Fig. 1. County map of northern California showing the site as well as some major cities from which advection of anthropogenic hydrocarbons can occur. Also shown are two 12-h isentropic back-trajectories calculated with HYSPLIT (National Oceanic and Atmospheric Administration Air Resources Laboratory, 1997) for 18 and 21 June 1999, arriving at Blodgett at 1900 h (first symbol at -1 h).