Hydrogeological characteristics of the Paraíba do Sul river flood plains: a case study of a mining area in the Tremembé municipality, SP, Brazil

Abstract

The sand mining activity in the São Paulo part of the Paraíba do Sul River Valley started in the 1950s, in the area of Jacareí municipality, located in the Southwestern part of the basin. From there, it advanced by the neighboring municipalities, fostered by urbanization and industrialization processes, especially of São Paulo Metropolitan Region. The lack of detailed hydrogeological studies in this area motivated this study that had the objective of verifying the hypothesis that sand mining pits interfere on the quality and storage of the water in the quaternary sedimentary aquifer. This study focused specifically in the eastern part of the Tremembé Municipality, São Paulo State, in a mining place called “Mineração Paraíso”. The investigation involved infiltration and flow tests, laboratory analyses of soil physical parameters and estimation, water quality indices and infrastructure construction for equipment installation including piezometers, dynamic penetration tests – (SPT - Standard Penetration Test), wells opening and office analyses. The soil physical parameters analyses revealed high porosity (66%) in the friable soils. This in addition to the high levels of transmissivity (15.5 m2/h measured in Well 1 and 33 m2/h in Well 3), obtained from the flow test, increases the speed of the transportation of solutes including bacteria, to the underground water. Results of the granulometric analyses showed that the sand mining technique is not only predatory, but also inefficient, since the sand at the bottom of the pit is left unexploited, as well as the bentonite clay found in the deeper layers. According to the penetration tests (SPT), the aquifer has an average thickness of 5.5 m and an upper layer of impermeable organic clays that confine the sand deposits. Furthermore, an aquiclude composed of green bentonitic clays was identified just below the confined layer. However, it is necessary to point out that the confinement condition is lost as soon as the organic soil layer is removed during mining exploitation and the sandy deposits starts to have direct contact with freely circulating air in the atmosphere, therefore the aquifer becomes unconfined. With reference to the results concerning the water quality, the ammoniac nitrogen element was found in a concentration greater than expected, both in the wells and in the active pits. This occurs due to the release of tillage soil, rich in urea (ammonium nitrate), which is removed for the installation of sand mining dredgers. Heterotrophic bacteria were found in the groundwater. Therefore, the negative interference of sand mining pits in the quality and storage of the water in the quaternary sedimentary aquifer was proved.