Hawthorn Lake IA 03-NSK-862
Mahaska County S10T77NR14W 1 mi S of Barnes City.
- 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
- Eutrophic
- Trend
- Improving
- Trend Comment
- Removal of an Algal Growth: Chlorophyll a impairment.
- Created
- 10/27/2025 3:11:41 PM
- Updated
- 1/12/2026 8:34:27 AM
| Cycle Added | Class | Cause | Data Source | Rationale |
|---|---|---|---|---|
| 2010 | Class A1 | Algal Growth: Chlorophyll a | Ambient monitoring: Iowa DNR-lakes | New data: WQ improvement (chemical / physical / bacterial) |
The Class A1 was assessed as “fully supported.” The Class BLW use was assessed (evaluated) as “WINOFI” due to cyanobacteria blooms. All other designated uses were “not assessed.” Note: A TMDL for algae at Hawthorn lake was prepared by DNR and approved by EPA in 2025.
Data Sources: Data Source Data Source ID Data Type Data Age Site ID Site
Name Site
Description Iowa DNR 6 WQ CY 2020-2024 22620001 Hawthorn
Lake
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 22620001 6 15 / 5 7.4 8.9 8.3 0 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 22620001 6 Chlorophyll a TSI 15 / 5 42 68 62 NO MONITORED FULL 22620001 6 Secchi TSI 15 / 5 49 70 64 NO MONITORED FULL 22620001 6 Total Phosphorus TSI 15 / 5 49 64 60 NA NA NA 22620001 6 Fixed suspended solids 15 / 5 0.7 6.5 3 NA NA NA DNR's assessment methodology indicates that a TSI value less
than or equal to 63 are necessary to suggest de-listing of an
impairment. The chlorophyll a TSI value for the 2010 assessment
listing cycle was 65, which based on DNR's assessment/listing methodology
indicated an impairment based on the narrative criteria protecting against
aesthetically objectionable conditions. Because the TSI value for the 2026
assessment/listing cycle is less than or equal to the delisting trigger of 63
(median TSI value for chlorophyll a was 62), based on DNR's methodology, these
values suggest "full support" of the Class A1 uses protecting against
aesthetically objectionable conditions and the impairment (5a) for chlorophyll
a is suggested for delisting for the 2026 assessment/listing cycle. Additionally, de-listing is suggested based on a TMDL for algae being completed and approved in 2025. 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 22620001 6 Dissolved Oxygen 14 / 5 5 9.87 7.18 0 NA / NA 4 NO MONITORED FULL 22620001 6 pH 15 / 5 7.4 8.9 8.3 0 NA / NA 4 NO MONITORED FULL 22620001 6 Temperature 15 / 5 19 29.1 24.2 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 22620001 6 4 193.1284 97.52% 39.8806 78.47% 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 22620001 6 Ammonia 15 / 5 0.345 0.07 0.02 0 / 0 NO MONITORED FULL
Class HH - Toxic Parameters: No Data