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