**2. Material and methods**

The Metropolitan Zone of Mexico Valley (MZMV), comprises Mexico City which is considered a Megacity (Molina & Molina, 2004) located in a basin on the central Mexican plateau with a population around 20 million, 4 million vehicles, and 35,000 industries

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

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,

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

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

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

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

The Metropolitan Zone of Mexico Valley (MZMV), comprises Mexico City which is considered a Megacity (Molina & Molina, 2004) located in a basin on the central Mexican plateau with a population around 20 million, 4 million vehicles, and 35,000 industries

ecosystems as well as for the human food chain (Liu & Korenaga, 2001).

the health of organisms.

water, soil and food.

transportation) (Mastral & Callen, 2000).

bioconcentration (Samsoe et al., 2002).

aquifer recharge zones.

**2. Material and methods** 

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

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 according to standard methods of conservation.

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

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)

Presence of Polycyclic Aromatic Hydrocarbons

**0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%**

**3.2 Crops** 

(PAHs) in Semi-Rural Environment in Mexico City 219

combustion process such as anthracene, benzo(a)pyrene and others compounds. Some investigations have suggested a monitoring of wastewater a long term domestic wastewater irrigation to evaluate risks about this resource for agricultural terrains (Chung et al., 2008)

 **Tlahuac Milpa Alta** 

Fig. 2. Distribution of PAHs in water for irrigation in crops semi-rural terrains in Mexico City.

Finally, these areas are considered areas of recharge for aquifers but the PAHs found in the water do not represent a risk for crops (toxicity by translocation) and soils (degradation). Despite the high contamination in Mexico City, wastewater use is adequate for crops although some work has shown that wastewater is a potential source of PAHs

Our study focus in two crops of economical importance for Tlahuac and Milpa further the government has impulse with economical, technical and material resources for development of farmer. Cactus stem has high acceptation in population because is traditional food from prehispanic to present time while apple crop recently is accepted as attractive and rentable crop. Although there are several agriculture products that grow in these areas and around such as spinach, broccoli, amaranth, olive, ornamental flowers and etc (Grupo Produce, 2006). Human exposure to PAHs is 88–98% connected with food (meat, seed, vegetable, fruit and others). PAHs can penetrate food indirectly (from air or water) and directly, e.g. during smoking. Once these compounds are released into the atmosphere they can be transported away from their emission sources over long distances and/or deposited to the terrestrial and aquatic environment through dry and wet deposition. A major issue associated with the emission of these compounds is the zone of influence, which determines whether the possible source has predominately local impacts or contributes to regional or global

According to Rey-Salgueiro et al. (2008) the form of vegetable or fruit influenced the PAH concentration over the surface as they found that leaves and quasispherical fruits (grapes

contamination (Escobedo et al., 2000; Chung et al., 2008).

background levels (Rey-Salgueiro et al., 2008).

6 rings 5 rings 4 rings 3 rings 2 rings

anthracene (BaA), chrysene (Cry), benzo(b)fluoranthene (BbF), benzo(k) fluoranthene (BkF), indeno(1,2,3-cd)pyrene (Ind), dibenzo(ah+ac)anthracene (DaA), and benzo(ghi)perylene (Bghi). In general the detection limits were 0.01 mg/kg.

Quality control was carried out by analysis of fortified blanks and samples together with the performance of the GC. Recoveries were 80-95% except for naphthalene for which a value of 50% was obtained. Quantification of individual PAHs was made by an external standard method.
