North Skunk River IA 03-NSK-854

from confluence with Cedar Cr. (S15 T75N R12W Keokuk Co.) to confluence with Middle Cr. in S35 T76N R14W Mahaska Co.

Assessment Cycle

2022

Release Status

Final

Data Collection Period

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 2018 - 2020
Stream/River Water Quality Data	CY 2018 - 2020
Lake Water Quality Data	CY 2016 - 2020
Biological Sampling Data	CY 2016 - 2020*
Fish Kill Data	CY 2016 - 2020
Fish Tissue Sampling Data	CY 2016 - 2020
Continuous Dissolved Oxygen Sampling Data	CY 2016 - 2020
*Biological sites sampled in both 2015 and 2019 were also assessed.

Overall IR Category: 5 - Water is impaired or threatened and a TMDL is needed.
Trend: Unknown

Created: 12/27/2021 8:20:02 AM
Updated: 1/12/2022 2:07:03 PM

Assessment conducted in accordance with Iowa's 2022 IR methodology

Use Support

Class A1

Recreation

Primary contact

Not Supported

Impairments

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2008

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Ambient monitoring: Iowa DNR-rivers

TMDL Priority

Tier IV

Class BWW2

Aquatic Life

Warm Water Type 2

Fully Supported

Impairment Delistings

Cycle Added	Class	Cause	Data Source	Rationale
2010	Class BWW2	Metals: Chromium	Ambient monitoring: Iowa DNR-rivers	New data: WQ improvement (chemical / physical / bacterial)

Documentation

Assessment Summary

The Class A1 use was assessed as "not supported" due to levels of indicator bacteria (E. coli) that exceeded state water quality criteria. The Class BWW2 use was assessed as "fully supported" based on biological sampling data and water quality monitoring data.

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	WQ	CY 2018-2020	10540001	North Skunk River
Iowa DNR	6	BIO	See Below	944	North Skunk River	Oskaloosa

Class A1 - Indicator Bacteria:

Site ID

Data Source ID

# Samples per Year (2018/ 2019/2020)

2018 Geometric Mean

2019 Geometric Mean

2020 Geometric Mean

Violates Annual Geometric Mean?

# 2018 SSM violations / SSM Violations needed for impairment

# 2019 SSM violations / SSM Violations needed for impairment

# 2020 SSM violations / SSM Violations needed for impairment

Violates Annual SSM Significantly >10% Rule?

Assessment Type

Support Level

10540001

8/8/8

247

544

169

YES

4 / 3

6 / 3

2 / 3

YES

Monitored

Not

Class A - pH

Site ID	Data Source ID	# Samples / # Years	Minimum Value	Maximum Value	Mean Value	# Violations	Violations needed for impairment	Violates Significantly >10% Rule?	Assessment Type	Support Level
10540001	6	36 / 3	6.7	8.5	7.9	0	7	NO	Monitored	Full

Class BWW2 - Conventional Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Minimum Value	Maximum Value	Mean Value	# Violations	# Acute / Chronic Violations	Violations needed for impairment	Violates Significantly >10% Rule?	Assessment Type	Support Level
10540001	6	Ammonia	36 / 3	0.02	0.53	0.07	NA	0 / 0	7	NO	Monitored	Full
10540001	6	Chloride	36 / 3	7.2	53	22.3	NA	0 / 0	7	NO	Monitored	Full
10540001	6	Dissolved Oxygen	36 / 3	4.2	13.6	10.1	1	NA / NA	7	NO	Monitored	Full
10540001	6	pH	36 / 3	6.7	8.5	7.9	0	NA / NA	7	NO	Monitored	Full
10540001	6	Sulfate	36 / 3	9.2	120	28.6	0	NA / NA	7	NO	Monitored	Full
10540001	6	Temperature	36 / 3	0.1	26.8	11.6	0	NA / NA	7	NO	Monitored	Full

Class BWW2 - Toxic Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Acute / Chronic Violations	>1 Acute / Chronic Violation	Assessment Type	Support Level
10540001	6	Aluminum	14 / 3	6.2	1.9	1.1	0 / 0	NO	Monitored	Full
10540001	6	Arsenic	13 / 3	2.2	1.2	1	0 / 0	NO	Monitored	Full
10540001	6	Cadmium	13 / 3	0.05	0.03	0.02	0 / 0	NO	Monitored	Full
10540001	6	Chromium	13 / 3	1.3	0.7	0.7	0 / 0	NO	Monitored	Full
10540001	6	Lead	13 / 3	0.27	0.12	0.1	0 / 0	NO	Monitored	Full
10540001	6	Nickel	13 / 3	4.3	2.9	2.8	0 / 0	NO	Monitored	Full
10540001	6	Silver	13 / 3	0.025	0.025	0.025	0 / 0	NO	Monitored	Full
10540001	6	Zinc	13 / 3	3.3	1.9	2	0 / 0	NO	Monitored	Full

Class BWW2 - Biological Sampling:

Benthic Macroinvertebrates:

Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage	BM Sample Gear	BMIBI score	BMIBI Class	47f BMIBI BIT	BMIBI Pass/Fail
944	8/29/19	631	47f	MS	Floating Art Sub	72	Good	51	Pass

Overall:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Evaluated	Pass (1/1)	Full		Assessment was evaluated because the drainage area of the site used for
Overall	Evaluated	Pass	Full	1	the assessment was greater than the calibrated range.

Monitoring and Methods

Assessment Key Dates

1/4/2018

Fixed Monitoring Start Date

8/29/2019

Biological Monitoring

12/8/2020

Fixed Monitoring End Date

Methods

230

Fixed station physical/chemical (conventional plus toxic pollutants)

315

Regional reference site approach

320

Benthic macroinvertebrate surveys

420

Indicator bacteria monitoring