# Groundwater Contamination by Gas Stations in Two Eastern Amazonian Towns (Northern Brazil)
## 1 Introduction  
Water is a vital resource for humans and is present in most aspects of modern life, ranging from the basic physiological needs of human beings to economic activities, such as farming, industrial production, and the generation of electricity [1]. Conflicts over land and access to water, the indiscriminate exploitation of underground water sources, the lack of basic sanitation, and the inadequate management of forestry resources are all part of the major environmental problems in the northern Amazonian region of Brazil [2]. In the specific case of the northern Brazilian state of Para, the past few decades have been marked by the progressive growth in the exploitation of natural resources. While anthropogenic modifications have yet to have irreversible impacts on the availability of water in the Brazilian Amazon region, the progressive growth of the pressures that affect the vulnerable local ecosystems area has been increasingly restrictive of the potential for social development and environmental sustainability [3]. The hydrographic basin of the Caete River is located in the northeastern extreme of Para and partially drains the areas of seven municipalities--Bonito, Santa Luzia doPara, Ourem, Capanema, Tracuateua, Braganca, and Augusto Correa. The Caete basin encompasses a number of potential sources of pollution, including (i) open-air refuse disposal; (ii) the lack of a sanitation system or effluent treatment plants; (iii) the discharge of the water used to clean the filters and decanters of the water treatment plant of the town of Braganca directly into the municipality's primary source of drinking water; and (iv) the lack of adequate measures of environmental control of the gas stations located on the margins of the Caete River [3]. The principal problems generated by the inadequate management of the gas stations located at or near the margins of the Caete River include the dilution and dispersal of contaminants derived from petroleum, such as monoaromatic hydrocarbons [4], which affect both underground reservoirs and terrestrial bodies of water and have the potential to affect large areas [3]. Gas stations are important local sources of contamination of the soil and underground water, which occurs primarily in urban areas [5]. The seepage of the hydrocarbons derived from petroleum into the soil is a significant environmental problem, due to the widespread occurrence of leaks and the highly toxic nature of the contaminants that reach the underground waters used for human consumption [5; 6]. These sources of contamination, contaminants derived from gasoline in particular, may also affect large numbers of workers and local residents in the area surrounding the gas stations [7; 8; 9; 10]. Leaks at gas stations, caused by problems with the underground storage tanks and their pipes (ruptures, corrosion) or during the transportation of petroleum products, may cause serious environmental problems [11; 12; 13; 14; 15; 16; 17; 18]. Even minor fuel leaks may contaminate shallow underground water sources [19]. These leaks may also arise due to the constant and almost uninterrupted handling of fuels [3; 7; 20]. The principal volatile organic compounds found in the soil and groundwater as the result of leaks from gas stations are the monoaromatic hydrocarbons or BTEX compounds (benzene, toluene, ethylbenzene, and xylene [o-, m- and p-xylene]) [21]. These substances are abundant in petroleum (e.g., fuel oil and gasoline) and petrochemical products [21; 22; 23]. BTEX substances are extremely toxic, and, once they are released into the environment, they evaporate rapidly into the air. These substances, as well as numerous other petroleum products, represent the fraction of crude oil that is most mobile and soluble in water [24; 25; 26; 27; 28]. BTEX petroleum products spread easily from sources of contamination [21]. A number of different types of detectors have been employed to determine the BTEX concentrations in field samples, in particular gas chromatography (GC) and mass spectrometry (MS) [29; 30; 31; 32].