Anomalous concentrations of hexavalent chromium have been detected in ground-water of the Adamantina Aquifer inat least 54 municipalities located in the northwestern region of the State of São Paulo, southeast Brazil, occasionallyexceeding the permitted limit for human consumption (0.05 mg.L-1). An investigation was conducted in the municipality of Urânia, where the highest concentrations of chromium were detected regionally. It was defined that the originof this contamination is natural, since high concentrations of chromium were detected in aquifer sandstones (averageof 221 ppm) and also in pyroxenes (6000 ppm), one of the main heavy minerals found in the sediments. Besides, noother possible diffuse or point sources of contamination were observed in the study area. Stratification of ground-waterquality was observed and the highest concentrations of Cr6+ were detected at the base of the aquifer (0.12 mg.L-1),where ground-water shows elevated values for redox potential (472.5 mV) and pH (8.61). The origin of Cr6+ in water may be associated with the weathering of pyroxene (augite), followed by the oxidation of Cr3+ by manganese oxides. The highest concentrations of Cr6+ are probably related to desorption reactions, due to the anomalous alkaline pHfound in ground-water at the base of the aquifer.

Two areas in the Atlantic Forest (São Paulo State, Brazil), with contrasting environments in respect of human occupation, were monitored from 1999 to 2001. One area named PEFI (23°38'08''-23°40'18'' S and 46°36'48''-46°38'00'' W) at an altitude of 798 m a.s.l., 526.4 ha in area and about 50 km from the sea, lies in a State Park within the largest metropolis of South America - São Paulo. The other area, named CUNHA (between 23°13'18'' and 23°16'10'' S and 45°02'53'' and 45°05'15'' W) about 1050 m a.s.l. with an area of 2854 ha and about 15 km from the sea, is also within a State Park in the Atlantic Forest, but is surrounded by rural areas and small villages. For each area, the rainfall and throughfall chemistry were examined and pH and Na⁺, K⁺, Mg²⁺, Ca²⁺, NH₄⁺, Cl^-, NO₃^- and SO₄^2- as well as trace metals were determined. Compared with PEFI, CUNHA is characterised by low chemical fluxes and the largest differences are for the ions such as Ca²⁺, H⁺, NO^3- and SO₄^2- which are mainly anthropogenic in origin. Differences in throughfall chemical fluxes are linked to the nutritional status of the trees.