Mercer County, Ohio · Water Quality Report Card
Grand Lake St Marys @ Intake
Grand Lake St Marys @ Intake in Mercer County, Ohio has a water quality grade of F (Very Poor). Phosphorus: 395.5 µg/L. Chlorophyll-a (algae): 116.5 µg/L.
- Mean depth
- ~12 ft est.
- Type
- Hypereutrophic
Grand Lake St Marys @ Intake 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 @ Intake ranks #2 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: 395.5 µg/L. Chlorophyll-a: 116.5 µg/L. Trophic State Index: 84.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | 395.5 µg/L | Poor |
| Chlorophyll-a (Algae) | 116.5 µg/L | Poor |
| Trophic State Index (TSI) | 84 | 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 @ Intake.
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Fishing Grand Lake St Marys @ Intake
Location
County Ranking
Ranked #2 of 2 lakes in Mercer County
Nearby Lakes in Mercer County
Explore Grand Lake St Marys @ Intake across LakeGrade
More Ohio Lake Rankings
Other Grade F Lakes in Ohio
Hypereutrophic Lakes in Ohio
More records, surveys & data sources
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
30 measurements from 1 monitoring station, 2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Division of Surface water (Ohio)
21OHIO_WQX30 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 (1)
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
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: 1