Heavy Metals in Saginaw Bay:

Evidence for Brine Seepage?
Central Michigan University, 1995 On-Campus Abstracts
Michele Struble

Faculty Sponsor: Jane Matty
Abstract: The Michigan Basin contains some highly concentrated brines. Geochemical and isotoptic evidence indicates that high values of chloride in the water of Saginaw Bay and Lake Huron may be caused by the direct discharge into the bay of saline groundwater from the Michigan Basin. If saline groundwater is seeping into these bodies of fresh water, the chloride could strip off any heavy metals within the sediment and carry them up into the water column.

In late May to early June of 1994, sediment samples from Saginaw Bay were retrieved by the process of gravity coring. Two sites were chosen for this study: Site SB-4 is located near the center of the bay and site SB-5b is located near the south east side of the bay. Pore water was extracted from the sediment cores using a squeezing apparatus. The following metals were analyzed using an ICP-MS, copper (Cu), lead (Pb), zinc (Zn), barium (Ba), and chromium (Cr). All of the metals that were analyzed for were detected in most of the samples from both sites.

If the heavy metals that are present in the pore water of Saginaw Bay today are resulting from pollution such as atmospheric deposition, non-point and point source pollution, one would expect to find concentrations highest near the top of the sediment. Core SB-4 has higher concentrations of all elements, except for lead, toward the bottom of the core (roughly 60 cm). These evidence suggests that the source of these meals is coming from below not through atmospheric deposition, non-point or point sources pollution.

Core SB-5b is very different from core SB-4. Lead is the only element in core SB-5b that gets higher toward the bottom of the core (roughly 200 cm). Differences in absorption could be the explanation for the difference in the trends of these elements. This is consistent with the hypothesis that brine may be moving up, into the sediment and desorbing some of the metals that are less tightly held to the sediment.