DISSOLVED-GAS RECHARGE TEMPERATURES FOR EVALUATING PALEO-WATER-TABLE RISES AND CLIMATE CHANGE

Recharge temperatures calculated from dissolved-gas concentrations (N, Ar, Kr, Ne) in ground water have often been used for paleo-temperature reconstruction. Dissolved-gas recharge temperatures cooler than average annual air temperatures are generally used to infer a previously colder climate. Water from 10 shallow wells screened within 7 m of the water table (currently 18 to 43 m below land surface) at Sand Hollow in southwestern Utah has a mean recharge temperature of 9 degrees Celcius (C). This is much cooler than measured annual average temperatures, which range from 17 C at land surface to 19 C in the deep vadose zone and shallow water table. Ground water in these wells has near-modern C-14 ages, indicating late Holocene recharge under a temperature regime similar to present day conditions. Recharge temperatures decrease from 14 C in shallow wells with modern tritium concentrations (recharged within the past 50 years) to as low as 1 C in shallow wells with older water (50 to a few thousand years). Wells screened deeper in the aquifer generally have warmer dissolved-gas recharge temperatures (7 to 18 C) and C-14 ages ranging throughout the Holocene. We believe the cooler dissolved-gas recharge temperatures of late-Holocene-age ground water at Sand Hollow represent winter recharge to a water table that had risen to near land surface. Winter infiltration rapidly recharging a shallow water table may preserve dissolved-gas concentrations in rain equilibrated with seasonally cool air temperatures. Darcy’s Law calculations indicate that recharge rates of as much as twice the current amounts are needed for the water table to rise to land surface. In addition to a shallow water table, this indicates that summer monsoonal precipitation in southwestern Utah may have been a smaller component of recharge than under current climatic conditions. This is supported by the correlation between cooler recharge temperatures and lighter stable-isotope ratios in ground water. These findings suggest the possibility that cool dissolved-gas recharge temperatures in other aquifers recharged through very shallow water tables may represent climatic shifts toward wetter winter-dominated precipitation rather than cooler paleo-climatic air temperatures.