Evolution of the chemical composition of water passing through the unsaturated zone to ground water at an experimental site at the University of São Paulo, Brazil

To study the evolution of the chemical composition of water infiltrating through the unsaturated zone to ground water, water samples were collected weekly over a 14-month period, from porous cups installed at depth intervals of 50 cm through ∼ 12 m of the unsaturated zone, at an experimental site at the University of São Paulo. Previous studies furnished the following data (mean) at the site: precipitation ( P) = 1400 mm year −1; hydraulic conductivity ( K) = 10 −4 cm s −1; zero-flux plane = 0.50–1.50 m; lithology = alternating sandy, clayey, siltic layers; geology = S a ̃ o Paulo sedimentary basin of Tertiary age. The analyses of chemical composition showed that at a depth of 0.5 m, the proportion of cations was exactly the same as that found in the rainwater, i.e. rCa > rMg > rNa > rK where r = reacting value or meq l −1. During percolation of water through the unsaturated zone, the proportion changed predominantly to rNa > rK > rMg > rCa, in the ground water at the site, to rNa > rCa > rK > rMg, and finally at a well further downslope to rCa > rNa > rK > rMg. The proportion of anions in the rainwater was rSO 4 > rNO 3 > rCl. Immediately after rainwater infiltration, this changed to rSO 4 ⩾ rCl > rNO 3 > rHCO 3 in the soil with the appearance of HCO − 3, which became predominant in the ground water, i.e. rHCO 3 > rCl > rSO 4 > rNO 3. Thus the chemical composition of water in the upper part of the unsaturated zone at the experimental site, is influenced by the climate whereas that in the lower part is affected more by local lithology. Three-dimensional, computer treatment of each variable (PLOT/SURF program) showed important variations in the chemistry of water with depth in the unsaturated zone. The variations are less pronounced over time, given the more-or-less uniform conditions of a subtropical climate throughout the year.