The Guarani Aquifer (SAG) in South America is a huge hydrogeological system that underlies an area of about 1,100,000 km2, mainly distributed in Brazil (62%), Paraguay, Uruguay and Argentina, is composed by a sequence of sandstone beds, mainly weakly-cemented, of Triassic-Jurassic age, formed by the processes of continental deposition (aeolic, fluvial and lacustrine). The average thickness is about 250 m, varying from < 50m to > 600m, and reaches depths of over 1,000 m (Foster et al., 2009).
The west of São Paulo State - Brazil, where is localized the study area, has a huge dependency on groundwater resources, since the surface water resources are distant from the urban centers. Around the Marilia municipality, within an area of approximately 800 km2, there are nine pumping wells in the SAG. The Marilia municipality, the biggest city at the study area, is supplied by SAG with 1,300 m3/h (33,3% of the total water resources), with five pumping wells.
In this region, the average thickness of SAG is about 218 m, with minimum of 180 m and maximum of 254 m, and the thickness of basalt varies between 564 and 785 m. This portion of SAG belongs to the IV resource management zone classified by Foster et al. (2009) as “Deep Confined Zone”, where pumped groundwater is being ‘mined’ from aquifer storage with continuously (but very gradually) falling potentiometric surface. Prandi et al. (2013) identified residual drawdown varying between 48 a 59.5 m in three of the SAG pumping wells located at Marilia since 1990.
Within this scenario, five SAG’s pumping wells were monitored for two years with the objective to measure the variation of the groundwater level with the pumping schemes. The activities developed consisted by the preparation of a database, well selection and installation of the monitoring equipments, download the monitoring data, consolidation and interpretation of data, and management recommendations were given.
Besides of many problems during the two years monitoring, some results could be described. The SAG drawdown is happening at the region and still in course. In approximately twenty years, the potenctiometric surface decline at maximum 60 m. The hydraulic gradient between the pumping wells is lower in comparison with at the beginning of the pumping, and bigger between these pumping wells and other closest confined SAG wells.
Considering that the SAG is a transboundary aquifer and the west of São Paulo State is very dependent of groundwater resources, this region demands a consistent and constant monitoring program. The aquifer system characterization (quantification of aquifer storage reserves – specific yields, assessment of contemporary recharge rates and appraisal of depletion trends) to provide adequate predictions is essential (Lloyd,1999) to define restriction areas and control of abstraction and use of groundwater.