2020 Final assessment for South Skunk River

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

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 2016 - 2018
Stream/River Water Quality Data	CY 2016 - 2018
Lake Water Quality Data	CY 2014 - 2018
Biological Sampling Data	CY 2014 - 2018*
Fish Kill Data	CY 2014 - 2018
Fish Tissue Sampling Data	CY 2014 - 2018
Continuous Dissolved Oxygen Sampling Data	CY 2014 - 2018
*Biological sites sampled in both 2013 and 2017 were also assessed.

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

Partially Supported

Bacteria: Indicator Bacteria- E. coli

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

WINOFI

Biological: low aquatic macroinvertebrate IBI

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

Fully Supported

Class HH

Fully Supported

General Use

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

Water Quality Assessments

Impaired Waters List

South Skunk River IA 03-SSK-927

Assessment Summary

Assessment Explanation

Assessment Key Dates

Methods