Natural background levels of inorganic elements in the groundwaters of the Lagoa Santa karst region, Minas Gerais, Brazil

• Autor(a) principal: Frederico Aragão Cardoso
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Institucional da UFMG
• Texto Completo: http://hdl.handle.net/1843/36904
• topic: Natural background levels; Hydrochemistry; Geogenic anomalies; Karst aquifers; Groundwater contamination; Hidrogeologia; Aquíferos – Lagoa Santa (MG); Carste – Lagoa Santa (MG)

Natural Background Levels (NBLs) were proposed to the groundwaters sampled from deep tubular wells in the karst region of Lagoa Santa, Minas Gerais, within an area of 505 km². The metalimestones are from the Neoproterozoic age, show a low-grade metamorphism and constitute the Sete Lagoas Formation, which is composed by impure limestones (Pedro Leopoldo Member), calcitic limestones (Lagoa Santa Member) and metapelites from Serra de Santa Helena Formation. Typical geomorphological features illustrate an evolved karst, with an elevated number of depressions, caves, ducts and upwelling zones. The NBLs calculation method was the determination of the 90º percentile from the probability distribution plots, related to each parameter. According to the number of samples with values above the quantification limit, the study was realized in two stages: firstly, NBLs were proposed to ions that are indicative of pollution in the collected waters only from deep tubular wells, for N-NO3- (1.0 mg/L), Cl- (2.4 mg/L), PO43- (0.1 mg/L) e SO42- (5.3 mg/L); secondly, NBLs were proposed only to major ions, namely, Ca2+ (96.99 and 91.52 mg/L), Mg2+ (4.88 and 2.88 mg/L), Na+ (6.15 and 3.75 mg/L), K+ (1.01 and 0.31 mg/L), HCO3- (282.52 and 254.55 mg/L), SO42- (9.35 and 13.64 mg/L), Cl- only for deep tubular wells (6.67 mg/L) and, finally, Si4+ (17.87 e 12.71 mg/L), which was used to the distinction of the siliciclastic units. As for the pollution indicators considered in the first stage, results have shown that, for all parameters, the majority of the areas with surpassed NBLs are in moderate vulnerability portions, indicating anthropic impacts represented by domestic sewage and agricultural and industrial activities. And, as for the major ions and silica from deep tubular wells and springs, results have shown that for all parameters, except for chloride, the elevated concentrations above the NBL hold a geogenic origin, influenced mainly by the calcitic pure composition of the Lagoa Santa Member and by the impure pelitic intercalations of Pedro Leopoldo Member and Serra de Santa Helena Formation, whereas the high Cl- levels might be due to sewage in the Mata river. The results contribute to the understanding of the natural inorganic hydrochemical levels of the region and orientate new studies in specific areas, according to the indicative parameters of pollution.