Gropp Lake
Mercer County, New Jersey41 acresView mapDirections
NJDEP publishes the water-quality measurements for Gropp Lake. Here is what has been measured.
- Eutrophic
- Impaired · Impaired biota (cause unknown)
- Public access
- Limited data
Gropp Lake in Mercer County, New Jersey has water-quality measurements published by NJDEP, shown here as reported. Water clarity: 3.3 ft visibility. Phosphorus: 35 µg/L. Chlorophyll-a (algae): 25.8 µg/L. Surface area: 41 acres.
Gropp Lake is a 41-acre productive, nutrient-rich eutrophic lake in Mercer County, New Jersey. Limited fish-species data available. Public access is available. Compared to the 14 other monitored lakes in Mercer County, Gropp Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, your state environmental agency, last sampled 2024-11-14. Methodology: how these readings are compared.
Swimming conditions
Readings onlyBacteria advisory
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Gropp Lake is listed impaired under Clean Water Act §303(d) (IR 5) for impaired biota (cause unknown), metals (other than mercury), nutrients (phosphorus, nitrogen), pathogens (E. coli, fecal), pesticides, pCBs, salinity / chlorides, turbidity. Check local advisories before you swim.
Whether Gropp Lake is safe to swim in today is a call for the New Jersey Department of Environmental Protection and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Gropp Lake
These are the measurements NJDEP publishes for Gropp Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 3.3 ft of visibility. Phosphorus 35 µg/L. Chlorophyll-a 25.8 µg/L. TSI 59.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 3.3ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 35µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 25.8µg/L | Fair GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 59 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Gropp Lake
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
41 acres
Public access
Yes
Listed by NJDEP
No bathymetric survey has been published for Gropp Lake, so its depth is not documented here.
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Fishing Gropp Lake
Reservoir Info (USACE NID)
Gropp Lake is a man-made reservoir impounded by the Gropps Lake Dam (completed 1900), built primarily for recreation on the Back Creek; earth-type dam, 25 ft tall and 485 ft long.
- Surface area
- 41 ac
- Normal storage
- 211 ac-ft
- Max storage
- 549 ac-ft
- Drainage area
- 7.9 sq mi
- Hazard class
- Low
- Owner
- Township of Hamilton
Source: USACE National Inventory of Dams, NID ID NJ00235 · Operator website · Matched by proximity (0.53 km)
EPA Impairment Status
Gropp Lake is officially listed as impaired under Clean Water Act §303(d) in the 2022 EPA reporting cycle (IR category 5).
Causes of impairment
Designated uses not supporting: Drinking water, Aquatic life, Fish consumption, Recreation.
Source: EPA ATTAINS assessment unit NJ02040201070020-01 · Official waterbody report
Nearby Lakes in Mercer County
Frequently asked questions about Gropp Lake
Where is Gropp Lake?
- Gropp Lake is a lake in Mercer County, New Jersey. Its water-quality measurements are published by NJDEP and shown on this page as reported.
What is the water quality of Gropp Lake?
- Gropp Lake in Mercer County, New Jersey has water-quality measurements published by NJDEP, shown here as reported. Water clarity: 3.3 ft visibility. Phosphorus: 35 µg/L. Chlorophyll-a (algae): 25.8 µg/L. Surface area: 41 acres.
Is Gropp Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Gropp Lake is listed impaired under Clean Water Act §303(d) (IR 5) for impaired biota (cause unknown), metals (other than mercury), nutrients (phosphorus, nitrogen), pathogens (E. coli, fecal), pesticides, pCBs, salinity / chlorides, turbidity. Check local advisories before you swim.
Can I fish at Gropp Lake?
- Public access to Gropp Lake is listed as yes on the lake record. Seasons, size and bag limits, and whether you need a license are set by New Jersey's fish and wildlife agency, so check its current regulations before you go.
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Where this data comes from
32 measurements from 2 monitoring stations, 2024. Retrieved from the EPA Water Quality Portal on 2026-08-21.
Collected and reported by
- NJDEP Bureau of Freshwater and Biological Monitoring
NJDEP_BFBM32 samples · 2024
Analytical methods
- Phosphorus by Colorimetry
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
Monitoring station identifiers (2)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database