Urban aquifer recharge: case study in Urânia (São Paulo State, Brazil)

Author(s)

Carlos Henrique Maldaner

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2010-10-06

Examining board members

Ricardo Cesar Aoki Hirata; Marly Babinski; Edson Cezar Wendland

• Advisor: Ricardo Cesar Aoki Hirata

Abstract

The focus of this project is to identify the source and quantify the recharge of the unconfined portion of the Bauru aquifer, present in the urban area of Urania, a small city located at the northwestern corner of São Paulo State, Brazil. The aquifer is composed by sandy sediments of the Vale do Rio do Peixe Formation, Bauru Group. Water balance and water table fluctuation were used to quantify the recharge. Hydrochemistry and isotopes, both stable and radioactive, were used to determine recharge sources. Rain water samples were collected, as well as from shallow and deep wells taping the Bauru aquifer and the confined Guarani Aquifer System (GAS), and from the public water supply system, which is a mixture of both aquifers. Climate is tropical, with two well defined seasons: the humid, from October to May, and the dry, from June to September. The total accumulated precipitation between September 2007 and august 2008 was 2498 mm. Using the water balance method, runoff accounted for 10% of this amount (247 mm), evapotranspiration 69% (1734 mm) and recharge 21% (517 mm). Water table was monitored in two wells with different depths in the unconfined aquifer, between September 2008 and April 2010. The total precipitation during this period was 2742 mm. Using the water table fluctuation method, recharge in the shallower well was 20% of this total (544 mm), and 16% in the deeper well (456 mm). Only rainfalls in excess of 100 mm/month caused an elevation in the water table. The local meteoric line was established (D = 8,5 18O + 17 ( VSMOW)). The stable hydrogen and oxygen isotopes indicated that the main recharge source is rainfall, and a secondary source is leakage from water and sewage mains. Radioactive Pb isotopes were not useful for recharge source definition. Water samples collected at different depths in gravel-packed wells show both distinctive radioactive Sr isotope and hydrochemical signatures. Thus, groundwater can classified as shallow, intermediary and deep. Most samples collected at gravel-packed wells had less than 8% of water coming from the shallow zone, but a few wells had up to 49% of shallow water, which indicates high vulnerability to contaminants. Therefore, Sr isotopes may be used as a tool in unconfined aquifer vulnerability assessments. (AU)