An evaluation of ground water-surface water interactions in Saginaw Bay, Lake Huron, with implications for trace metal cycling (Michigan)

A suite of water and sediment samples was collected to characterize the extent of interaction between saline ground water (with solutes derived from Michigan basin brines) and surface water in Saginaw Bay and the Saginaw Lowland Area (SLA). Porewater chemistry in sediment cores from both the bay and SLA indicate the presence of a saline, isotopically light source at depth. Cl:Br ratios indicate formation brine is the source for salinity. Mixing models constrained by Cl-δ18O data indicate that porewater chemistry in both areas may be the result of similar multi-stage mixing process(es), but they do not appear to be interacting with the same water masses. 1-D transport simulations indicate that the SLA porewater profiles required approximately 8000 a to develop and that advective transport is an important contributor. Significantly shorter times are needed (<300 a) for bay core porewater profiles; diffusion dominates transport in these cores. A number of samples from the Saginaw Bay watershed also have elevated chloride levels, but Cl:Br ratios indicate that interaction with saline ground water is limited. Agricultural drains appear to be a much more significant source for chloride and bromide to the watershed. However, a number of sediment cores collected from riverbeds do indicate the presence of a high-chloride source at depth. The Cl:Br ratios in porewater from these cores are low (generally less than 30) and may reflect the influence of organic matter decomposition.; Sediment cores collected from Saginaw Bay were used to evaluate trace metal loading histories, study metal partitioning and diagenesis, and assess the potential for remobilization due to saline water intrusion. Trace metal mass balances constructed for cadmium and zinc indicate that benthic fluxes could be a significant source for trace metals to the water column. Trace metal partitioning studies, used as a proxy for saline water, indicate that porewater concentrations of cadmium and copper could increase significantly in response to saline water intrusion, however, only the cadmium cycle would be strongly affected by such interaction.; Porewater chemistry in Saginaw Bay, combined with new and historical data regarding porewater chemistry in Lake Michigan, indicate that saline fluids, derived from Michigan basin brines, are interacting with surface sediments on a regional scale. Saline porewater in both Saginaw Bay and Lake Michigan is likely derived from two different geologic formations. A conceptual model for solute transport in the Michigan basin is proposed to account for the presence of saline porewater in sediments. A combination of advection (driven by topography) and diffusion likely accounts for the observed chemistry in the Saginaw Bay area. There exists the potential for topographically-driven flow to transport solutes to Lake Michigan, but advection (as a result of compaction) might also be responsible for the observed porewater chemistry.