**1. Introduction**

214 Fossil Fuel and the Environment

N.H Wonga, D.K.W Cheonga, H Yana, J Soha, C.L Ongb, A Siab, The effects of rooftop

NZ Herald, Urban sprawl not the answer for Auckland, says councillor, Dec 17, 2011, www.nzherald.co.nz/nz/news/article.cfm?c\_id=1&objectid=10773736 Ocean warming's effect on phytoplankton / NASA satellite data show how global climate

Richard B. Howarth, Brent M. Haddad, Bruce Paton, The economics of energy efficiency:

S. Raeissi, M. Taheri, Energy saving by proper tree plantation, Building and Environment,

Shahriar Khan, "Planting Trees for Mitigating Flooding, Erosion, and Cyclonic Damage,"

Shahriar Khan, An energy saving program for bangladesh, for reducing load shedding,

Shahriar Khan, An Energy Saving Program for Bangladesh, Journal of Petroleum Technology and Alternative Fuels, Vol. 2, No. 6, pp. 86 - 94, June 2011. Shahriar Shafiee, Erkan Topal, (2009). "When will fossil fuel reserves be diminished?"

Stephen J DeCanio, The efficiency paradox: bureaucratic and organizational barriers to

The Great Reversal, an increase in forest density worldwide, is under way, Posted June 6,

Wikipedia.com, Urban Sprawl, accessed Dec. 20, 2011. Sprawl plunges Canadians into

Wokje Abrahamse, Linda Steg, Charles Vlek, Talib Rothengatter , "A review of

Yates, Suzanne M.; Aronson, Elliot, A social psychological perspective on energy

YunXia Wangb, Tao Zhao, Analysis of interactions among the barriers to energy saving in

Energy Policy, Volume 37, Issue 1, Pages 181-189, January 2009.

2011, The Rockefeller University, newswire.rockefeller.edu.

Debt, The Times Colonist, Dec. 18, 2011

www.marinebio.org, Ocean Pollution, Accessed Oct. 9, 2011.

and Buildings, Volume 35, Issue 4, May 2003, pp. 353-364.

change hurts marine food chain, SFGate.com, Dec. 7, 2011.

Volume 34, Issue 5, 1 September 1999, pp. 565-570

6-7, June 2000, Pages 477-486

Ottawa.

Bangladesh.

1998, Pages 441-454

1983, 435-444

1889

garden on energy consumption of a commercial building in Singapore, Energy

insights from voluntary participation programs, Energy Policy, Volume 28, Issues

Proceedings of Natural Disasters in Bangladesh, 1991, Carleton University,

and for continuity of power for IT sector, Proceedings of the 11th International Conference on Computers and Information Technology, 2008, Khulna,

profitable energy-saving investments, Energy Policy, Volume 26, Issue 5, April

intervention studies aimed at household energy conservation," Journal of Environmental Psychology, Volume 25, Issue 3, September 2005, Pages 273-291.

conservation in residential buildings. American Psychologist, Vol 38(4), Apr

China, GuoHong Wanga, Energy Policy, Volume 36, Issue 6, June 2008, pp. 1879-

The quality of the environment in big cities depends on its population and their domestic, transport, and industrial activities. In some places agricultural land use coexists with urban areas and as a result of this urbanization and the presence of infrastructure for services like water, electricity, drainage, and the use of fossil fuels etc, contamination problems in the atmosphere, soil and water (Wilcke, 2000), that lately lead some ills on organisms such as respiratory malaises, liver-lung-skin cancer, irritation on eyes and others discomforts. The growth of urban environments presents a major challenge. However, Mexico City as center of economic growth, education, technological advancement, and culture, large city also offer opportunities to manage the growing population in a sustainable way.

These concentrations of people and activity are exerting increasing stress on the natural environment, with impacts at urban, regional and global levels. In the last few decades, air pollution has become one of the most important problems of megacities. The nitrogen and sulphur compounds are main air pollutants, photochemical smog-induced primarily from traffic, but also from industrial activities, power generation, and solvents-has become the main source of concem for air quality. Air pollution has serious impact on public health, causes urban and regional haze, and has the potential to contribute significantly to climate change (Molina & Molina, 2004).

Mexico City and metropolitan area (MCMA), often simply called Mexico City, consists of 16 delegations of the Federal District and 37 contiguous municipalities from the State of Mexico and one municipality from the State of Hidalgo, some with populations over 1 million, that make up the total population of above 20 million for this megacity (Escobedo et al., 2000).

Polycyclic aromatic hydrocarbons (PAHs) are compounds with two or more aromatic rings (benzene) produced by both natural and anthropogenic pathways although anthropogenic

Presence of Polycyclic Aromatic Hydrocarbons

according to standard methods of conservation.

poor air quality (Fast et al., 2007).

(PAHs) in Semi-Rural Environment in Mexico City 217

(Figure 1). It is situated at a tropical latitude, has an urban area of about 3500 km2, is at 2240 m altitude, and is surrounded by high mountains on three sides, all of which contribute to

We collected a composite sample (2 kg) of apple for each location from an area of approximately 1500 m2 during both dry and wet months and steam cactus stem in Milpa Alta in 2008-2009. For irrigation water we took 1 L during 2008 from each location with glass previously cleaned with solvents. Finally, we collected a composite sample (1 kg) of soil from each location during both dry and wet months in 2009. The samples were conserved

Fig. 1. Distribution of urban and green-conservation terrains in Mexico City (From GDF, 2003).

Soxhlet extraction was used for fruits and soil to extract 10 g of sample mixed with anhydrous sodium sulfate, using a solvent mix of hexane-dichloromethane (1:1) according to the method of Samsoe et al. (2002). For water samples liquid-liquid extraction was used with a mix of hexane-acetone. Chromatographic columns were prepared with chromatographic absorbents to obtain PAH extracts. The organic extract was concentrated in a rotary evaporator to 1 mL and transferred to a vial for gas chromatographic analysis according to EPA method 8100. The concentrations and profiles of PAH compounds were analyzed using a Perkin Elmer AutoSystem gas chromatograph with capillary column HP-5. The oven temperature was initially set at 90°C and the final temperature was 300ºC. Detector and injector temperature were 320ºC. The carrier gas was high purity helium

Identification of PAH compounds was based on matching their retention time with a mixture of PAH standards (Chem Service). The 16 PAH compounds were naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), fluorene (Flu), phenanthrene (Phe), anthracene (Ant), fluoranthene (Fla), pyrene (Pyr), benzo(a)fluorene (BaF), benzo(a)

(99.99%). A sample of 1 L was injected in splitless mode.

activities generally release much greater amounts to the environment (Eom et al., 2007). They originate from combustion, coke production, oil derivates and high temperature industrial processes. PAHs are considered as persistent organic pollutants (POPs) according to the Stockholm Convention. In many studies of contamination, they have been found in air, water, food and soil. There is evidence that some PAHs are carcinogenic, mutagenic and toxic. Monitoring of the PAHs in the environment is important in the evaluation of risk to the health of organisms.

With this problematic situation, the food production may be contaminated with different classes of organic and inorganic residues and contaminants (García-Alonso et al., 2003). For Mexico City case, the presence of contaminants in rural environment highlights persistent organic pollutants (POPs), for example polycyclic aromatic hydrocarbons (PAHs). The sources of these compounds are variable, for example vegetation and fossil fuel combustion, heating (Finizio et al., 1998). PAHs with high persistent in the environment are benze(a)pyrene, anthracene, crysene and others with molecular high weight (> 4 rings aromatics). The occurrence of PAHs is widespread in environmental compartments as air, water, soil and food.

Soils are large reservoirs of hazardous contaminants derived from anthropogenic activities. Some studies of wet and dry atmospheric deposition of PAHs have found values of >10 mg/kg, mainly in urban soils and tropical areas, for example in Brazil (Krauss et al., 2000; Wilcke, 2000). Soils are contaminated with PAHs mainly from atmospheric deposition from stationary sources (gas burning, industrial and municipal organic residues incineration, forest fires) and mobile sources (mainly from fuel fossil combustion for terrestrial transportation) (Mastral & Callen, 2000).

The presence of PAHs in soils has been found to be increasing in industrial and urban developments over the last few decades. Some studies have indicated that vehicle exhausts are major sources of PAHs in soils along with increased use of wastewater for irrigation of crops. Soil contamination by PAHs is considered to be a good indicator of the level of environmental pollution by human activities (Chung et al., 2008). PAHs from soil and water are possibly dangerous to human health because plant root uptake can result in bioconcentration (Samsoe et al., 2002).

The quality of air, water and soil are important for the production of vegetables and animals, and of course for humans as well. The occurrence of contaminants in the environment above certain levels may entail multiple negative consequences in the ecosystems as well as for the human food chain (Liu & Korenaga, 2001).

Our objectives were to investigate the occurrence of PAHs on semi rural terrains within Mexico City and identify the sources of these organic contaminants in crops, water for irrigation and soil in two areas (Tlahuac and Milpa Alta), which are considered important as aquifer recharge zones.
