Groundwater with anomalous fluoride content and water mixture patterns were studied in the fractured Serra Geral Aquifer System, a basaltic to rhyolitic geological unit, using a principal component analysis interpretation of groundwater chemical data from 309 deep wells distributed in the Rio Grande do Sul State, Southern Brazil. A four-component model that explains 81% of the total variance in the Principal Component Analysis is suggested. Six hydrochemical groups were identified. δ18O and δ2H were analyzed in 28 Serra Geral Aquifer System samples in order to identify stable isotopes patterns and make comparisons with data from the Guarani Aquifer System and meteoric waters. The results demonstrated a complex water mixture between the Serra Geral Aquifer System and the Guarani Aquifer System, with meteoric recharge and ascending water infiltration through an intensive tectonic fracturing. 1. Introduction In the last several decades, the global water consumption has dramatically increased, especially for agriculture, water supply, and industrial uses. This paper examines the fluoride content in water from the southernmost region of the fractured subcontinental Serra Geral Aquifer System (SGAS), an important aquifer that supplies a large amount of water for an economic developed region in Southern Brazil. Fluoride content in water is beneficial to human health and in a moderate concentration (0.7–1.2？mg/L) prevents dental cavities. When used in excess can be toxic causing human and animal dental and skeletal fluorosis, which has been detected in China [1, 2], India [3], Kenya [4], and Israel [5], among other countries. The drinking water limit recommended by the World Health Organization for fluoride is 1.5？mg/L [6]. In the study area, the SGAS fluoride average concentrations are around 0.24？mg/L, with a minimum value of 0.02？m/L and the highest at 3.03？mg/L. The SGAS overlies the Guarani Aquifer System (GAS; [7]), which has been the focus of several recent studies due to its spatial extent and storage potential as a transboundary aquifer [8]. The area covered by the SGAS in Brazil, Uruguay, Argentina, and Paraguay is equivalent to 1,200,000？km2 (Figure 1). In Brazil, these groundwater resources should be efficiently managed to protect its water potential and quality. The climate in the study area ranges from subtropical to temperate, with precipitation average of 1,550？mm/year. Figure 1: Location map showing the Serra Geral Formation (SGF) in South America (modified from [ 9]). The SGAS reaches its maximum thickness of about 1,800？m along the central
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