Isotope hydrology and residence times of the unimpounded Meramec River Basin, Missouri

Strong isotopic forcing of hydrologic systems in eastern Missouri, caused by the large seasonal variations in the δ18O values of meteoric precipitation, can be used to determine numerous characteristics of hydrologic systems including the residence time of the water. The normal annual average δ18O value of meteoric precipitation in this region is about −6.9‰, but during the period June 1997–May 1998, which incorporates an El Niño event, the average δ18O of precipitation was −9.4‰. Monthly averages are highly variable, ranging from −2.8‰ in August 1996 to −15.1‰ in January 1998, and define a cycloid-like annual pattern. This meteoric forcing gives rise to similar patterns of isotopic variation in springs and rivers, but with greatly reduced amplitudes. Thus the δ18O variations for the precipitation have an amplitude exceeding 10‰, yet the annual amplitudes of the variations in the unimpounded Meramec and Big Rivers are only about 3‰, and the amplitudes of several karst springs, including the `first magnitudeʹ Maramec Spring, are even smaller at about 1‰. Most of the isotopic variation in streamflow can be explained by a simple exponential weighting of the preexisting rainfall events, such that the most recent precipitation more greatly influences the flow than earlier precipitation events, according to our formulation:δ18Oflow=∑δiPie−ti/τ∑Pie−ti/τwhere δi and Pi are the δ18O value and amount for a given rain event, ti is the time interval between the storm and the stream or spring sample, and τ is the residence time. For the Meramec and Big Rivers, τ takes on a value of close to 100 days, whereas it is 1–2 years for several springs. Smaller contributions with τ on the order of 1–10 days are superimposed, representing the latest storm events. This method has a significant advantage over the standard mixing arguments for overland flow and baseflow contributions to hydrologic systems, in that it not only demonstrates the dominant contributions of `pre-eventʹ water in the systems, but it implicitly accounts for the variability of baseflow and also provides the approximate time scale for subsurface mixing.