HYDROGEOLOGICAL CHARACTERISTICS AND THE STRATIGRAPHIC TRANSITION FROM LACUSTRINE MARL TO PEAT IN A RIPARIAN WETLAND, MANISTEE NATIONAL FOREST, MICHIGAN

A shallow groundwater monitoring network in the Huron-Manistee National Forest of Central Michigan provides resource managers with long-term data on groundwater and surface water interactions within a riparian wetland-upland system. Continuously recorded groundwater levels, precipitation data, and stream discharge measurements help to characterize the linkage of a shallow unconfined and artesian aquifer with the headwaters of the White River. Weekly discharge measurements, taken during the 2010 growing season, range from 0.36m³/sec to 0.76m³/sec. Stage-discharge relations are complex during late summer; a rise in stage appears to correlate to declines in stream discharge and local groundwater levels. This increase in stream stage in the drier months is attributed to decreased velocities as a function of an increase in vegetation within the stream channel. The direct relation between stream discharge and wetland groundwater levels and field observations of numerous discharge points within the channel, indicate that this section of the White River is dominantly sustained by groundwater. Groundwater levels in wells show seasonal water table fluctuations of 70 cm in the upland and 19 cm in the wetland. Evapotranspiration demands are significant during the summer months, with water table declines as much as 3.5 cm/day within the wetland and 4.6 cm/day in the adjacent upland. Vibracoring within the wetland suggests that the local hydrostratigraphy is defined by a basal, well sorted, medium grained sand, confined by an overlying sequence of lacustrine marl with abundant woody debris and shell fragments and a surficial peat. This stratigraphy represents an outwash sand deposited during deglaciation of the Laurentide Ice Sheet, followed by development of a shallow lake that transitioned into a peat accumulating wetland. Radiocarbon-age dating of basal peat indicates the lake-wetland transition occurred 6,050 +/- 203 yrs BP. AMS dating of a bivalve fragment at the sand-marl boundary and a wood fragment within the marl-peat transition is in progress.