Williamson Pond IA 04-LDM-1058

Lucas County S27T73NR21W 2 mi. E of Williamson.

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: Hypereutrophic
Trend: Stable
Created: 10/27/2025 3:09:51 PM
Updated: 11/18/2025 11:10:26 AM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Turbidity

Support Level

Not Supported

Impairment Code

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

Cause Magnitude

High

Status

Continuing

Source

Other: Sediment Resuspension

Source Confidence

High

Cycle Added

2002

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

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

New

Source

Natural: Internal Nutrient Recycling (Primarily Lakes)

Source Confidence

Moderate

Cycle Added

2026

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier IV

Class BLW

Aquatic Life - Lakes and wetlands

Fully Supported

Class HH

Human Health -

Fully Supported

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed (monitored) as “not supported” due to poor water transparency and algae that violates Iowa’s narrative criteria protecting against aesthetically objectionable conditions. The Class BLW use was assessed (monitored) as “fully supported.” The Class HH use was assessed (monitored) as "fully supported" based on fish tissue monitoring. Note: A TMDL for turbidity and nutrients at Williamson Pond was prepared by DNR and approved by EPA in 2006. Because not all Section 303(d) impairments (algae) identified for the current assessment/listing cycle are potentially addressed by the TMDL, this waterbody be place in IR Category 5a (impaired; TMDL needed).

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	WQ	CY 2021-2024	22590001	Williamson Pond Max Depth
Iowa DNR	6	FT	See Below	1340	Williamson Pond

Class A1 - Indicator Bacteria: No Data

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
22590001	6	12 / 4	7.46	9.17	8.37	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
22590001	6	Chlorophyll a TSI	12 / 4	32	80	74	YES	MONITORED	NOT
22590001	6	Secchi TSI	12 / 4	50	80	70	YES	MONITORED	NOT
22590001	6	Total Phosphorus TSI	12 / 4	64	94	83	NA	NA	NA
22590001	6	Fixed suspended solids	12 / 4	1.7	16	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
22590001	6	Dissolved Oxygen	12 / 4	2.2	13.1	7.4	1	NA / NA	4	NO	MONITORED	FULL
22590001	6	pH	12 / 4	7.46	9.17	8.37	1	NA / NA	4	NO	MONITORED	FULL
22590001	6	Temperature	12 / 4	19.3	27.2	24.6	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
22590001	6	5	18.0712	83.84%	5.6057	57.96%	NO	EVALUATED	NA

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
22590001	6	Ammonia	12 / 4	0.708	0.19	0.07	0 / 0	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)
1340	fillet	2024	channel catfish	3	52	0.051	<0.05	<0.0246
Support Level	Full

Monitoring and Methods

Assessment Key Dates

5/25/2021

Monitoring Start

10/2/2024

Monitoring End

3/21/2006

TMDL Completed

8/21/2024

Fish Tissue Monitoring

Methods

200

Physical/Chemical Monitoring

260

Fish tissue analysis

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)