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Hennepin Point history

Aerial view of Hennepin Point, a 225-acre island in the Detroit River.
Environment, Great Lakes, and Energy

Hennepin Point history

Michigan Alkali Company bought Hennepin Point in the early 1900s to use as a dumping ground for waste from the production of soda ash and propylene oxide, known as “distiller blow off” (DBO). The island was also used for brine mining, in which water is pumped into an underground slab of salt, forcing brine back to the surface. Michigan Alkali merged with another firm and was renamed Wyandotte Chemical Company in 1943. BASF bought the company in 1969. Brine mining on the island ended in the 1970s. BASF was allowed to inject mercury-containing waste into the bedrock for storage for one year in 1971.

Contamination Map

Sinkholes

Three sinkholes, labeled as North Sinkhole, Central Sinkhole, and South Sinkhole on the Hennepin Point Contamination Map, formed in the 1960s because of the effects of brine mining and DBO dumping. The mining extended to a depth of more than 1,600 feet below the surface, creating caverns in the bedrock. That, and the weight of the DBO dumped on the island, caused the ground to give way.

Environmental Issues

The major risk currently is hazardous substances getting into the Detroit River. Soil and groundwater sampling found ammonia, phenols, and cresol exceeding EGLE groundwater-to-surface-water interface (GSI) criteria. DBO has an elevated pH; groundwater from within the DBO on the island was measured at a pH of 12, which exceeds the EGLE GSI screening level of 9. Surface water in contact with the shoreline also exceeds the pH screening level. Effects of higher pH include increased stress on organisms which can lead to decreased reproduction, decreased growth, and disease. Higher pH can also increase the rate of corrosion and the release of metals into the surface body, reducing water quality. Phenols and cresols are organic compounds which, when ingested, have been found to result in kidney problems, mouth and throat burns, and abdominal pain. They can also have negative effects on the blood and nervous systems.

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Hennepin Point Contamination Map Preview

2022-2025 BASF Investigation

BASF conducted a geophysical investigation of Hennepin Point. The results identified two parts of the island with a higher degree of interaction between groundwater and the Detroit River. These areas have greater potential for groundwater impacted by DBO to be discharged into the river and are identified as purple lines on the Hennepin Point Contamination Map. That finding is based on BASF’s evaluation of the following:

  • Island groundwater elevation above or equal to the surface of the Detroit River
  • Detroit River surface water flowing into the island groundwater
  • Flow of groundwater outward from the island to the river
  • Specific conductivity of groundwater and surface water
  • Temperature comparison between groundwater and surface water

Analysis of island groundwater interaction with the Detroit River was conducted for the central-southern portion of the island during 3rd and 4th quarters of 2024 and 1st quarter of 2025. This analysis identified limited final acute value (FAV) exceedances for un-ionized ammonia and manganese. Data indicates that native microbe population is degrading the presence of un-ionized ammonia for the majority of the island. The natural attenuation model is being used to evaluate continued degradation.  Elevated levels of pH were limited to the shallow groundwater interval.

In addition, an evaluation was completed look at the potential impact to surrounding areas from dust (air particulates) generated from the DBO impacted soils at Hennepin Point. Due to vegetative cover, limited exposed soil surface area, no construction activities, and low probability of windblown particulate matter, Arcadis/BASF determined the risk of impact to surrounding areas from air particulates generated from the site is minimal. In addition, analytical results of DBO indicated that maximum concentrations were significantly less than the EGLE Particulate Soil Inhalation Criteria. Based on analytical results and evaluation of wind distribution, EGLE concurs with the evaluation that airborne particulate impact is minimal. 

 

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Aerial view of Hennepin Point, a 225-acre island in the Detroit River.