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Investigation of Sediment and Water Chemistry at Grassy Island, Shiawassee National Wildlife Refuge, Wyandotte Unit, Wyandotte Michigan, 1998

By: Sweat, M.J.


Grassy Island lies in the Detroit River, 1 kilometer (km) east of Wyandotte. The island has an area of 29 hectares (h) surrounded by 93 h of shoal. A population of five million people lives near the island. The U.S. Army Corps of Engineers (USACOE) used Grassy Island as a confined disposal facility (CDF) for sediment dredged from the Rouge River between 1961 and 1983. The CDF is comprises 2 tiers of dikes composed of riprap and rubble that contain the dredged sediments. As the first CDF constructed on the Detroit River, it lacks the confinement technology required in more recent CDFs.

In 1961, the island became part of the Wyandotte National Wildlife Refuge because of the large population of migratory waterfowl that feed on wild celery in the shoals near the western and southern shores. Previous investigations by USACOE, Michigan Department of Environmental Quality (MDEQ), U.S. Environmental Protection Agency (USEPA), and the Detroit River Remedial Action Plan (RAP) have found the sediments and soils deposited on the island to contain concentrations of metals and organic compounds that exceed USEPA limits.

From November 18 through 20, 1996, continuous seismic-reflection profiles (CSP) were collected along the shoals of Grassy Island to determine general lithologic properties, the thickness of riverine sediments, and depth to underlying bedrock. Profiles were collected using both 7 kilohertz (kHz) and 14 kHz energy sources to obtain adequate depth penetration and profile resolution. Profiles were collected in both upstream and downstream directions (north-south), as well as normal to the flow direction (east-west). Sediments are generally glacio-lacustrine clays, and range in thickness from about 1 to 6 meters (m), although they are locally absent.

Between May 20 and June 9, 1997, 16 piezometers were installed. Water levels indicate ground-water flow is radial, from the center of the island toward the perimeter. Water and sediment samples were collected at 10 sites on Grassy Island and in the Detroit River, both north and south of the island, and analyzed for selected volatile organic compounds, semi-volatile organic compounds, pesticides, inorganics, and physical characteristics. Of the 37 samples collected, including blanks and duplicates, 6 were water matrix and 31 were sediment matrix.

Iron was the most frequently detected inorganic analyte, and was present in all water samples and in all but one sediment sample. Calcium, magnesium, sodium and zinc were also detected in all water samples, but only zinc was detected in sediment samples. Manganese was detected in all but one sediment sample and in all but one water sample. Other frequently detected inorganic materials include lead, aluminum and mercury. Few organic analytes were detected in water samples aside from methylene chloride and acetone; both were also present in most sediment samples. Numerous semivolatile organics and pesticides were present at detectable levels in sediments.

Sweat, M.J., 1998, Investigation of Sediment and Water Chemistry at Grassy Island, Shiawassee National Wildlife Refuge, Wyandotte Unit, Wyandotte Michigan, in: Michigan: Its Geology and Geologic Resources - Fifth Symposium Abstracts, p. 21.

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