In April 1997, samples of soil were collected at 10 sites, distributed in a grid over Grassy Island for analysis of pesticides, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and heavy metals. The objective of the project was to characterize the chemical composition and distribution of contaminants in sediments at various depths and locations on the Island, and to investigate the potential for contaminant transport in ground water. Sediment samples were collected at 2 offshore sites and 8 onshore sites; water samples were collected at 1 offshore and 4 on- shore sites. Water levels were measured at all sites. A total of 30 samples were collected for analysis. 

Analyses of sediment samples were made for 28 pesticides, of which 25 were detected; analyses of sediment samples were made for 64 semivolatile organic compounds, of which 24 were detected; and analyses of sediment samples were made for 33 volatile organic compounds, of which 9 were detected. Analyses of sediment samples were made for 23 metals, of which 9 were detected. Analyses of water samples were made for 28 pesticides, of which 11 were detected; analyses of water were made for 64 semivolatile organic compounds, of which 6 were detected; and analyses of water were made for 33 volatile organic compound, of which 4 were detected. Analyses of water samples were made for 23 metals, of which 16 were detected.

Water levels were measured in an attempt to determine hydraulic gradients on Grassy Island. In general water levels in the river sediments were lower than those in the sediments situated between the inner and outer dikes; likewise, water levels in the sediments between the dikes were lower than were water levels in the sediments within the inner dike. Average water levels were all below land surface. 

Contaminant concentrations show little, if any, trend with depth. Native parent materials generally contain only background levels of most trace metals, although the level of some organic contaminants is elevated. Using guidelines developed by the Michigan Department of Environmental Quality, the sediments were rated with respect to their level of contamination. Sediments were found to range from not impacted by arsenic and manganese, to extremely contaminated by mercury. They were moderately impacted by chromium and iron, and severely contaminated by lead and zinc. Sediments were found to be extremely contaminated by polychlorinated biphenyls and polycyclic aromatic hydrocarbons. 

Contaminant concentrations in water exceeded United States Environmental Protection Agency (U.S. EPA) safe-drinking water guidelines for cadmium, iron, lead, and manganese. They were also exceeded for aluminum at 3 sites and barium at 1 site. Contaminant concentrations in water also exceed the guidelines for polychlorinated biphenyls. 

There are 2 surface-water impoundments on Grassy Island with the potential to transmit contaminants off the Island during periods of precipitation and overland flow. There is an overflow weir on the northeast corner of the Island. Large numbers of waterfowl were observed on these impoundments. These surface-water impoundments were not sampled. 

Additional samples of sediments and water are needed upstream, downstream, lateral to, and on the Island. Further studies of water quality are needed to identify the contaminants, if any, that are present in ponded surface water, and in water being discharged from the Island through the overflow weir. Event sampling of surface- water flow and quality in overland runoff and at the overflow weir is needed to document any connection between ponds on the north end of the Island and the overflow weir, and to document the quality and quantity of discharges to the Detroit River through the weir. Additional piezometers and wells are needed on the Island, around its perimeter, and in the Detroit River to determine the ground- water surface and the potential for movement of ground water into either the Detroit River or underlying sediments.