ISOTOPIC COMPOSITION OF PRECIPITATION IN CENTRAL IOWA

The isotopes of hydrogen and oxygen have been used extensively as indicators of groundwater age (H-3) and paleotemperature of groundwater recharge (O-18/O-16, H-2/H-1); however, comparisons of values in groundwater with those in recent precipitation are few. This study describes and interprets the isotopic composition of precipitation in central Iowa for the year 1992. An unusually mild winter driven by south-westerly flow of the jet stream was followed by an unusually cool and predominantly dry summer that was caused by a persistent northwesterly flow in the upper atmosphere. Temperature was below normal (9.0 degrees C); precipitation was above normal (868.4 mm) and was mostly derived from a Gulf of Mexico moisture source. Fifty-one precipitation samples were collected from 37 events and analyzed for hydrogen and oxygen isotopes. Tritium activities ranged from 0 to 44 tritium units (TU) and the weighted mean H-3 activity was 11.02 TU, a value consistent with International Atomic Energy Agency (IAEA) data. Anomalously high H-3 activities suggested input of technogenic H-3. Weighted mean values of delta(18)O and delta(2)H were -8.02 parts per thousand and -53.62 parts per thousand, respectively, and the equation of the local Meteoric Water Line was delta(2)H = 7.266(18)O + 4.65. Weighted mean delta(18)O and delta(2)H values from the Gull-Pacific moisture source (-10.80 parts per thousand, -74.82 parts per thousand) were more depleted than those ascribed solely to a Gulf of Mexico moisture source (-7.24 parts per thousand, -47.39 parts per thousand). Seasonal differences in temperature primarily controlled these differences in isotopic composition.