Union Grove Lake is a 117-acre highly nutrient-loaded hypereutrophic lake in Tama County, Iowa. Water quality is graded F based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Compared to the 2 other monitored lakes in Tama County, Union Grove Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-08-27. Grade methodology: LakeGrade methodology.
Watch Union Grove Lake water quality
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Swimming Safety
Based on E. coli testing, Union Grove Lake frequently exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 24.8% of 105 samples across 7 seasons.
This is a historical summary of past testing, not a current condition — bacteria levels change quickly, especially after rain. Always check Iowa’s current beach and health-department advisories before swimming.
Water Quality Grade: F, Very Poor
Very murky, less than 1.6 ft of visibility. Chlorophyll-a: 88.5 µg/L. Trophic State Index: 73.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.6 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 88.5 µg/L | F |
| Trophic State Index (TSI) | 73 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 117 acres |
Where to stay near Union Grove Lake
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Fishing Union Grove Lake
Water Quality Trend: ↑ Improving
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.077 m/yr | 5 |
| Chlorophyll-a | ↑ Improving | -8.2 µg/L/yr | 5 |
Water Temperature
Union Grove Lake averages about 77°F at the surface in summer, usually warmest in Jul (about 80°F) and coldest in Sep (about 67°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 110 field readings (2010–2023) via the EPA Water Quality Portal. See the full Union Grove Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Union Grove Lake, reaching a maximum surveyed depth of 12 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #2 of 3 lakes in Tama County
Cleaner Lakes Within 30 Miles
Union Grove Lake holds Grade F. 3 nearby lakes hold higher grades.
- BSand LakeMarshall County · 9.1 mi · Two grade letters higher (F → B)
- CGreen Castle LakeMarshall County · 15.3 mi · Two grade letters higher (F → C)
- DOtter Creek LakeTama County · 11.7 mi · Higher grade (F → D)
Nearby Lakes in Tama County
Explore Union Grove Lake across LakeGrade
More Iowa Lake Rankings
Other Grade F Lakes in Iowa
Hypereutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Union Grove Lake is a man-made reservoir impounded by the Union Grove Lake Dam (completed 1920), built primarily for recreation on the DEER CREEK; earth-type dam, 29 ft tall and 1,205 ft long.
- Surface area
- 117 ac
- Normal storage
- 770 ac-ft
- Max storage
- 2,120 ac-ft
- Drainage area
- 11 sq mi
- Hazard class
- Significant
- Owner
- Iowa Department of Natural Resources
Source: USACE National Inventory of Dams, NID ID IA01373 · Operator website
EPA Impairment Status
Union Grove Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IA 02-IOW-724 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Union Grove Lake under Clean Water Act (NPDES) discharge permits. 2 facilities have a reported Clean Water Act violation in the past three years.
- Garwin City Of StpGarwin · 3.6 mi away · NPDES IA0025925Recent violation
- Gladbrook City Of StpGladbrook · 4.4 mi away · NPDES IA0025330Recent violation
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2024-08-27
Monitoring stations: 1