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

Stable oxygen and hydrogen isotopes have been used in ground water studies to investigate recharge, mixing, ground water/surface water interaction, ad vective-diffusive transport, paleohydrogeologic interactions and to estimat e ground water ages. Such studies require that the isotopic composition of precipitation be known, as precipitation is a major input to ground water a nd surface water systems. As oxygen-18 and deuterium data for precipitation are lacking across much of the United States, there is need to establish a dditional 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 t his problem is to determine the delta O-18 and delta H-2 values of precipit ation using archive samples collected at monitoring stations managed by the National Atmospheric Deposition Program (NADP), This study describes and i nterprets the seasonal delta O-18 and delta H-2 composition of archived pre cipitation samples collected in eastern Nebraska near the town of Mead duri ng the years 1992-1994. Values for delta O-18 range from -23.6 to -0.7 part s per thousand. Values for delta H-2 range from -172 to 0 parts per thousan d. Yearly arithmetic mean delta O-18 and delta H-2 values for the Mead stat ion are -8.1 parts per thousand and -53 parts per thousand, respectively. W eighted yearly means for delta O-18 and delta H-2 were -7.4 parts per thous and and -48 parts per thousand respectively. Mead values show a strong isot opic enrichment between winter and summer precipitation, and a strong delta O-18-T correlation (r(2) = 0.91) for mean monthly values of about 0.5 part s per thousand per degree Celsius. The local meteoric water line for the Me ad site is delta H-2 = 7.40 delta O-18 + 7.32. Deuterium excess values sugg est 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 a bsence of long-term monitoring stations such as those operated globally by the International Atomic Energy Association, NADP archive samples can be us ed to determine the isotopic composition of precipitation, to characterize the local meteoric water line and establish the various climatic relationsh ips, and define the meteoric input function for use in ground water studies .