Walnut Creek IA 04-RAC-1120

mouth (S13 T78 R25W Polk Co.) to Interstate 35/80 in S33 T79N R25W Polk 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.

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Cycle: 2018
Release Status: Final
Overall IR: 4 - Water is impaired or threatened and a TMDL has been completed or is not needed.
Trend: Unknown
Created: 5/15/2019 10:23:36 AM
Updated: 8/1/2019 2:28:54 PM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

High

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2010

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Volunteer monitoring

Class BWW2

Aquatic Life - Warm Water Type 2

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 (evaluated) as “not supported” based on data from snapshot monitoring. The Class B(WW-2) aquatic life remain “not assessed” due to lack of data upon which to base an assessment. The source of data for this assessment remains the results for E. coli monitoring from 2014-2016 at Three monitoring stations sampled as part of Snapshot monitoring: station 977111 (Polk County Snapshot (Site WC2 - Walnut Creek)), station 977113 (Polk County Snapshot (Site WC4 - Walnut Creek)), and station 977115 (Polk County Snapshot (Site WC6 - Walnut Creek)).

Assessment Explanation

Note: A TMDL for pathogen indicator (E. coli) impairments in the Raccoon River basin was prepared by DNR and approved by EPA in June 2008. Thus, this impairment is placed into Category 4a of Iowa’s Integrated Report.

The Class A1 (primary contact recreation) uses are assessed (evaluated) as "partially supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 6 samples collected during the recreational seasons of 2014 through 2016 at station 977111 (Polk County Snapshot (Site WC2 - Walnut Creek)) were as follows: the 2014 geometric mean was 192 orgs/100 ml, the 2015 geometric mean was 322 orgs/100 ml, and the 2016 geometric mean was 173 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Three of the combined 6 samples (50%) exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml. The geometric means of indicator bacteria (E. coli) in the 6 samples collected during the recreational seasons of 2014 through 2016 at station 977113 (Polk County Snapshot (Site WC4 - Walnut Creek)) were as follows: the 2014 geometric mean was 124 orgs/100 ml, the 2015 geometric mean was 401 orgs/100 ml, and the 2016 geometric mean was 249 orgs/100 ml. Two of the three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Four of the combined 6 samples (67%) exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml.The geometric means of indicator bacteria (E. coli) in the 6 samples collected during the recreational seasons of 2014 through 2016 at station 977115 (Polk County Snapshot (Site WC6 - Walnut Creek)) were as follows: the 2014 geometric mean was 177 orgs/100 ml, the 2015 geometric mean was 401 orgs/100 ml, and the 2016 geometric mean was 228 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Four of the combined 6 samples (67%) exceeded Iowa’s 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 the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as "partially supported."

Monitoring and Methods

Assessment Key Dates

5/7/2014

Non-fixed Monitoring Start Date

10/4/2016

Non-fixed Monitoring End Date

6/1/2008

TMDL Completed

Methods

150

Monitoring data more than 5 years old

220

Non-fixed station physical/chemical monitoring (conventional pollutant only)

420

Indicator bacteria monitoring