Use of NADP archive samples to determine the isotope composition of precipitation: Characterizing the meteoric input function for use in ground water studies

Abstract

Stable oxygen and hydrogen isotopes have been used in ground water studies to investigate recharge, mixing, ground water/surface water interaction, advective-diffusive transport, paleohydrogeologic interactions and to estimate ground water ages. Such studies require that the isotopic composition of precipitation be known, as precipitation is a major input to ground water and surface water systems. As oxygen-18 and deuterium data for precipitation are lacking across much of the United States, there is need to establish additional local meteoric water lines as isotope input functions across the region, as well as to develop better understanding of the isotopic climate linkages that control oxygen and hydrogen isotope ratios in precipitation. In the absence of long-term monitoring stations, one possible solution to this problem is to determine the δ18O and δ2H values of precipitation using archive samples collected at monitoring stations managed by the National Atmospheric Deposition Program (NADP). This study describes and interprets the seasonal δ18O and δ2H composition of archived precipitation samples collected in eastern Nebraska near the town of Mead during the years 1992-1994. Values for δ18O range from -23.6 to -0.7‰. Values for δ2H range from -172 to 0‰. Yearly arithmetic mean δ18O and δ2H values for the Mead station are -8.1‰ and -53‰, respectively. Weighted yearly means for δ18O and δ2H were -7.4 ‰ and -48‰, respectively. Mead values show a strong isotopic enrichment between winter and summer precipitation, and a strong δ18O-T correlation (r2 = 0.91) for mean monthly values of about 0.5‰ per degree Celsius. The local meteoric water line for the Mead site is δ2H = 7.40 δ18O + 7.32. Deuterium excess values suggest that most of the moisture across the region is derived primarily from a Gulf of Mexico source. The results of this study demonstrate that in the absence of long-term monitoring stations such as those operated globally by the International Atomic Energy Association, NADP archive samples can be used to determine the isotopic composition of precipitation, to characterize the local meteoric water line and establish the various climatic relationships, and define the meteoric input function for use in ground water studies.