixed Monitoring Start Date

11/20/2013

Fixed Monitoring End Date

Methods

240

Non-fixed station physical/chemical (conventional + toxicants)

420

Indicator bacteria monitoring