# Groundwater Contamination by Gas Stations in Two Eastern Amazonian Towns (Northern Brazil)
## 4 Discussion  
BTEX compounds are the contaminants found most frequently in environmental disasters. These compounds are commonly found in gasoline and other derivatives of petroleum and make up a major part of its soluble fraction [11; 52; 53]. The percentage per weight of BTEX in gasoline is 11% benzene, 26% toluene, 11% ethylbenzene, 31% meta-xylene, 12% ortho-xylene, and 9% para-xylene [54]. One of the principal sources of contamination of the subsoil and underground waters is the accidental release of petroleum-based fuels from storage tanks or pipelines [11; 55]. These highly toxic compounds tend to enter the environment through spills or leaks and then spread rapidly. These accidental discharges typically occur in the vicinity of oil and gas wells, fuel stations, and storage tanks, e.g., [14; 16; 17; 56; 57; 58]. These substances are highly volatile and are extremely similar to one another in their physicochemical characteristics (e.g., solubility, volatility) and their dispersal routes into the environment [22; 59]. The maximum BTEX concentrations (mg L\({}^{-1}\)) that can dissolve in pure water at a specific temperature (25 \({}^{\circ}\)C) are 1740-1860 mg L\({}^{-1}\) of benzene, 500-627 mg L\({}^{-1}\) of toluene, 131-208 mg L\({}^{-1}\) of ethylbenzene, and 167-196 mg L\({}^{-1}\) of xylene, e.g., [16; 22; 60]. The solubility of these substances in water greatly affects their movement and their dispersal through the soil and underground water [54]. BTEX contaminants are dangerous substances, given their enormous capacity for movement through water, the ground, and the air, due to the density, solubility, and molecular weight of these compounds [61]. Each compound may provoke a specific type of toxicological damage, with benzene, for example, having deleterious impacts on the hematopoietic, central nervous, and reproductive systems, while toluene may provoke damage to the hematopoietic and reproductive systems. Ethylbenzene and xylene, in turn, may cause disorders of the respiratory and neurological systems [9]. Benzene is considered to be the most toxic and dangerous compound of the BTEX group and is a major risk factor for the development of cancer [7]. In a study of underground water reserves in northern Fortaleza, a city in northeastern Brazil [62], samples were collected from tubular wells at gas stations and in the surrounding area. In this study, ethylbenzene and o-xylene had the highest concentrations of the compounds analyzed. Ref. [63] applied the CG/SM approach in their investigation of contamination by BTEX at six points, using samples of underground water collected from wells excavated at a gas station with a history of leaks in the metropolitan region of Yogyakarta, in Indonesia. In this study, the majority of the BTEX components were found in the samples at concentrations of between 0.008 and 25.631 \(\upmu\)g L\({}^{-1}\). The concentration of benzene at one of the sampling points exceeded the threshold for drinking water established by both the Indonesian government and the WHO. Ref. [64] investigated the existence of BTEX in samples of shallow underground water from 16 wells monitored in the region of Songyuan, in the basin of the Songhua River in China. The mean BTEX concentrations were 1.53 \(\upmu\)g L\({}^{-1}\) (B), 1.76 \(\upmu\)g L\({}^{-1}\) (T), 2.11 \(\upmu\)g L\({}^{-1}\) (E), and 0.30 \(\upmu\)g L\({}^{-1}\) (X). Ref. [65] also applied the GC/MS approach to analyze the occurrence of BTEX contaminants in samples of underground and surface waters in the city of Guangzhou, in the Chinese province of Guangdong, but found that the samples were within the legal thresholds. A proportion (14.63%) of the samples was contaminated, with concentrations of less than 9.5 \(\upmu\)g L\({}^{-1}\). Toluene (12.20% of the samples) and benzene (3.65%) were the predominant contaminants. Ref. [66] evaluated the degree of contamination of the subsoil in the vicinity of an oil spill in the town of Osubi, in Okpe, the Delta state, in southeastern Nigeria.