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 delta(18)O 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 delta(18)O value of meteoric precipitation in this region is about -6.9 parts per thousand, but during the period June 1997-May 1998, which incorporates an Fl Nino event, the average delta(18)O of precipitation was -9.4 parts per thousand. Monthly averages are highly variable, ranging from -2.8 parts per thousand in August 1996 to -15.1 parts per thousand 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 delta(18)O variations for the precipitation have an amplitude exceeding 10 parts per thousand, yet the annual amplitudes of the variations in the unimpounded Meramec and Big Rivers are only about 3 parts per thousand, and the amplitudes of several karst springs, including the 'first magnitude' Maramec Spring, are even smaller at about 1 parts per thousand. 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: delta(18)O(flow) = Sigma delta(i)P(i)e(-ti/tau)/Sigma P(i)e(-ti/tau) where delta(i) and P-i are the delta(18)O value and amount for a given rain event, t(i) is the time interval between the storm and the stream or spring sample, and tau is the residence time. For the Meramec and Big Rivers, tau takes on a value of close to 100 days, whereas it is 1-2 years for several springs. Smaller contributions with tau 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-event' water in the systems, but it implicitly accounts for the variability of baseflow and also provides the approximate time scale for subsurface mixing. (C) 1999 Elsevier Science B.V. All rights reserved.