δD and δ18O evidence for inputs to groundwater at a wetland coastal boundary in the southern Great Lakes region of Canada

Abstract

The stable isotope compositions ( δ 18O and δD) of water have been used to determine the relative contributions of different water sources to the groundwater in a barrier sand-bar that separates a coastal freshwater marsh from Lake Erie, Canada. An extensive groundwater study was initiated by Environment Canada at Point Pelee National Park after elevated nutrient concentrations were measured in the park’s marsh, located on a spit of land that extends 15 km south into Lake Erie. As part of this larger study, which includes groundwater and nutrient modelling, the stable isotope compositions ( δ 18O and δD) of water have been used to independently determine the nature and extent of groundwater flow within sand deposits that separate the marsh from Lake Erie. One of the two study sites chosen for this investigation is located near a large septic-system tile-bed that receives human waste from a public toilet facility within the park, and could potentially release nutrients to the marsh via the groundwater. Both transects studied are well suited to isotopic investigation because they are located between two potentially recharging surface water bodies with temporally variable isotopic compositions, and because the transects are subject to recharge by local precipitation, which exhibits large seasonal isotopic variations. These differing isotopic compositions, when considered spatially and temporally, make it possible to establish the source and movement of groundwater within the transects. The oxygen and hydrogen isotopic compositions of surface waters from the Point Pelee marsh and Lake Erie lie on an evaporation line with a slope of 5.7. The isotopic composition of Lake Erie remained relatively stable for over 21 months ( δ 18O=−7.5‰ to −6.7‰) whereas the marsh exhibited considerable spatial and temporal variability ( δ 18O=−8.4‰ to −0.1‰). The δD and δ 18O values of precipitation samples vary seasonally with local surface temperatures, and constitute a local meteoric water line of δD=8.0( δ 18O)+9.8. While influx of precipitation is evident and similar in the upper two metres of both transects, the relative contribution to groundwater from Lake Erie and the Point Pelee marsh appears to differ. The average δ 18O and δD groundwater values measured for the narrower transect were −7.0‰ and −54‰ respectively, and reflect significant groundwater recharge from both Lake Erie (∼67%) and the marsh (∼28%). Precipitation constitutes the primary source of groundwater recharge to the wider transect, located near the large septic-system tile-bed, with average groundwater values of δ 18O=−9.1‰ and δD=−65‰. The isotopic profile of groundwater in this area is very stable, both with depth and over time, and contrasts sharply with the narrower northern transect which is much more dynamic. These findings suggest that the septic-system tile-bed is well situated in the wider portion of the barrier sand-bar, where a larger head difference between the lake and the marsh would be required to produce groundwater flow equivalent to that observed in the narrower transect. The differences in groundwater recharge between the two sites arise primarily from the difference in width, and account for the 2‰ and 10‰ differences in δ 18O and δD values, respectively, measured in the base-flow groundwater of the two transects.