The water source to a wetland determines hydrology and nutrient content. Interdunal wetlands may have a mix of groundwater, precipitation, and Lake Michigan water sources making them uniquely susceptible to variations in, alterations to, and pollutants from all sources. The dearth of information known about interdunal wetlands is a hindrance to coastal management in Michigan. The proposed research aims to elucidate the water source of interdunal wetlands both spatially along Lake Michigan’s eastern shoreline, and temporally through seasonal variation. During June, August, and October 2020, we collected a total of 137 shallow groundwater samples from 24 wetlands in 4 parks along the Lake Michigan coastline and 14 surface water samples from 6 potential water sources. Samples were analyzed for hydrogen and oxygen stable isotopes to isolate water sources of interdunal wetlands. The isotopic compositions of shallow groundwaters from the interdunal wetlands and Lake Michigan lie on an evaporation line with a slope of ~5, with parks showing different slopes from the local meteoric water line suggesting evaporation. The isotopic composition of Lake Michigan was stable, whereas wetlands exhibited considerable spatial variability along the coastline and among wetlands (range d18O -10.3‰, range d2H 53.4‰). We did not see evidence of seasonal shifts in isotopic composition. Wetlands vary in main water source, with some being solely Lake Michigan, while others are a mix of local rivers and unknown water sources. Groundwater below interdunal wetlands in the dune landscape was unevenly distributed and not a simple lake-wetland connection. Climate change is impacting Great Lakes water levels, which given our data indicates will directly impact the number, size, hydroperiod, and distribution of interdunal wetlands in the coastal dunes.