Rock Creek IA 02-CED-585

mouth (S24 T97N R17W Floyd Co.) to confluence with unnamed tributary in NW 1/4 SE 1/4 S17 T97N R17W 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 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
View assessment methodology in new window

Cycle: 2018
Release Status: Final
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trend: Unknown
Created: 7/17/2019 8:52:21 AM
Updated: 7/17/2019 8:56:47 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

2008

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Special project/study

TMDL Priority

Tier III

Class BWW1

Aquatic Life - Warm Water Type 1

Fully Supported

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 presumptive Class A1 uses are assessed (monitored) as “partially supported” (IR 5p) due to levels of indicator bacteria that exceed state water quality criteria. The Class B(WW1) aquatic life uses are assessed (monitored) as “fully supported” (IR 2a) based on results of chemical/physical water quality monitoring from 2012-2014. Fish consumption uses remain not assessed (IR 3a) due to the lack of fish contaminant monitoring in this assessment segment. Sources of data for this assessment include the results of ambient chemical/physical water quality monitoring conducted at monitoring station 15340005 at County Road T38 monitored from 2014 through 2016.

Assessment Explanation

The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) 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 18 samples collected during the recreational seasons of 2014 through 2016 at DNR station 15340005 at county road T38 were as follows: the 2014 geometric mean was 321 orgs/100 ml, the 2015 geometric mean was 127 orgs/100 ml, and the 2016 geometric mean was 413 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Twelve of the combined 18 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 Iowa 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."

Results of chemical/physical water quality monitoring at DNR station 15340005 at county road T38 suggest “full support” of the aquatic life uses. Monitoring showedno violations of Class B(WW1) water quality criteria for 5 Dissolved Oxygen samples (minimum = 7.5 mg/L), 11 pH samples (range = 7.7 to 8.5), or 11 Temperature samples (maximum = 23.9°C) occurred during monitoring from July 2015 to October 2016.According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated. Thus, these results thus suggest “full support” of the Class B(WW1) aquatic life uses.

Fish consumption uses remain not assessed due to the lack of fish contaminant monitoring in this assessment segment.

Monitoring and Methods

Assessment Key Dates

4/22/2014

Fixed Monitoring Start Date

10/10/2016

Fixed Monitoring Start Date

Methods

220

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

420

Indicator bacteria monitoring