Geosciences, Vol. 8, Pages 9: Groundwater Natural Contamination by Toluene in Beja and Faro Districts, Portugal Geosciences doi: 10.3390/geosciences8010009 Authors: Gabriel Barberes Rui Pena dos Reis André Spigolon Paulo Fonseca Carlos Bandeira de Mello Maria Barata The focus area is located along Beja and Faro districts (southwestern of Portugal), where the Baixo Alentejo Flysch Group exists, is composed of the Mértola, Mira, and Brejeira Formations. The aim of this article is to characterize the groundwater contamination by toluene, one of the petroleum products that may originate from natural petroleum systems and have been introduced into the environment by natural seepage. For this study, twenty soil samples and thirty-one water samples were collected, in 2016, from artesian wells, boreholes, and springs, and analyzed in a GC-FID system. Sampling was performed using headspace vials, with distilled water and a bactericide to inhibit any bacterial activities. The results showed that 93.5% of water samples range between 1000 and 6000 μg/L, and 55% are higher than 3000 μg/L of toluene concentration. In soil samples, amounts of toluene are less than 1000 μg/L in general. These levels are much higher than the World Health Organization health-based guideline for drinkable water (700 μg/L) and higher than those usually found in other places, being only comparable with values from severe industrial contamination. Contaminated groundwater represents a serious concern for human health, and residents should be informed to avoid drinking contaminated ground water or using it for other domestic proposes.