Lake Keomah IA 03-SSK-930

Mahaska County S13T75NR15W 4.5 mi E of Oskaloosa.

This is a DRAFT assessment

Draft assessments may change without notice

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 2022 - 2024
Stream/River Water Quality Data	CY 2022 - 2024
Lake Water Quality Data	CY 2020 - 2024
Biological Sampling Data	CY 2020 - 2024*
Fish Kill Data	CY 2020 - 2024
Fish Tissue Sampling Data	CY 2020 - 2024
Continuous Dissolved Oxygen Sampling Data	CY 2020 - 2024
*Biological sites sampled in both 2019 and 2023 were also assessed.

Open Segment Map
View 2026 assessment summary page

Cycle: 2026
Current Phase: Public Comment
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trophic: Eutrophic
Trend: Stable
Created: 10/27/2025 3:09:44 PM
Updated: 11/24/2025 2:25:39 PM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Turbidity: Secchi Disk Transparency

Support Level

Not Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

Slight

Status

Continuing

Source

Other: Sediment Resuspension

Source Confidence

Low

Cycle Added

2022

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

Algal Growth: Chlorophyll a

Support Level

Not Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

Slight

Status

Continuing

Source

Agriculture

Source Confidence

Low

Cycle Added

2024

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Moderate

Cycle Added

2008

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Beach monitoring: Iowa DNR WQMA

Class BLW

Aquatic Life - Lakes and wetlands

WINOFI

Algal Growth: Cyanobacteria

Support Level

Further Investigation Needed

Impairment Code

3b - Use potentially impaired based on an evaluated assessment.

Cause Magnitude

Slight

Status

Continuing

Source

Agriculture

Source Confidence

Low

Cycle Added

2022

Impairment Rationale

Adverse impacts on plant/animal communities

Data Source

Ambient monitoring: Iowa DNR-lakes

Class C

Drinking Water -

Fully Supported

Class HH

Human Health -

Not Supported

Fish Consumption Advisory: Mercury

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Moderate

Cycle Added

2012

Impairment Rationale

Fish consumption advisory in effect: no more than 1 meal/week

Data Source

Fish contaminant monitoring: Iowa DNR

TMDL Priority

Tier IV

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed as “not supported” due to levels of indicator bacteria (IR 5a) and "not supported” due to poor water transparency caused by algal (IR 4a) and non-algal turbidity (IR 4a). The Class BLW use was assessed as “WINOFI” based on cyanobacteria blooms. The Class C use was assessed as “fully supported” based on water quality monitoring data. The Class HH (fish consumption) use remains assessed as "not supported" based on fish tissue sampling data and due to the existence of a fish consumption advisory at this lake (IR 5a). Note: A TMDL for siltation at Lake Keomah was prepared by DNR and approved by EPA in 2002. In September 2012, DNR received EPA approval of a TMDL for the impairments of the Class A1 and Class BLW uses caused by algae and pH. Additionally, a TMDL for indicator bacteria E. coli at Lake Keomah was prepared by DNR and approved by EPA in 2023.Not all of the section 303(d) impairments identified for the current assessment/listing cycle (mercury), however, are addressed in the TMDL. Thus, this waterbody remains in Category 5a (impaired; TMDL required) for the current assessment/listing cycle.

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	WQ	CY 2022-2024	21620001	Lake Keomah Beach
Iowa DNR	6	WQ	CY 2020-2024	22620002	Lake Keomah
Iowa DNR	6	FT	See Below	1166	Lake Keomah	Near Oskaloosa

Class A1 - Indicator Bacteria:

Site ID

Data Source ID

# Samples per Year (2022/ 2023/2024)

2022 Geometric Mean

2023 Geometric Mean

2024 Geometric Mean

Violates Annual Geometric Mean?

# 2022 SSM violations / SSM Violations needed for impairment

# 2023 SSM violations / SSM Violations needed for impairment

# 2024 SSM violations / SSM Violations needed for impairment

Violates Annual SSM Significantly >10% Rule?

Assessment Type

Support Level

21620001

15/14/2

116

481

YES

1 / 4

6 / 4

1 / NA

YES

MONITORED

NOT

Class A1 - 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
22620002	6	17 / 5	7.4	9.2	8.4	1	4	NO	MONITORED	FULL

Class A1 - Lake TSI's

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Minimum Value	Maximum Value	Median Value	Median TSI Violation?	Assessment Type	Support Level
22620002	6	Chlorophyll a TSI	13 / 5	31	77	66	YES	MONITORED	NOT
22620002	6	Secchi TSI	17 / 5	45	80	68	YES	MONITORED	NOT
22620002	6	Total Phosphorus TSI	13 / 5	55	77	71	NA	NA	NA
22620002	6	Fixed suspended solids	13 / 5	0.7	12	3	NA	NA	NA

Class BLW - Binomial 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
22620002	6	Dissolved Oxygen	15 / 4	2.5	12.9	8.5	1	NA / NA	4	NO	MONITORED	FULL
22620002	6	pH	17 / 5	7.4	9.2	8.4	1	NA / NA	4	NO	MONITORED	FULL
22620002	6	Temperature	17 / 5	19.1	29.7	24.8	0	NA / NA	4	NO	MONITORED	FULL

Class BLW - Lake Planktonic Cyanobacteria:

Site ID	Data Source ID	# Samples	Maximum Cyanobacteria Wet Mass	Maximum Cyanobacteria % Composition	Median Cyanobacteria Wet Mass	Median Cyanobacteria % Composition	Median Cyanobacteria Concentration = 75th Percentile	Assessment Type	Support Level
22620002	6	5	42.5837	99.46%	16.036	58.57%	YES	EVALUATED	WINOFI

Class BLW - 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
22620002	6	Ammonia	13 / 5	0.5801	0.11	0.03	0 / 0	NO	MONITORED	FULL

Class C - Binomial Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Minimum Value	Maximum Value	Mean Value	Median Value	# Violations	Violates Significantly >10% Rule?	Assessment Type	Support Level
22620002	6	Inorganic nitrogen (nitrate and nitrite)	12 / 5	0.03	0.99	0.24	0.18	0	NO	MONITORED	FULL
22620002	6	pH	17 / 5	7.4	9.2	8.4	8.5	1	NO	MONITORED	FULL

Class HH - Fish Tissue Sampling:

Site ID	Biopart	Year	Species	# in sample / # of samples	Avg Length (cm)	(Avg) Hg (ppm)	Chlordane (ppm)	Sum PCBs (ppm)
1166	tissue plug	2015	largemouth bass	4	38.8	0.3
Support Level	Not	A one meal/week mercury consumption advisory for largemouth bass exists on this lake. Based on IR Methodology, the Class HH (fish consumption) use was assessed as “not supported.”

Monitoring and Methods

Assessment Key Dates

6/10/2020

Monitoring Start

5/29/2024

Monitoring End

10/28/2002

TMDL Completed

9/24/2012

TMDL Completed

7/24/2015

Fish Tissue Monitoring

11/21/2023

TMDL Completed

Methods

200

Physical/Chemical Monitoring

260

Fish tissue analysis

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)

420

Indicator bacteria monitoring