Isotope characterization of the hydrologic cycle components in four sub-basins of the Piracicaba river basin (SP)

Author(s)

Carlos Eduardo Beduschi

• Document type: Master's Dissertation
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2008-06-20

Examining board members

Luiz Antonio Martinelli; Osny Oliveira Santos Bacchi; Jean Pierre Henry Balbaud Ometto

• Advisor: Luiz Antonio Martinelli

Abstract

This project had the objective to investigate the oxygen (?18O) and hydrogen (?D) isotopic variation of shallow unconfined aquifers in four regions, in transects defined from the river to the limit of the drainage area. The transects were established within a topographical gradient in four sub-catchments of the Piracicaba river hydrographic basin. The population in this basin is almost three and a half millions inhabitants and an economy based on agriculture and industry is responsible for about 10% of the total Brazilian gross production. Based on a well documented isotopic variation of rainwater and river water done previously (Martinelli et al. 2004), where data seasonality and a possible variation related to water use in urban areas were identified, the studies were extended to the groundwater (baseflow) sources to the major channel (Piracicaba river) of the basin. The seasonality defined by a wet summer (november - april) and a dry winter (may - september) was covered with monthly (end of dry season of 2005; rainy season of 2005-2006 and dry season of 2006) and every two weeks sampling (rainy season 2006-2007) of rain water, shallow wells, springs and surface water (rivers) that drain the studied sub-basins. The ?18O and ?D individual values of composite precipitation in the studied sub-areas presented higher variation (extreme values; mean ?18O from -0,4 to -13,0? ) when compared to the values found for surface water (mean ?18O from -5,1 to -9,2? ) and groundwater (wells and springs; mean ?18O from -6,9 a -7,1? ), being the values of groundwater almost constant within the studied period. Precipitation and baseflow have relative different isotopic composition of ?18O, once the weighted average of precipitation (-8,6? observed for the whole studied period) is different from the average values found for unconfined groundwater (-7,0? in average). Values of surface runoff to the main channel (Piracicaba river) were similar to those found in precipitation in the rainy season (presenting variations within this period), otherwise, the isotopic signals of surface water were rather more similar to those found in baseflow, characterizing the contribution of baseflow in the lower flows. However, average value of ?18O of all rivers and streams studied (-5,7? ) were about 1,2? more enriched than the average value of wells and springs (-6,9? ) in the dry season. The amplitudes of ?18O found in precipitation and groundwater were used as a first aproximation to estimate mean residence time of unconfined groundwater. (AU)