Journal of Conference Abstracts

Biogeochemical Responses to Human-Induced Changes in a Meso-Scale River (Piracicaba River) in Southern Brazil

Alex V. Krusche (avkrusch@carpa.ciagri.usp.br),
M. Victoria R. Ballester (mvrballe@carpa.ciagri.usp.br),
Marcelo C. Bernardes (bernardes@cena.usp.br), Jorge M. Moraes (jmmoraes@cena.usp.br),
Luiz A. Martinelli (zebu@cena.usp.br) & Reynaldo L. Victoria (reyna@cena.usp.br)

CENA - University of São Paulo, Piracicaba, SP, Brazil.

The Piracicaba River basin (12,000 km²), is located in one of the most developed regions of Brazil, the southern State of São Paulo. Besides industrial and urban complexes, land use in the basin has long been driven by agricultural practices (mainly sugar-cane plantations). As a result, only 1 % of the remaining land in the basin is pristine. Human pressure on the riverine system also includes the presence of reservoirs at the upstream reaches, as well as the discharge of both untreated industrial and domestic sewage produced in the basin (10 % and 90 % of the total discharge, respectively).

To understand the responses of the river system to these perturbations in the basin, a bi-weekly monitoring programme was started in 1995. Ten sampling stations were used along the river, consisting of two in the main tributaries (Jaguari and Atibaia Rivers), one in each of the minor tributaries (Camanducaia and Corumbatai Rivers), and 4 along the Piracicaba River itself. Parameters analysed include discharge, quantity, quality, and isotopic composition of particulate and dissolved phases.

In general, oscillations in these parameters were related to two main controlling factors: (1) changes in discharge, associated with rainy and dry season, and (2) evolution along the river, associated with increasing pollution in the basin. For example, annual averages for SO₄^2-/Cl^- ratios, which are good indicators of S loadings from industrial activities, increased from 0.6-1.0 upstream to 1.7-1.8 downstream, a pattern also observed for the composition of other major ions and nutrients. Seasonally, the most striking differences were observed in sediment concentrations, for which dry and rainy season averages ranged, respectively, from 23 to 53 mg L^-1 upstream and from 27 to 157 mg L^-1 downstream.

Concentrations of dissolved organic carbon and respiration rates increased downstream and are correlated within the 95 % confidence level, resulting in changes in average dissolved oxygen (DO) concentrations from oversaturation upstream during the rainy season to partial anoxia (25 % DO saturation) downstream during the dry season. At the same time, the existence of waterfalls among the last two stations downstream results in an increase in average DO saturations from 25 to 52 % during the dry season and from 53 to 69 % during the rainy season, demonstrating one alternative for the restoration of water quality conditions in this system.

These examples will be discussed together with the complete dataset (including never before analysed long-term data provided by CETESB - Sao Paulo State Department for Water Quality Control) for the Piracicaba river. The results show that human pressure in this basin has lead to a significant shift in the biogeochemistry of the Piracicaba River, caused by changes in the patterns of element cycling and metabolic rates. Some alternatives for restoration will be drawn in light of these results.

Acknowledgments

Financial Support: FAPESP, CNPq, ESSO Brazileira de Petroleo.