Stable Isotope Hysteresis Loops Associated with Flash Flooding, St. Louis County, Missouri

Oxygen and hydrogen isotopes undergo significant excursions, commonly tens of permil for D and several permil for 18-O, during flash flood conditions on small rivers and streams. Most interestingly, we do not observe simple linear mixing lines between pre-storm baseflow and recent storm precipitation, as is normally assumed in hydrograph separation techniques. Instead, we observe hysteresis loops that feature excursions from the meteoric water line which are associated with variations in turbidity. This behavior requires either a minimum of three separate components, complex isotopic variations in the causal storm event, or D/H exchange with soils and soil-associated waters. We suggest that the complex hysteresis loops are a result of the input of isotopically-enriched soil waters into the mix of storm waters and displaced groundwaters. Incorporation of soil waters and interactions with clay particles would help explain many characteristics of flood waters, including the pH excursions of 0.5 to 1 unit, the large (>100x) turbidity variations, and the rapid variations in minor and trace metal concentrations. In the St. Louis area, we observe hysteresis effects in two watersheds that vary greatly in size and degree of urbanization in which high frequency sampling was conducted. Black Creek, a 15 km2, highly urbanized watershed, had the largest excursions from the meteoric water line with differences of up to 10‰ for H and 1.5‰ for 18-O at the widest parts of the hysteresis loops. Grand Glaize, a suburban, 60 km2 watershed, had smaller excursions with differences of up to 5‰ for hydrogen and 0.5‰ for oxygen. Their hysteresis loops are mostly centered on the MWL but vary in width, rotation sense (clockwise versus counterclockwise), and position relative to MWL depending on basin conditions and the storm characteristics.