Iowa DNR
Iowa DNR
ADBNet
Water Quality Assessments
Impaired Waters List

South Skunk River IA 03-SSK-927

from the Highway 63 bridge north of Oskaloosa (S25 T76N R16W Mahaska Co.) to confluence with Elk Cr. in NE 1/4 S19 T77N R17W Mahaska Co.

Assessment Cycle
2020
Release Status
Final
Data Collection Period
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Trend
Unknown
Created
8/12/2020 12:17:50 PM
Updated
10/16/2020 10:41:00 AM
Assessment conducted in accordance with Iowa's 2020 IR methodology
Use Support
Class A1
Recreation
Primary contact
Partially Supported
Impairments
Support Level
Partially 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 III
Class BWW1
Aquatic Life
Warm Water Type 1
WINOFI
Support Level
Water in Need of Further Investigation (WINOFI)
Impairment Code
3b-u - Use potentially biologically impaired based on uncalibrated IBI metrics.
Cause Magnitude
Moderate
Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
Low
Cycle Added
2014
Impairment Rationale
Low Biotic Index
Data Source
Biological monitoring: Iowa DNR WQMA
Class C
Drinking Water
Fully Supported
Class HH
Human Health
Fully Supported
General Use
General Use water
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Documentation
Assessment Summary

The Class A1 use was assessed as "partially supported" due to levels of indicator bacteria (E. coli) that violated state water quality criteria. The Class BWW1 use was assessed as "partially supported" based on biological sampling data. The Class C and the Class HH uses were assessed as "fully supported" based on water quality monitoring data.

Assessment Explanation

Sources of Data:

Data Source

Data Source ID

Data Type

Data Age

Site ID

BioNet Site ID

Site Name

Site Description

Iowa DNR

6

WQ & BIO

CY 2016-2018

10620001

945

South Skunk River near Oskaloosa


Class A1 - Indicator Bacteria:

Site ID

Data Source ID

# Samples / # Years

2016 Geometric Mean

2017 Geometric Mean

2018 Geometric Mean

Annual Geometric Mean Violation

# Violations

% Violations

Significantly >10% Violations

Assessment Type

Support Level

10620001

6

22 / 3

277.78

383.88

326.79

Yes

11

50%

Yes

Monitored

Partial


Class A - pH:

Site ID

Data Source ID

# Samples / # Years

Maximum Value

Mean Value

Median Value

# Violations

% Violations

Significantly >10% Violations

Assessment Type

Support Level

10620001

6

36 / 3

8.6

8.2

8.3

0

0%

No

Monitored

Full


Class BWW1 - Conventional Parameters:

Site ID

Data Source ID

Parameter Name

# Samples / # Years

Maximum Value

Mean Value

Median Value

# Acute / Chronic Violations

% Acute/ Chronic Violations

Significantly >10% Violations

Assessment Type

Support Level

10620001

6

Ammonia

36 / 3

0.74

0.06

0.02

0 / 0

0% / 0%

No

Monitored

Full

10620001

6

Chloride

36 / 3

50

23.9

21.5

0 / 0

0% / 0%

No

Monitored

Full

10620001

6

Dissolved oxygen

36 / 3

14.7

10.2

10.1

NA / 0

NA / 0%

No

Monitored

Full

10620001

6

pH

36 / 3

8.6

8.2

8.3

0 / NA

0% / NA

No

Monitored

Full

10620001

6

Sulfate

36 / 3

66

31.1

28.5

0 / NA

0% / NA

No

Monitored

Full

10620001

6

Temperature

36 / 3

25.4

12.1

9.9

0 / 0

0% / 0%

No

Monitored

Full


Class BWW1 - Toxic Parameters:

Site ID

Data Source ID

Parameter Name

# Samples / # Years

Maximum Value

Mean Value

Median Value

# Acute / Chronic Violations

% Acute/ Chronic Violations

>1 Acute / Chronic Violation

Assessment Type

Support Level

10620001

6

Aluminum

9 / 2

3.5

1.15

1.1

0 / 0

0% / 0%

No

Evaluated

Full

10620001

6

Arsenic

9 / 2

3.4

2.1

2.1

0 / 0

0% / 0%

No

Evaluated

Full

10620001

6

Cadmium

9 / 2

0.07

0.03

0.025

0 / 0

0% / 0%

No

Evaluated

Full

10620001

6

Chromium

9 / 2

1.2

0.85

0.97

0 / 0

0% / 0%

No

Evaluated

Full

10620001

6

Copper

9 / 2

3

1.4

1.2

0 / 0

0% / 0%

No

Evaluated

Full

10620001

6

Lead

9 / 2

0.99

0.191

0.07

0 / 0

0% / 0%

No

Evaluated

Full

10620001

6

Nickel

9 / 2

4.4

3.2

3.2

0 / 0

0% / 0%

No

Evaluated

Full

10620001

6

Selenium

9 / 2

2.7

1.24

0.89

0 / 0

0% / 0%

No

Evaluated

Full

10620001

6

Zinc

9 / 2

4.3

1.52

1.4

0 / 0

0% / 0%

No

Evaluated

Full


Class BWW1 – Biological Sampling:


Benthic Macroinvertebrates:

BioNet Site ID

Date

Drainage Area (mi2)

Ecoregion

Major Drainage

BM Sample Gear

BMIBI score

BMIBI Class

47f BMIBI BIT

BMIBI Pass/ Fail

945

9/6/16

1635

47f

MS

Floating Art Sub

51

Fair

51

Pass

945

10/7/13

1635

47f

MS

Art Sub

42

Fair

51

Fail

945

10/8/12

1635

47f

MS

Art Sub

18

Poor

51

Fail

Community

Assessment Type

Assessment Result

Support Level

Earned IR Category

Actual IR Category

Reason

Benthic Macroinvertebrate

Evaluated

Fail (1/3)

Partial

3bu

3bu

Assessment is evaluated because segment did not have two BM and/or fish samples collected in last five years and watershed size was larger than calibrated range.

Fish

NA

NA

NA

Overall

Evaluated

Fail

Partial


Class C - Conventional Parameters:

Site ID

Data Source ID

Parameter Name

# Samples / # Years

Maximum Value

Mean Value

Median Value

# Violations

% Violations

Significantly >10% Violations

Assessment Type

Support Level

10620001

6

Chloride

36 / 3

50

23.9

21.5

0

0%

No

Monitored

Full

10620001

6

Inorganic nitrogen (nitrate and nitrite)

36 / 3

13

5.58

5.55

1

3%

No

Monitored

Full


Class C - Toxic Parameters:

Site ID

Data Source ID

Parameter Name

# Samples / # Years

Maximum Value

Mean Value

Median Value

Number of Non - Detects

Mean Value > Criterion or MCL

Assessment Type

Support Level

10620001

6

Alachlor

3 / 1

0.05

0.05

0.05

3

No

All Non-Detect

Full

10620001

6

Atrazine

3 / 1

0.2

0.12

0.1

1

No

Evaluated

Full

10620001

6

Cadmium

9 / 2

0.07

0.03

0.025

8

No

Evaluated

Full

10620001

6

Chromium

9 / 2

1.2

0.85

0.97

0

No

Evaluated

Full

10620001

6

Lead

9 / 2

0.99

0.191

0.07

3

No

Evaluated

Full

10620001

6

Silver

9 / 2

0.025

0.025

0.025

9

No

All Non-Detect

Full

10620001

6

Simazine

3 / 1

0.05

0.05

0.05

3

No

All Non-Detect

Full


Class HH-FW - Toxic Parameters:

Site ID

Data Source ID

Parameter Name

# Samples / # Years

Maximum Value

Mean Value

Median Value

Number of Non - Detects

Mean Value > Criterion or MCL

Assessment Type

Support Level

10620001

6

Copper

9 / 2

3

1.4

1.2

0

No

Evaluated

Full

10620001

6

Nickel

9 / 2

4.4

3.2

3.2

0

No

Evaluated

Full

10620001

6

Selenium

9 / 2

2.7

1.24

0.89

0

No

Evaluated

Full

10620001

6

Zinc

9 / 2

4.3

1.52

1.4

0

No

Evaluated

Full

Monitoring and Methods
Assessment Key Dates
10/7/2013
Biological Monitoring
10/8/2012
Biological Monitoring
1/6/2016
Fixed Monitoring Start Date
12/10/2018
Fixed Monitoring End Date
9/6/2016
Biological Monitoring
Methods
150
Monitoring data more than 5 years old
230
Fixed station physical/chemical (conventional plus toxic pollutants)
315
Regional reference site approach
320
Benthic macroinvertebrate surveys
420
Indicator bacteria monitoring