Unnamed Tributary to Silver Creek IA 01-UIA-6569

from mouth (T99N R11W Sec13) to headwaters (T99N R11W Sec23) Howard 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: 7/19/2019 9:24:05 AM
Updated: 7/19/2019 9:25:50 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

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2014

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Watershed project monitoring

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 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 assessed as “fully supporting: based on results of chemical/physical monitoring. The source of data for this assessment remains the results of monitoring at STORET stations 15450017 (Trib to Silver Creek DS of WWTP) and STORET station 15450018 (Trib to Silver Creek US of WWTP) conducted as part of a Clean Water Act Section 319 water quality project from April 2011 through July 2012.

Assessment Explanation

The presumptive Class A1 uses remain assessed (monitored) as “not supported" due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion. The geometric means of indicator bacteria (E. coli) in the 22 samples collected at Site 2 during the recreation seasons of 2011 and 2012 were as follows: the 2011 geometric mean was 272 orgs/100 ml and the 2012 geometric mean was 2,186 orgs/100 ml. Fifteen of the 22 samples (68%) exceeded the Class A1 single sample maximum criterion of 235 orgs/100 ml. Similarly, the geometric means of indicator bacteria (E. coli) in the 22 samples collected at Site 1 during the recreation seasons of 2011 and 2012 were as follows: the 2011 geometric mean was 315 orgs/100 ml and the 2012 geometric mean was 557 orgs/100 ml. Sixteen of the 22 samples (73%) 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.

Results of chemical/physical monitoring at Sites 1 and 2 during the 2011-12 period suggest "full support" of the presumptive Class B(WW1) aquatic life uses. None of the approximately 15 samples analyzed from May to November 2011 at Sites 1 and 2 violated the Class B(WW1) criteria for ammonia, dissolved oxygen, pH, or temperature. Based on DNR’s assessment methodology, these results suggest that the presumptive Class B(WW1) uses should be assessed as "fully supporting."

Monitoring and Methods

Assessment Key Dates

4/18/2011

Fixed Monitoring Start Date

7/11/2012

Fixed Monitoring End Date

Methods

220

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

420

Indicator bacteria monitoring