USING STABLE ISOTOPES TO TEST ALTERNATIVE HYPOTHESES OF THE SIZE AND COMPLEXITY OF A SHALLOW SANDY AQUIFER, OTTAWA COUNTY, MICHIGAN, USA

Shallow sandy aquifers with discharging seeps and springs are common within the Great Lakes region, providing a vital source of water for stream and wetland environments. In this study, we measured oxygen and hydrogen isotopes of precipitation and three spring-fed streams, and monitored spring-fed stream stage and groundwater levels at Hemlock Crossing Park in Ottawa County, Michigan to test alternative hypotheses of aquifer size and complexity. A rapid response to precipitation in groundwater levels and a noticeable seasonal difference in spring-fed stream isotope values would suggest a small simple aquifer, whereas the opposite would suggest a larger and more complex aquifer.

From late 2019 to early 2021, groundwater level and stream stage were measured weekly along with sampling of stream water and precipitation for isotope analysis. Results of the δ¹⁸O values for precipitation vary from -0.6 to -29.7‰, while spring-fed stream values vary from -8.5 to -9.8‰. δ²H values for precipitation vary from +4.45 to -198.5‰, while spring-fed stream values vary from -53.8 to -63.7‰. The δ¹⁸O stream values were found to generally trend upwards by ~1‰ and δ²H by ~10‰. This shift may be correlated to changing aquifer and river levels later in the year. The isotopic ratios do not reflect seasonal variation thus suggesting that there is sizeable mixing within a larger and more complex aquifer. Groundwater levels and stream stage show a limited response to only the largest precipitation events.

Variability within the stream isotope values may be attributed to return flow and shallow subsurface flow during and after precipitation events that release water into the streams, slightly altering the isotopic chemistry from that of the discharging groundwater springs. Analysis, however, suggests that these events do not significantly change mean δ¹⁸O and δ²H values and produce similar variation in isotope values compared to values not affected by precipitation events. Our results suggest that the studied aquifer is larger and more complex than originally hypothesized, and it appears that there is minimal impact on stream isotope values due to shallow subsurface flow and return flow during rainfall events.