Figure 1: (a) Mezquital Valley topography with sampling sites (circles), major population centers, and the refinery-thermoelectrical plant complex. (b) Trajectory of the dispersion plume in SE direction observed on April 4, 2006 at 16:00 and 17:00 hours. APX: Apaxco, ATO: Atoyac, ATI: Atitalaquia, CRZ: Cruz Azul, JAS: Jasso; PXT: PEMEX gas substation; RTPPC: refinery thermoelectrical power plant complex; SMA: Santa María Apaxco; TEP: Tepeji; TLX: Tlaxcoapan; TUL: Tula.