Geochemical evolution of groundwater in a basaltic aquifer based on chemical and stable isotopic data: Case study from the Northeastern portion of Serra Geral Aquifer, Sao Paulo state (Brazil)

Author(s)

Gastmans, Didier ^[1] ; Hutcheon, Ian ^[2] ; Menegario, Amauri Antonio ^[1] ; Chang, Hung Kiang ^{[1, 3]}

Affiliation

^[1] UNESP Univ Estadual Paulista, Ctr Estudos Ambientais, Av 24A, 1515 Bela Vista, BR-13506900 Rio Claro, SP - Brazil

^[2] Univ Calgary, Appl Geochem Grp, Dept Geosci, 2500 Univ Dr NW, Calgary, AB T2N 1N4 - Canada

^[3] UNESP Univ Estadual Paulista, Lab Estudos Bacias, Av 24A, 1515 Bela Vista, BR-13506900 Rio Claro, SP - Brazil

• Document type: Journal article
• Source: Journal of Hydrology; v. 535, p. 598-611, APR 2016.
• Web of Science Citations: 9

Abstract

Groundwater from the fractured basalt Serra Geral Aquifer (SGA) represents an important source for water supply in Northeastern Sao Paulo state (Brazil). Groundwater flow conditions in fractured aquifers hosted in basaltic rocks are difficult to define because flow occurs through rock discontinuities. The evaluation of hydrodynamic information associated with hydrochemical data has identified geochemical processes related to groundwater evolution, observed in regional flowpaths. SGA groundwaters are characterized by low TDS with pH varying from neutral to alkaline. Two main hydrochemical facies are recognized: Ca-Mg-HCO3, and Na-HCO3 types. Primarily, the geochemical evolution of SGA groundwater occurs under CO2 open conditions, and the continuous uptake of CO2 is responsible for mineral dissolution, producing bicarbonate as the main anion, and calcium and magnesium in groundwater. Ion exchange between smectites (Na and Ca-beidelites) seems to be responsible for the occurrence of Na-HCO3 groundwater. Toward the Rio Grande, in the northern portion of the study area, there is mixing between SGA groundwater and water from the sandstones of the Guarani Aquifer System, as evidenced by the chemical and isotopic composition of the groundwater. Inverse mass balance modeling performed using NETPATH XL produces results in agreement with the dissolution of minerals in basalt (feldspars and pyroxenes) associated with the uptake of atmospheric CO2, as well as the dissolution of clay minerals present in the soil. Kaolinite precipitation occurs due to the incongruent dissolution of feldspars, while Si remains almost constant due to the precipitation of silica. The continuous uptake of CO2 under open conditions leads to calcite precipitation, which in addition to ion exchange are responsible by Ca removal from groundwater and an increase in Na concentrations. Down the flow gradientCO(2) is subject to closed conditions where the basalts are covered by the sediments of Bauru Group or associated with deeper isolated discontinuities. A decrease in the amount of dissolution of labradorite and augite is observed, associated with precipitation of carbonates and kaolinite. Stable isotope ratios of SGA groundwater vary from -37.8 parts per thousand to -61.3 parts per thousand VSMOW for delta H-2 VSMOW, and -5.7 parts per thousand to -8.9 parts per thousand VSMOW for delta O-18, indicating temporal variations in climatic conditions during recharge. (C) 2016 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 12/00241-5 - Hydrogeochemistry and isotopic hydrology of the Serra Geral Aquifer groundwaters in São Paulo State
• Grantee: Didier Gastmans
• Support Opportunities: Regular Research Grants