**3.3 Soils**

According to the results we found higher values of PAHs in crops than soil. For the case of crops, the individual PAHs were slightly lower in wet than dry season; in the rainy season the irregular intensity and duration of rainfall cleans the atmospheric of suspended particles. In figure 4 the results show greater values in apple from Tlahuac and cactus stem from Milpa Alta in the wet season. In general, the compounds > 3 aromatic rings were

Presence of Polycyclic Aromatic Hydrocarbons

(PAHs) in Semi-Rural Environment in Mexico City 225

Fig. 5. Values of PAHs in soils from semi-rural terrains from Mexico City.

volatile compound.

as a source of contamination.

Nap not found in our analysis we supposed a loss in handling of sample due that is high

The most prominent source of PAHs in the urban environment is the incomplete combustion of biomass (such as vegetation) and fossil fuels (petroleum). Vehicular traffic (mainly diesel-powered) is considered to be the most significant contributor to the atmospheric PAH load within urban areas (Marr et al., 2004). Atmospheric deposition is the most common source of pollution in soil and it is expected that most combustion derived PAHs will be restricted to the top layer of the soil. Urban areas generally have high traffic density which results in heavy contamination of surface soils (Agarwal, 2009). For our study areas there are several asphalt ways that communicate with population areas, it considering

According Cram et al (2008), the high concentration of these contaminants in atmospheric is concentrate in South of Mexico City, mainly due to the direction of wind from North to South. As appreciate in figure 1 Tlahuac and Milpa are located in Southeast. These soils are considering as conservation, recharge of aquifer and recreational areas further gives other environmental services as retention, regulation and alleviates the deposition of contaminants derived of fossil fuels combustion that origins heavy metals and ethers, hydrocarbons, etc.

present in crops and soil in both season such as BaA, Cry, Fla, Pyr, DaA, BaP and BkF are dominant in apple while cactus stem. According García-Alonso et al. (2003) the presence of Fla and Bghi in the two areas may indicate a common vehicular emission source.

There are many factors influencing the distribution of different PAHs in the environment, such as physicochemical properties of PAHs, physicochemical properties of soil, sources of emission of PAHs, and photochemical degradation of atmospheric PAHs. Two to three ring PAHs are subject to atmospheric transport to remote areas and are considered ''multi-hop'' chemicals, while higher ring PAHs are associated with particles and undergo ''single hop'' transport behavior, and higher ring PAHs are prone to rapid deposition and retention close to source regions. So, PAHs may become fractionated from source regions to remote regions during atmospheric transport (Wang et al., 2010).

The concentrations of BaP in the soils is an indication of both pyrogenic and petrogenic sources of PAH pollution on the environment (Essumang et al., 2010) for both seasons. BaP is considered within permissible limits (2 mg/kg) for agricultural and residential land use according to Mexican regulation NOM 138-SEMARNAT/SS-2003. However, the levels of BaP surpassed the limit established by the Danish Environmental Protection Agency (0.1 mg/kg) and Canadian Council of Ministers of the Environment (0.26 mg/kg; Essumang et al., 2010). The BaP concentrations are considered to be a risk to human health due to its potential exposure to carcinogenic PAHs for those living in those areas. According other international regulations our soils has severe problems of contamination, where some prevent measurements have applied such as strict control of emissions of industries, vehicular park and matter combustion.

Within polycyclic aromatic hydrocarbons considered in Mexican regulation, the concentrations of DaA, BaA, BbF, BkF and Ind were lower than the permissible limit for agricultural land use according NOM-138-SEMARNAT/SS-2003 (2 mg/kg). These compounds do not represent a risk for human health in comparison to BaP.

In figure 5 we appreciated the distribution of PAHs in dry and wet season found it a relative high concentration of chrysene in wet season in both sites. In general dominant PAHs for dry season were BaA, Cry, BaP, Fla, Ind and BkF with range of 0.5 to 3.5 mg/kg; Milpa Alta has slightly high BaA, BaP and Fla and Tlahuac were Cry and DaA. In wet season dominant PAHs were Cry and BaP mainly, with range of 1 to 9.5 mg/kg; Tlahuac has high concentration of Cry and BaA and Milpa Alta were Fla, BaP, Bghi, BbF, BkF, Ind and DaA. The variability of individual PAHs is key to recognize the possible source of contamination in the semi-rural area further the environmental condition defines distribution and concentration of contaminants in the soil and other media.

The presence of 4-6 aromatic rings compounds in soils in Mexico City was similar to results from a survey in soil described by Wang et al. (2010) in urban soils from North China. In semirural soil of Mexico the compounds predominant were Cry, BaP and BaA different at found it in North of China. For dry season we found similar percent of four (50 % approximately); five (25 % approximately) and six (10 %) ring aromatic compounds in Tlahuac and Milpa Alta (Figure 6). In wet season, there are significant changes over aromatic compounds, in Tlahuac has 90 % of four ring aromatic compounds while others compounds diminished to 8 % (5 rings compounds) and 2 % (6 ring compounds). So, Milpa Alta shows similar percent of aromatic compounds than dry season.

present in crops and soil in both season such as BaA, Cry, Fla, Pyr, DaA, BaP and BkF are dominant in apple while cactus stem. According García-Alonso et al. (2003) the presence of

There are many factors influencing the distribution of different PAHs in the environment, such as physicochemical properties of PAHs, physicochemical properties of soil, sources of emission of PAHs, and photochemical degradation of atmospheric PAHs. Two to three ring PAHs are subject to atmospheric transport to remote areas and are considered ''multi-hop'' chemicals, while higher ring PAHs are associated with particles and undergo ''single hop'' transport behavior, and higher ring PAHs are prone to rapid deposition and retention close to source regions. So, PAHs may become fractionated from source regions to remote regions

The concentrations of BaP in the soils is an indication of both pyrogenic and petrogenic sources of PAH pollution on the environment (Essumang et al., 2010) for both seasons. BaP is considered within permissible limits (2 mg/kg) for agricultural and residential land use according to Mexican regulation NOM 138-SEMARNAT/SS-2003. However, the levels of BaP surpassed the limit established by the Danish Environmental Protection Agency (0.1 mg/kg) and Canadian Council of Ministers of the Environment (0.26 mg/kg; Essumang et al., 2010). The BaP concentrations are considered to be a risk to human health due to its potential exposure to carcinogenic PAHs for those living in those areas. According other international regulations our soils has severe problems of contamination, where some prevent measurements have applied such as strict control of emissions of industries,

Within polycyclic aromatic hydrocarbons considered in Mexican regulation, the concentrations of DaA, BaA, BbF, BkF and Ind were lower than the permissible limit for agricultural land use according NOM-138-SEMARNAT/SS-2003 (2 mg/kg). These

In figure 5 we appreciated the distribution of PAHs in dry and wet season found it a relative high concentration of chrysene in wet season in both sites. In general dominant PAHs for dry season were BaA, Cry, BaP, Fla, Ind and BkF with range of 0.5 to 3.5 mg/kg; Milpa Alta has slightly high BaA, BaP and Fla and Tlahuac were Cry and DaA. In wet season dominant PAHs were Cry and BaP mainly, with range of 1 to 9.5 mg/kg; Tlahuac has high concentration of Cry and BaA and Milpa Alta were Fla, BaP, Bghi, BbF, BkF, Ind and DaA. The variability of individual PAHs is key to recognize the possible source of contamination in the semi-rural area further the environmental condition defines distribution and

The presence of 4-6 aromatic rings compounds in soils in Mexico City was similar to results from a survey in soil described by Wang et al. (2010) in urban soils from North China. In semirural soil of Mexico the compounds predominant were Cry, BaP and BaA different at found it in North of China. For dry season we found similar percent of four (50 % approximately); five (25 % approximately) and six (10 %) ring aromatic compounds in Tlahuac and Milpa Alta (Figure 6). In wet season, there are significant changes over aromatic compounds, in Tlahuac has 90 % of four ring aromatic compounds while others compounds diminished to 8 % (5 rings compounds) and 2 % (6 ring compounds). So, Milpa

compounds do not represent a risk for human health in comparison to BaP.

concentration of contaminants in the soil and other media.

Alta shows similar percent of aromatic compounds than dry season.

Fla and Bghi in the two areas may indicate a common vehicular emission source.

during atmospheric transport (Wang et al., 2010).

vehicular park and matter combustion.

Fig. 5. Values of PAHs in soils from semi-rural terrains from Mexico City.

Nap not found in our analysis we supposed a loss in handling of sample due that is high volatile compound.

The most prominent source of PAHs in the urban environment is the incomplete combustion of biomass (such as vegetation) and fossil fuels (petroleum). Vehicular traffic (mainly diesel-powered) is considered to be the most significant contributor to the atmospheric PAH load within urban areas (Marr et al., 2004). Atmospheric deposition is the most common source of pollution in soil and it is expected that most combustion derived PAHs will be restricted to the top layer of the soil. Urban areas generally have high traffic density which results in heavy contamination of surface soils (Agarwal, 2009). For our study areas there are several asphalt ways that communicate with population areas, it considering as a source of contamination.

According Cram et al (2008), the high concentration of these contaminants in atmospheric is concentrate in South of Mexico City, mainly due to the direction of wind from North to South. As appreciate in figure 1 Tlahuac and Milpa are located in Southeast. These soils are considering as conservation, recharge of aquifer and recreational areas further gives other environmental services as retention, regulation and alleviates the deposition of contaminants derived of fossil fuels combustion that origins heavy metals and ethers, hydrocarbons, etc.

Presence of Polycyclic Aromatic Hydrocarbons

and recharge of aquifers with low density of population.

Compounds Soils (µg/kg)

Tláhuac Milpa Alta Fangcun Rural

study, 2009; 2. Chen et al., 2005; 3. Agarwal, 2009; 4. Nam et al, 2003.

employ for irrigation for crops or human and animal consumption

Ingestion of particles or food on contaminate soils

Inhalation of smallest particles

(Sabroso & Pastor, 2004).

**3.4 Sources of PAHs in semirural sites** 

The importance of contamination of soil is due serious risks on population health due:

Direct contact with skin in workers of field or direct markets of commercialization

Further the contamination of water resource in form superficial or underground that are

Government hopes that in medium time the contaminants reduce significantly in favour the environment and health population of Mexico by programs of improvement of quality air.

We used the Flu/(Flu+Pyr), Ant/(Ant+Phe), Ind/(Ind+Bghi), BaP/(BaP+Cry), BkF/Bghi and BaP/Bghi ratios and the majority of samples fell into the section identifying pyrogenic sources (fossil fuel, grass and garbage combustion) (Table 3). This is logical considering the

(PAHs) in Semi-Rural Environment in Mexico City 227

industrialisation (Marr et al, 2004; Maliszewska-Kordybach et al., 2009). The spatial distribution of soils contaminated with PAHs reflected that rural/forested/ recreation areas decreasing along the South to North of Mexico City. Due in North there high concentration of industries and urban areas while in South of City the soils are considered conservation

Nap --- --- 131 --- 78.5 Acy --- --- 50 120 317 41.5

Ace 60 180 9 198 298 33.7 36.0 Flu --- 90 40 48 152 37.6 9.3 Phe 150 330 281 39 259 141 254 Ant --- --- 34 25 135 33.7 9.8 Fla 500 740 583 101 599 353 8.3 Pyr 30 250 492 45 363 317 581 BaA 1620 2180 232 47 521 284 244 Cry 5400 3150 693 23 332 267 319 BbF 270 720 267 36 540 431 313 BkF 270 630 101 37 661 138 139 BaP 920 2030 136 35 461 294 249 Ind 250 750 47 --- 621 248 145 DaA 420 380 42 --- 623 120 21 Bghi 380 1150 70 --- 1618 221 239 Total 10270 12580 3077 885 7501 2834 3390 Table 2. Comparison of PAH concentrations in rural soils near to urban areas. Note: 1. This

Mexico, D.F.1 China2 India3, Delhi Korea4 Francia

zone. Highway Rural

zone

Park

Fig. 6. Distribution of PAHs in soils according to its number of aromatic rings in Tlahuac and Milpa Alta.

Important aspect for these soils in Tlahuac and Milpa are addition of farmyard manure and compost where we appreciated low values of PAHs in comparison crops (Cai et al., 2007). The organic matter plays a role to catch contaminants where can be stabilized within its structure. A long term, organic matter is long reservoirs of several contaminants which must be practice good management of this parameter of soil.

The presence of low molecular weight PAHs (2-3 aromatic rings) is indicative of petroleum (fossil fuel combustion) while high molecular weight PAHs (> 4 aromatic rings) are more likely to be derived from organic material combustion (Ma et al., 2005). There are likely to be multiple contamination sources in vegetables and garbage from these conservation terrains.

The contamination in Mexico City is high due to the environmental conditions, for example high population density and geography (such as altitude and temperature), where atmospheric deposition (diffuse and point sources) leads to high concentrations of PAHs. Similar contamination situations have been reported in Chinese soils (Ping et al., 2007).

In table 2, we appreciated high values of individual PAHs than others areas where compounds with high molecular weight surpass values of others studies of different countries. We supposed that high concentrations are due to pull of contaminants to South of City together certain activities of specific areas as possible explication.

Maliszewska-Kordybach (1996) suggested a soil contamination classification system based on ΣPAH16 as follows: noncontaminated soil (<200 ng/g), weakly contaminated soil (200– 600 ng/g), contaminated soil (600–1000 ng/g) and heavily contaminated soil (>1000 ng/g). According to this classification system, all samples in this study were heavily contaminated. This discrepancy among European and Mexican regulations are based in diverse criteria of analytical techniques mainly.

Studies made in past years for Mexico City has showed differences in abundance, type and distribution of PAHs according landscape type and territories of intensive urbanisation/

Milpa Alta Dry season

Fig. 6. Distribution of PAHs in soils according to its number of aromatic rings in Tlahuac

Important aspect for these soils in Tlahuac and Milpa are addition of farmyard manure and compost where we appreciated low values of PAHs in comparison crops (Cai et al., 2007). The organic matter plays a role to catch contaminants where can be stabilized within its structure. A long term, organic matter is long reservoirs of several contaminants which must

The presence of low molecular weight PAHs (2-3 aromatic rings) is indicative of petroleum (fossil fuel combustion) while high molecular weight PAHs (> 4 aromatic rings) are more likely to be derived from organic material combustion (Ma et al., 2005). There are likely to be multiple contamination sources in vegetables and garbage from these conservation terrains. The contamination in Mexico City is high due to the environmental conditions, for example high population density and geography (such as altitude and temperature), where atmospheric deposition (diffuse and point sources) leads to high concentrations of PAHs. Similar contamination situations have been reported in Chinese soils (Ping et al., 2007).

In table 2, we appreciated high values of individual PAHs than others areas where compounds with high molecular weight surpass values of others studies of different countries. We supposed that high concentrations are due to pull of contaminants to South of

Maliszewska-Kordybach (1996) suggested a soil contamination classification system based on ΣPAH16 as follows: noncontaminated soil (<200 ng/g), weakly contaminated soil (200– 600 ng/g), contaminated soil (600–1000 ng/g) and heavily contaminated soil (>1000 ng/g). According to this classification system, all samples in this study were heavily contaminated. This discrepancy among European and Mexican regulations are based in diverse criteria of

Studies made in past years for Mexico City has showed differences in abundance, type and distribution of PAHs according landscape type and territories of intensive urbanisation/

City together certain activities of specific areas as possible explication.

 Tlahuac Wet season

Milpa Alta Wet season

6 rings 5 rings 4 rings 3 rings 2 rings

0%

and Milpa Alta.

analytical techniques mainly.

 Tlahuac Dry season

be practice good management of this parameter of soil.

20%

40%

60%

80%

100%

industrialisation (Marr et al, 2004; Maliszewska-Kordybach et al., 2009). The spatial distribution of soils contaminated with PAHs reflected that rural/forested/ recreation areas decreasing along the South to North of Mexico City. Due in North there high concentration of industries and urban areas while in South of City the soils are considered conservation and recharge of aquifers with low density of population.


Table 2. Comparison of PAH concentrations in rural soils near to urban areas. Note: 1. This study, 2009; 2. Chen et al., 2005; 3. Agarwal, 2009; 4. Nam et al, 2003.

The importance of contamination of soil is due serious risks on population health due:


Further the contamination of water resource in form superficial or underground that are employ for irrigation for crops or human and animal consumption

Government hopes that in medium time the contaminants reduce significantly in favour the environment and health population of Mexico by programs of improvement of quality air.
