Mercer County, Ohio · Water Quality Report Card
Grand Lake St Marys
Grand Lake St Marys in Mercer County, Ohio has a water quality grade of F (Very Poor). Phosphorus: 350 µg/L. Chlorophyll-a (algae): 120.5 µg/L.
- Mean depth
- ~12 ft est.
- Type
- Hypereutrophic
Grand Lake St Marys is a highly nutrient-loaded hypereutrophic lake in Mercer County, Ohio with an estimated average depth of about 12 feet. Water quality is graded F based on 2 sampling years through 2022. Compared to the 1 other monitored lakes in Mercer County, Grand Lake St Marys ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Ohio EPA, last sampled 2022-09-21. Grade methodology: LakeGrade methodology.
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Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F (Very Poor)
No clarity data. Phosphorus level: 350 µg/L. Chlorophyll-a: 120.5 µg/L. Trophic State Index: 83.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | 350 µg/L | Poor |
| Chlorophyll-a (Algae) | 120.5 µg/L | Poor |
| Trophic State Index (TSI) | 83 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Average Depth | ~12 ft est. |
Depth marked “est.” is a modeled estimate from HydroLAKES, not a measured survey. Ohio has not published bathymetry for Grand Lake St Marys.
Where to stay near Grand Lake St Marys
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Fishing Grand Lake St Marys
Location
County Ranking
Ranked #1 of 2 lakes in Mercer County
Nearby Lakes in Mercer County
Explore Grand Lake St Marys across LakeGrade
More Ohio Lake Rankings
Other Grade F Lakes in Ohio
Hypereutrophic Lakes in Ohio
More records, surveys & data sources
EPA Impairment Status
Grand Lake St Marys is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit OH051201010204 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Where this data comes from
90 measurements from 3 monitoring stations, 2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Division of Surface water (Ohio)
21OHIO_WQX90 samples · 2022
Analytical methods
- Total Phosphorus After Block Digestion
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
- 2130 B ~ Nephelometric Method
Monitoring station identifiers (3)
Grade thresholds come from the U.S. EPA National Lakes Assessment 2022 Technical Support Document (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated.
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion
Most recent sample: 2022-09-21
Monitoring stations: 3