**3.1 Water**

The occurrence of low weight molecular compounds in water is due to wet and dry deposition of particles from the atmospheric that contain adsorbed PAHs such as naphthalene and phenanthrene. The probable source of these compounds is organic matter combustion to low temperatures (Nagy et al., 2007); relatively high concentrations in comparison to high molecular weight compounds can be explained by the relative solubility of the PAHs. This pattern of concentrations has been seen before (Bishnoi et al., 2005; Ma et al., 2005; Chung et al., 2008)

The mean value of total PAHs in Tlahuac and Milpa Alta water samples for 2008 were both 0.04 µg/L. This did not exceed the permissible value of 1.0 µg/L proposed by European Union for irrigation of crops.

In 2009, the values were lower than detection limits. The PAHs were concentrated in suspended particles or in sediment particles that accumulated in the reservoirs with high content of fine sediments and organic matter.

Within results in water samples, we observed that values changed according to the wet and dry season. In the dry season, concentration and number of PAHs were higher than in the wet season, probably due to a dissolution effect where concentration reduced. There is a great interaction of contaminants between atmospheric, water superficial bodies and soils. The scarcity of water in some months of year for crops is limiting in these rural terrains

The use of wastewater for this zone does not present a problem of contamination for crops and soils, as there is no association between the PAHs and type of water. In some areas of China there a direct association between residual water and contamination and degradation of arable soils and an associated drop in quality of crops over the medium to long term (Cai et al., 2007).

In the figure 2 we appreciated the distribution of PAHs according number of aromatic rings, where in Tlahuac and Milpa has similar percentage of 3, 4 and 6 aromatic rings, the high values were in 3 (33% in both sites) and 4 (35% approximately in both sites). Similar studies have determined that those compounds are derived of combustion of organic matter (vegetation and fuels), so PAHs occurrence in water in both zones is by combustion mainly via deposition for case of Mexico City.

The occurrence of PAHs in water generally is associated by suspended particles or sediments due the high affinity to organic carbon further low solubility of many aromatic compounds. The main source of contamination of PAHs in water is by deposition of airborne and urban storm water runoff. Many PAHs found in water are derived of

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

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

The occurrence of low weight molecular compounds in water is due to wet and dry deposition of particles from the atmospheric that contain adsorbed PAHs such as naphthalene and phenanthrene. The probable source of these compounds is organic matter combustion to low temperatures (Nagy et al., 2007); relatively high concentrations in comparison to high molecular weight compounds can be explained by the relative solubility of the PAHs. This pattern of concentrations has been seen before (Bishnoi et al., 2005; Ma et

The mean value of total PAHs in Tlahuac and Milpa Alta water samples for 2008 were both 0.04 µg/L. This did not exceed the permissible value of 1.0 µg/L proposed by European

In 2009, the values were lower than detection limits. The PAHs were concentrated in suspended particles or in sediment particles that accumulated in the reservoirs with high

Within results in water samples, we observed that values changed according to the wet and dry season. In the dry season, concentration and number of PAHs were higher than in the wet season, probably due to a dissolution effect where concentration reduced. There is a great interaction of contaminants between atmospheric, water superficial bodies and soils. The scarcity of water in some months of year for crops is limiting in these rural terrains

The use of wastewater for this zone does not present a problem of contamination for crops and soils, as there is no association between the PAHs and type of water. In some areas of China there a direct association between residual water and contamination and degradation of arable soils and an associated drop in quality of crops over the medium to long term (Cai et al., 2007). In the figure 2 we appreciated the distribution of PAHs according number of aromatic rings, where in Tlahuac and Milpa has similar percentage of 3, 4 and 6 aromatic rings, the high values were in 3 (33% in both sites) and 4 (35% approximately in both sites). Similar studies have determined that those compounds are derived of combustion of organic matter (vegetation and fuels), so PAHs occurrence in water in both zones is by combustion mainly

The occurrence of PAHs in water generally is associated by suspended particles or sediments due the high affinity to organic carbon further low solubility of many aromatic compounds. The main source of contamination of PAHs in water is by deposition of airborne and urban storm water runoff. Many PAHs found in water are derived of

(Bghi). In general the detection limits were 0.01 mg/kg.

standard method.

**3.1 Water** 

**3. Results and discussion** 

al., 2005; Chung et al., 2008)

Union for irrigation of crops.

content of fine sediments and organic matter.

via deposition for case of Mexico City.

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)

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