The city of Salamanca, Mexico is well known for its history of environmental pollution, as it is a center of industrial activity at the national level. Such activities, when not carried out based on regulatory standards, or when this does not exist, unfortunately leads to problems associated with the introduction of dangerous substances into the environment, with the risks involved. One of the most worrying cases, which so far has no definitive solution, is the presence of environmental liabilities within the vicinity of the former Tekchem Industrial Unit, which, when in operation, manufactured a wide range of OCPs and other synthetic chemical compounds. The environmental liabilities generated were the result of an accumulation of several years due to inconveniences in terms of waste disposal. Previous studies have characterized the polluting substances in the site, mainly reporting the presence of OCPs, which constitute 70% of the polluting mass (Beltrán Hernández et al. 2019; SEMARNAT 2018).
In this study, levels of γ-HCH, heptachlor, heptachlor epoxide, aldrin, endrin, as well as DDT and its metabolites were detected and quantified in soil and in school-age children in the city of Salamanca. Said compounds correspond to those that have been identified in the environmental liabilities of Tekchem (SEMARNAT 2018). DDT and its metabolites were the compounds that showed the highest levels of concentration in soil, followed by aldrin and heptachlor epoxide (Table 2). Comparing the total DDT concentration obtained in our study with the Canadian Environmental Quality Guideline, which establishes a maximum value of 700 ng/g of total DDT for residential soil (Canadian Council of Ministers of the Environment 1999), we find that the level did not exceed this value. The California Human Health Screening Levels guide, on the other hand, establishes a maximum value of 1900 ng/g for 4,4’-DDT, 2000 ng for 4,4’-DDE, and 2300 ng/g for 4,4’-DDD (California Department of Toxic Substances Control 2020), which were not exceeded in the soil samples analyzed. In the case of aldrin, the same guide establishes a maximum value of 3300 ng/g for residential soil, so that the levels obtained in this study did not exceed this limit.
In our country, monitoring studies have been carried out on OCPs in soil, mainly in agricultural areas and in those with a high prevalence of vector-borne diseases such as malaria. The total DDT concentration obtained in this study was higher than the levels reported in urban soil of San Luis Potosí (6.10 ng/g) (Perez-Vazquez et al. 2015), and in the Mexicali Valley (20 ng/g) and the Yaqui Valley (3.6 ng/g) in northwestern Mexico (Sánchez-Osorio et al. 2017). In this last work mentioned, lower heptachlor epoxide levels (Mexicali Valley: 0.013 ng/g; Yaqui Valley: 0.022 ng/g) (Sánchez-Osorio et al. 2017) were also reported compared to ours. Another study performed in soil from communities from Tabasco reported lower levels of total DDT compared to ours (median concentrations ranged from 4 to 38 ng/g), except for the community Cardenas whose level was higher (91 ng/g) (Torres-Dosal et al. 2012). In another monitoring carried out in urban soils of Chihuahua, the range of total DDT values (2 – 141.7 ng/g) was lower than that of our study (Díaz-Barriga Martínez et al. 2012). On the other hand, Orta-García et al. (2016) found higher median levels of total DDT (79.5 ng/g) compared to the present study, in urban soil from Monterrey. Likewise, the levels within the resulting range for total DDT in soil from various communities in Chiapas (2 – 26,980 ng/g) exceeded the maximum value obtained for total DDT in our study (Martínez-Salinas et al. 2011).
Cantu-Soto et al. (2011) reported ranges with very high levels of OCPs with respect to those obtained in the present work, in residential soils of the Yaqui Valley (4,4’-DDT: <LOD - 679,700 ng/g; 4,4’-DDE: <LOD - 621,300 ng/g; 4,4’-DDD: <LOD - 197,300 ng/g; aldrin: <LOD - 25,800 ng/g) and the Mayo Valley (4,4’-DDT: <LOD - 301,200 ng/g; 4,4’-DDE: <LOD - 226,300 ng/g; 4,4’-DDD: <LOD - 39,300 ng/g; aldrin: <LOD - 74,000 ng/g), in southern Sonora, Mexico.
Comparing with soil monitoring for OCPs in other countries, we found that the ranges obtained in this study had higher levels than the resulting maximum concentrations in urban soils in Pristina, Serbia (DDT: 2.41 - 18.8 ng/g; DDE: 1.19 - 30.5 ng/g; DDD: 0.97 - 8.38 ng/g; aldrin: 1.94 - 3.28 ng/g) (Gulan et al. 2017). Likewise, the maximum total DDT value from our study was higher than that reported in agricultural soils from Wuhan, China (<LOD - 1198 ng/g), while on the other hand aldrin range resulted in higher values than our maximum value obtained (<LOD - 21.56 ng/g) (Zhou et al. 2013). Another study performed in agricultural soil from Shanghai, China, reported maximum values for total DDT and heptachlor epoxide (0.44 – 247.45 ng/g and <LOD – 4.78 ng/g, respectively) lower than those from our study, while for aldrin was obtained a similar range (<LOD – 6.62 ng/g) (Jiang et al. 2009).
With respect to the levels of OCPs in the infant population, the compounds that resulted in the highest concentrations were 4,4’-DDT and its metabolites, as well as heptachlor and heptachlor epoxide (Table 3). By comparing our results with the median concentrations obtained in the German Environmental Survey for Children and Adolescents (GerEs) 2014-2017 for 4,4’-DDT (1.94 ng/g lipid), 4,4’-DDE (26.7 ng/g lipid) and 4,4’-DDD (1.88 ng/g lipid) (Bandow et al. 2020), we can see that our levels exceed the previous ones by 10, 2.6 and 8 times, respectively. On the other hand, the Fourth National Report on Human Exposure to Environmental Chemicals (population aged 12 to 19) reports heptachlor epoxide and 4,4’-DDT as undetectable, while for 4,4’-DDE the median concentration was 93.6 ng/g lipid (CDC 2009), slightly exceeding our found median value.
As well as in soil, in our country monitoring has also been carried out in children exposed to OCPs. Flores-Ramírez et al. (2017) reported a median value of 4,4’-DDE slightly lower than ours (58.77 ng/g lipid), in children living near contaminated sites in Mexico. Another study in children from pollution hot spots in Mexico showed median levels of DDE (387 ng/g lipid) and DDT (189 ng/g lipid) higher than ours, while heptachlor epoxide was not detected (Trejo-Acevedo et al. 2009). Studies conducted in children in malaria-endemic areas of the country show levels of DDT and its metabolites that far exceed those in this study. In Chiapas and Oaxaca, in southeastern Mexico, median levels of total DDT ranged from 11,353.8 ng/g lipid to 34,189 ng/g lipid between three assessed communities (Pérez-Maldonado et al. 2013). In Quintana Roo, in southern Mexico, median levels of DDT of 5,646 ng/g lipid and DDE of 39,934 ng/g lipid were found, while heptachlor epoxide was not detected (Trejo-Acevedo et al. 2012). In children from Chihuahua, ranges with higher levels than those in our study were reported, both for DDT (711 - 68, 669 ng/g lipid) and for DDE (271 - 170,596 ng/g lipid) (Díaz-Barriga Martínez et al. 2012).
Regarding monitoring in other countries, OCPs levels in children from our study were higher than those found in children in South Korea (heptachlor: <LOD; heptachlor epoxide: <LOD; 4,4’-DDT: 3.2 ng/g lipid; 4,4’-DDE: 44.7 ng/g lipid; 4,4’-DDD: <LOD) (Park et al. 2016). The ranges we obtained in this study for 4,4’-DDT and 4,4’-DDE include values that exceed the maximum concentrations reported in children from Canada (4,4’-DDT: 2.3 - 62.7 ng/g lipid; 4,4’-DDE: 5.6 – 1864.4 ng/g lipid) (Turgeon O’Brien et al. 2012). On the other hand, Windham et al. (2010) reported a median 4,4’-DDE value of 101.5 ng/g lipid in US girls, which is slightly higher than that obtained in our study.
The DDT/DDE ratio for soil resulted in a median value of 1.04, while in children it was 1.72. According to Tavares et al. (1999), a value of this ratio ≥ 1 indicates the recent incorporation of DDT into the environment, which is usually due to recent applications. The half-life time for DDT in terms of its persistence in the environment is 2 to 15 years, depending on the factors that are present in the site influencing its degradation (ATSDR 2019; Jayaraj et al. 2016). According to the previous studies performed both in soil and children, the concentrations of OCPs obtained in our study were lower, in general, than those reported in endemic areas of malaria, where large amounts of these compounds were applied in the past, mainly DDT (Díaz-Barriga et al. 2003). However, in the case of OCPs in blood, they are higher compared with baseline levels from general population data, such as those from the GerEs (Bandow et al. 2020). In the case of Tekchem, DDT was discontinued in 1997, however, as mentioned above, to date residues are present within the facilities, which are likely dispersing to the soils of the urban area of Salamanca, and this would explain the value obtained for DDT/DDE ratios in both soil and children, indicating both present and past exposure to DDT (Tables 2 and 3).
In our study, heptachlor was detected in 100% of children, however, in soil it was not detected significantly (less than 50% of the samples). It is important to mention that very few studies have reported levels of heptachlor and heptachlor epoxide in the human population, and in most cases, these compounds are not detected. Morgan et al. (2014) quantified heptachlor in various environmental media such as soil, dust, outdoor air and indoor air, finding the highest levels in air, mainly in indoor environments. The same study determined that the predominant route of exposure for this compound in preschool children was inhalation of air both outdoors and indoors (98%) (Morgan et al. 2014). This could explain our findings, in terms that in the case of heptachlor the soil is not the main route of exposure.
About the spatial distribution of the total sum of OCPs quantified in soil (Figure 1), it is noted that the highest concentrations were determined in the vicinity of Tekchem, so in this case there is a well identified source for the emission of these substances into the environment, and thus for human exposure. However, it is important to consider the possible influence of other OCPs sources that may be contributing to the concentrations of OCPs found in Salamanca soil. On the other hand, this type of compounds have the characteristic that they can be transported by the wind and travel great distances, being later in places distant to their origin (Heidelore Fiedler 2002; Sparling 2016). In the case of the total sum of OCPs in blood, the spatial distribution is interesting (Figure 2), since the highest concentrations were mainly identified in the northern area of Salamanca, however, there are also high concentration points in the vicinity of Tekchem. This variability in distribution could be explained by the different routes of exposure through which OCPs can be incorporated into the infant population. Although outdoor soil is one of the most important routes of exposure to OCPs (Herrera-Portugal et al. 2005), other routes such as food, breast milk and indoor soil and dust can also contribute significantly, which were not considered in this study. For example, it has been established that high concentrations of OCPs in indoor spaces, and longer residence times, can increase exposure to these pollutants up to 1000 times compared to outdoor exposure (Hwang et al. 2008). Another important point to consider in our study is the mobility of children within the city, which could also explain the variability in the spatial distribution of OCPs. Finally, as already mentioned, although the compounds studied are subject to either prohibition or restriction regulations (Stockholm Convention), unfortunately to date they are still dispersed globally and ubiquitously, regardless of their origin (Sparling 2016).
We cannot know specific risks to the health of children with the results obtained in our study, because cut-off levels have not been established for the compounds studied. However, various health effects associated with exposure to OCPs have already been reported in infant populations (Segal and Giudice 2019; Vrijheid et al. 2016).
Our work has several limitations, such as sample size, and the fact that other exposure routes to quantify compounds were not considered. In addition, due to the cross-sectional nature of the study, there may be biases in terms of spatio-temporal variability of the concentrations of the compounds analyzed, both in soil and in children.