Distribution of atmospheric SF6 near a large urban area as recorded in the vadose zone

Local emissions of SF6 are of interest for studying their impact on the use of SF6 as a groundwater-dating tool near source regions as well as for investigating the spatial distributions of (inert) gaseous compounds spreading from urban or industrial centers. A precondition for the use of SF6 in such studies is the capability to document the temporal and spatial evolution of SF6 in and around source regions with sufficient resolution. Here we present a time series of SF6 measurements in soil air at a site (Sparkill, NY) about 25 km north of New York City carried out between May 2000 and January 2002. The data show that, below about 2 m depth, the vadose zone integrates atmospheric SF6 mixing ratios over time scales greater than 1 month. SF6 mixing ratios in soil air at these depths match averaged high-resolution atmospheric measurements performed at Lamont-Doherty Earth Observatory in Palisades, NY, located about 3 km south of Sparkill. To a first-order approximation, a simple one-dimensional diffusion model reproduces the measured SF6 profiles in the vadose zone, suggesting that the soil indeed acts as a low-pass filter for inert atmospheric gases. These findings indicate that measurements of soil air can be used to determine the spatial pattern of SF6 excess relative to the remote atmosphere for a given region. A transect of soil profiles from Manhattan to the tip of Long Island indicates that emissions from sites close to New York City lead to significant SF6 excesses (ca. 25% or more) above the clean air mixing ratios over distances of the order of 80 km.