Black Pit IA 02-WIN-1920

Mason City T96N R20W Sec17 NE and T96N R20W Sec8 SE Cerro Gordo Co.

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: 2 - Some of the designated uses are met but there is insufficient data to determine if remaining designated uses are met.
Trophic: Unknown
Trend: Unknown
Created: 10/27/2025 3:10:38 PM
Updated: 1/12/2026 12:09:55 PM

Use Support

Class BLW

Aquatic Life - Lakes and wetlands

WINOFI

Fish Kill: Caused By Other

Support Level

Further Investigation Needed

Impairment Code

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

Cause Magnitude

Slight

Status

Continuing

Source

Natural: Ordinary Stratification and Bottom-Water Hypoxia/Anoxia

Source Confidence

Low

Cycle Added

2016

Impairment Rationale

Non Pollutant-caused fish kill

Data Source

Fish kill investigation: Iowa DNR

Class HH

Human Health -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class BLW use was assessed (monitored) as "fully supported." Additionally, the Class BLW use was assessed (evaluated) as “WINOFI” due to a fish kill investigation. Because the majority of the results were assessed (monitored) as "fully supported", Black Pit was place in category 2. All other designated uses were “not assessed.”

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	WQ	CY 2018-2022	22170003	Black Pit

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
22170003	6	Dissolved Oxygen	15 / 5	3.1	18	11	1	NA / NA	4	NO	MONITORED	FULL
22170003	6	pH	12 / 4	7.5	8.5	8	0	NA / NA	4	NO	MONITORED	FULL
22170003	6	Temperature	15 / 5	17.3	27.7	22	0	NA / NA	4	NO	MONITORED	FULL

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
22170003	6	Ammonia	12 / 4	0.2523	0.06	0.03	0 / 0	NO	MONITORED	FULL

Class BLW - Fish Kill:

The Class BLW use was assessed (evaluated) as “WINOFI” due to a fish kill that occurred in July of 2021. Because the cause of the fish kill was not identified, or the fish kill was attributed to a non-pollutant cause (e.g., winterkill), the assessment type is “evaluated.” Segments assessed using “evaluated” assessments are placed in Category 3b and are added to the state list of waters in need of further investigation. If no additional fish kills are reported in the segment for a 5 year period, the Category will be changed from 3b to 3a (water not assessed).

Details about the 2021 fish kill can be found here: https://programs.iowadnr.gov/fishkill/events/1010

Class HH - No Data

Additional Information: 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
22170003	6	Chlorophyll a TSI	15 / 5	19	76	56	NA	NA	NA
22170003	6	Secchi TSI	15 / 5	47	93	60	NA	NA	NA
22170003	6	Total Phosphorus TSI	15 / 5	56	85	65	NA	NA	NA
22170003	6	Inorganic suspended solids	15 / 5	0.6	73.7	3	NA	NA	NA

Monitoring and Methods

Assessment Key Dates

9/12/2022

Monitoring End

6/4/2018

Monitoring Start

2/15/2021

Fish Kill

Methods

120

Surveys of fish and game biologists/other professionals

140

Incidence of spills and/or fish kills

200

Physical/Chemical Monitoring