Journal of Conference Abstracts

Hydrogeochemistry of a Small Catchment in Northeastern Amazonia (Serro do Navio, Amapá State, Brazil)

M. C. Forti^1,3 (forti@met.inpe.br), R. Boulet^2,3 (fax: 55+11 210 4958),
A. J. Melfi³ (ajmelfi@usp.br) & C. Neal⁴ (C.Neal@unixa.nerc-wallingford.ac.uk)

¹ Instituto Nacional de Pesquisas Espacias, INPE, CP 515,
São José dos Campos, SP, CEP 12201-970, Brazil.

² OSTROM/NUPEGEL, Inst. De Geociéncias - USP, CP 10899, CEP 10498-979, São Paulo/SP, Brazil.

³ NUPEGEL, Instituto Astronômico e Geofísico USP, CP 9638, CEP 10165-970, São Paulo/SP, Brazil.

⁴ Institute of Hydrology, Maclean Building, Wallingford, Oxon OX10 8BB, U.K.

The initial findings of a comprehensive study of the effects of mining activities (manganese ore exploitation) on the chemical composition of the drainage waters of a small catchment in northeastern Amazonia are presented. The chemical composition of rainwater, throughfall, and stream water was determined weekly over a one year period. The total catchment area is 164 ha; the headwaters area (34 ha) is covered with natural tropical forest, and the lowest part (34 ha) is affected by activities related to Mn ore mining. The temporal chemical variability of these waters is evaluated along with a comparison between the waters from the pristine forested area and the altered one.

Rain waters are slightly acidic (pH ~5.4) with low alkalinity, while the stream water is circumneutral (pH ~6.9). Alkalinity is generated within the soil-soil water system. The rainfall-throughfall relationship of the chemical species in the pristine area shows strong internal cycling and a small net export from the catchment. For the area altered by mining, almost all chemical species show a net export in surface waters. These originated mainly in the mined area of the basin, sue to enhanced mechanical erosion and soil leaching as vegetation lost during the mining has not yet become reestablished.

Trends in Soil Percolate Chemistry at the Turkey Lakes Research Watershed (1983 to 1994)

Neil W. Foster (nfoster@glfc.forestry.ca) & Paul W. Hazlett (phazlett@glfc.forestry.ca)

Great Lakes Forestry Centre, Canadian Forestry Service, P.O.Box 490, Sault Ste.Marie,
Ontario, P6A 5M7, Canada.

Year-to-year variation in sulphate, nitrate, calcium, magnesium, and potassium concentrations in soil solutions from podzolic soils supporting maple-dominated forest, at the Turkey Lakes Watershed, was assessed for monotonic trends. Our objective was to examine whether soil solution ion concentrations decreased in response to reductions in sulphate and base cation concentrations in precipitation. Soil percolate was sampled at 60 cm depth, the bottom of the effective rooting zone, biweekly in the snow-free period and weekly during snowmelt. Trends in soil solution were compared to those in wet-only precipitation from the Algoma CATMON station at the Turkey Lakes Watershed.

Atmospheric induced changes in sulphate concentrations in soil percolate were site specific. At one site, similarities in significant negative trends for sulphate, calcium and magnesium, suggest that precipitation and soil percolate chemistry are strongly linked. At a second site, variable trends were observed for each of the five ions. Retention/release of sulphate by the soil, at the second site, was strongly influenced by nitrification in the soil. Acid deposition to the latter site, may acidify soil and water more slowly during nitrate flushes and more rapidly when nitrification rates decline. The ecological benefits, that are expected to acrue from SO₂ controls, may not be immediately apparent in watersheds with soils that nitrify readily.