Epsilon Archive for Student Projects

Abstract

The goal of this project was to investigate the influence of a large inland lake on
adjacent coastal freshwater peatlands. The specific aim was to determine the source of
groundwater for three differently formed peatlands located on the southern shore of
Lake Superior. The groundwater study was conducted at Bete Grise, a peatland
complex in a dune-swale system; Pequaming, a peatland developed in the swale of a
tombolo; and Lightfoot Bay, a peatland developed in a barrier beach wetland
complex.
To determine the source of groundwater in the peatlands, transects of six
groundwater monitoring wells were established at each study site, covering distinctly
different vegetation zones. At Pequaming and Lightfoot Bay the transects monitored
two vegetation zones: transition zone from upland and open fen. At Bete Grise, the
transects monitored dunes and swales. Additionally, at all three sites, upland
groundwater was monitored using three wells that were installed into the adjacent
upland forest. Biweekly measurements of well water pH and specific conductance
were carried out from May to October of 2010. At each site, vegetation cover, peat
depths and surface elevations were determined and compared to Lake Superior water
levels. From June 14 – 17, July 20 – 21 and September 10 – 12, stable isotopes of
oxygen (18O/16O) ratios were measured in all the wells and for Lake Superior water. A
mixing model was used to estimate the percentage of lake water influencing each site
based on the oxygen isotope ratios.
During the sampling period, groundwater at all three sites was supported
primarily by upland groundwater. Pequaming was approximately 80 % upland
groundwater supported and up to 20 % Lake water supported in the uppermost 1 m
layer of peat column of the transition zone and open fen. Bete Grise and Lightfoot
Bay were 100 % upland groundwater supported throughout the season. The height of
Lake Superior was near typical levels in 2010. In years when the lake level is higher,
Lake water could intrude into the adjacent peatlands. However, under typical
hydrologic conditions, these coastal peatlands are primarily supported by upland
groundwater.