RAINFALL CHEMISTRY IN TWO TEMPERATE FORESTS IN THE MÉXICO CITY BASIN

In this study chemical changes in rainfall water were evaluated in two temperate forests: (Desierto de los Leones: DL; high atmospheric deposition) and (Zoquiapan: ZQ; low deposition) in the basin of Mexico City. During the rainy season (August- October, 2003), concentrations of the major ions of anthropogenic origin ( SO 4− , NO3 , NH4 + and H + ) in rainfall, its variability and the effect of precipitation quantity on ionic concentrations were studied. Total atmospheric deposition were collected on a weekly basis by placing rainfall collectors in forest clearings. At DL the total amount of rain collected was 758 mm and 329 mm at ZQ. Ionic concentrations in rain water were also greater at DL, with 43.3, 33.2 and 49 μeq L −1 of SO 4− , NO3 − and NH 4 + , respectively, compared to 28.8, 16.3 y 34.6 μeq L −1 at ZQ. A significant negative correlation (p≤0.0001) was also found between total rain volume and ionic concentration. These findings provide evidence of the influence of atmospheric pollution from the metropolitan zone of Mexico City on forest ecosystems at DL and on water quality from these forested watersheds.