Hidrogeoquímica dos Aquíferos da Região da APA Carste de Lagoa Santa, MG

Abstract

This study at Environmental Protection Area of Lagoa Santa Karst on the north of Belo Horizonte aimed the geochemical characterization of rocks and groundwaters with intention to assist a later hydrodynamic and Hydrochemical models structuration and in the implementation of a monitoring network. Three main lithostratigraphic units outcrop in the region and form distinct aquifers. The granite-gneisses basement of Belo Horizonte Complex, recrystallized limestones of Sete Lagoas Formation, main aquifer of the area and divided in Pedro Leopoldo Member, inferior, composed by fine limestones with pelitic intercalations, and Lagoa Santa Member, superior, with coarse limestones of pure calcite composition; and pelitic rocks of Serra de Santa Helena Formation on the top. Water samples of springs and tube wells corresponding to shallow and deep circulation were chemically analyzed and data statistic treated and plotted in binary, ternary, Piper and Stiff diagrams for interpretation.. This waters are mainly calcium bicarbonated, occurring mixed bicarbonated facies limited to basement waters and sodium bicarbonated relative to colluvium. These are easily distinguishable from the others, as well as those associated with the pelitic rocks, by chemical composition with low concentrations of Ca2+, observed by the median (18,5 mg/L), and high concentrations of Na+ (7,9 mg/L) - referring only to granites - and Si4+ (35,5 mg/L). The granite-impure limestones and pelitic rocks-pure limestone mixes maintain high concentration of Si4+ (26,7 mg/L) but increase concentrations of Ca2+ (55,9 mg/L) due to carbonate contribution. The waters of strict circulation in Lagoa Santa Member reflect the calcite composition of rock showing higher concentrations of Ca2+ (80,2 mg/L) and HCO3- (243,4 mg/L) than the others and low concentrations of Mg2+ (1,1 mg/L), Na+ (0,3 mg/L) and Si4+ (9,4 mg/L). The presence of pelitic intercalations in impure limestones impart higher concentrations of Mg2+ (3,2 mg/L), Na+ (1,6 mg/L) and Si4+ (14,5 mg/L) to its waters, but lower of Ca2+ (59,8 mg/L) and HCO3- (205,0 mg/L) than the superior member. The faciological variability of this member allows the subdivision of its waters in four hydric types with variable concentrations of Ca2+ and Si4+ that also could be observed in the average water input quotas and tube wells productivity of this unit. The hydrochemistry subject to dissolution by the high concentration of Ca2+ associated to pure portions of the rock confers in part high productivity of wells. The primarily factor is its location near by faults zone and lithologic contact between the members, as well as the occurrence of higher flows in higher quotes in Pedro Leopoldo Member and lowers in Lagoa Santa Member. The intensity of karstification controls the shallow circulation that shows higher flows in superior member rocks. The concentration of Si4+ is the primarily indicative of horizontal and vertical aquifers compartmentation with increase in depth and from NW to SE and NE.