Measurement of chemical elements in rain from Rancho Viejo, a rural wooded area in the State of Mexico, Mexico

A rural forested site was selected to measure Al, Cd, Cr, Mn, Ni, Pb and V (soluble and insoluble fractions), and the major inorganic ions Na+, K+, Ca+, Mg2+, NH4+, H+, Cl−, NO3− and SO42−. A total of 44 samples were collected during the rain seasons of 2001–02. Trace metals were analyzed by atomic absorption spectroscopy with a graphite furnace accessory. As for ion analysis a Perkin Elmer liquid chromatograph was used. The volume-weighted mean concentration (VWMC) results indicated that highest concentration was that of aluminum followed in decreasing order by Mn, V, Pb, Cr and Cd. Solubilities were calculated in rain samples at pH<5 and pH>5, solubilities at pH<5 were higher than the solubilities at pH>5, as it has been reported by other authors. The rains from Rancho Viejo had a pH of 4.5 indicating an acid nature. Comparing the VWMC of Ca2+ (7.79 μeq l−1) with NH4+ (46.36 μeq l−1), indicated that the NH4+ was the main neutralizing compound. The scatter plots showed that the acidity of rain was due mainly to H2SO4 plus HNO3 (55% and 22.7%, respectively). Backward trajectories using the NOAA HYSPLIT Model were calculated at 3000 m above mean sea level (AMSL) and indicated significant differences in the mean concentrations that depended on the wind direction. Good and positive correlations were found among all trace metals that indicate a common origin. Factor Statistical Analysis 6.4 was applied to all data to have a better knowledge of the origin of the trace metals in rainwater. To evaluate the contribution of non-crustal sources an enrichment factor was also applied.