The distribution of meteoric ³⁶Cl/Cl in the United States: A comparison of models

The natural distribution of ^36Cl/Cl in groundwater across the continental United States has recently been reported by Davis et al. (2003). In this paper, the large-scale processes and atmospheric sources of ^36Cl and chloride responsible for controlling the observed ^36Cl/Cl distribution are discussed. The dominant process that affects ^36Cl/Cl in meteoric groundwater at the continental scale is the fallout of stable chloride from the atmosphere, which is mainly derived from oceanic sources. Atmospheric circulation transports marine chloride to the continental interior, where distance from the coast, topography, and wind patterns define the chloride distribution. The only major deviation from this pattern is observed in northern Utah and southern Idaho where it is inferred that a continental source of chloride exists in the Bonneville Salt Flats, Utah. In contrast to previous studies, the atmospheric flux of ^36Cl to the land surface was found to be approximately constant over the United States, without a strong correlation between local ^36Cl fallout and annual precipitation. However, the correlation between these variables was significantly improved (R ²=0.15 to R ²=0.55) when data from the southeastern USA, which presumably have lower than average atmospheric ^36Cl concentrations, were excluded. The total mean flux of ^36Cl over the continental United States and total global mean flux of ^36Cl are calculated to be 30.5±7.0 and 19.6±4.5 atoms m^-2 s^-1, respectively. The ^36Cl/Cl distribution calculated by Bentley et al. (1996) underestimates the magnitude and variability observed for the measured ^36Cl/Cl distribution across the continental United States. The model proposed by Hainsworth (1994) provides the best overall fit to the observed ^36Cl/Cl distribution in this study. A process-oriented model by Phillips (2000) generally overestimates ^36Cl/Cl in most parts of the country and has several significant local departures from the empirical data.