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Environmental Pollution 158 (2010) 749–755

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Environmental Pollution

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Organochlorine pesticides in soils of Mexico and the potential forsoil–air exchange

Fiona Wong a,b, Henry A. Alegria c, Terry F. Bidleman a,*

a Centre for Atmospheric Research Experiments, Science and Technology Branch, Environment Canada, 6248 Eighth Line, Egbert, Ontario L01 1N0, Canadab Department of Chemistry, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canadac Department of Environmental Science, Policy and Geography, University of South Florida St. Petersburg, 140 7th Ave. S., St. Petersburg, FL 33701, USA

Chemical profiles of residues and soil–air fugacities are used to assessMexico.

the potential of soil as a source of organochlorine pesticides to the air of

a r t i c l e i n f o

Article history:Received 8 May 2009Received in revised form14 September 2009Accepted 5 October 2009

Keywords:Organochlorine pesticidesMexicoSoilChiralFugacity

* Corresponding author. Tel.: þ1 705 458 3322; faxE-mail address: terry.bidleman@ec.gc.ca (T.F. Bidle

0269-7491/$ – see front matter Crown Copyright � 2doi:10.1016/j.envpol.2009.10.013

a b s t r a c t

The spatial distribution of organochlorine pesticides (OCs) in soils and their potential for soil–airexchange was examined. The most prominent OCs were the DDTs (Geometric Mean, GM ¼ 1.6 ng g�1),endosulfans (0.16 ng g�1), and toxaphenes (0.64 ng g�1). DDTs in soils of southern Mexico showed freshersignatures with higher FDDTe ¼ p,p0-DDT/(p,p0-DDT þ p,p0-DDE) and more racemic o,p0-DDT, while thesignatures in the central and northern part of Mexico were more indicative of aged residues. Soil–airfugacity fractions showed that some soils are net recipients of DDTs from the atmosphere, while othersoils are net sources. Toxaphene profiles in soils and air showed depletion of Parlar 39 and 42 whichsuggests that soil is the source to the atmosphere. Endosulfan was undergoing net deposition at mostsites as it is a currently used pesticide. Other OCs showed wide variability in fugacity, suggesting a mix ofnet deposition and volatilization.

Crown Copyright � 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Among the Latin American countries, Mexico was the largestconsumer of organochlorine pesticides (OCs) for sanitary andagricultural purposes (Li and Macdonald, 2005; Lopez-Carrilloet al., 1996). Even though the use of dichlorodiphenyltrichloro-ethane (DDT) was stopped in 2000 under the North AmericanRegional Action Plan (NACEC, 2003, 1997a,b), air samplings con-ducted between 2002 and 2005 have shown that DDT concentra-tions in the air of southern Mexico were two orders of magnitudeabove those in the North America Great Lakes region (Wong et al.,2009a; Alegria et al., 2008, 2006). Concentrations of DDT, rangingfrom hundreds to thousands of ng g�1 dry weight, were reported inresidential soils of a highly exposed community in Chiapas, thesouthernmost state of Mexico as well as a region where malaria hashistorically been endemic (Wong et al., 2008; Herrera-Portugalet al., 2005). As the use of most OCs has been restricted, the role ofsecondary emission from soils may become an important source tothe atmosphere. Our earlier studies have shown that soils insouthern Mexico serve as a source of toxaphene and DDT to the air

: þ1 705 458 3301.man).

009 Published by Elsevier Ltd. All

in some areas and as a sink in others (Wong et al., 2008). To ourknowledge, OC data have been published only for soils of southernMexico (Wong et al., 2008; Herrera-Portugal et al., 2005), and someagricultural soils in central Mexico (Waliszewski et al., 2004). Thisstudy was conducted to examine the spatial distribution of OCs inrural, urban and agricultural soils of Mexico and to investigate thenet direction of soil–air exchange by coupling soil residue data withair concentrations from colocated samplers (Wong et al., 2009a,2008) using the fugacity approach.

2. Methods

2.1. Sample collection and analysis

Soil samples were collected at 18 sites across 9 states of Mexico during 2005. Thesoils came from various land use types, i.e. urban, agricultural and rural (no agri-cultural activity and away from urban centres). Each sample was a composite of 3–6individual soil cores collected at the 0–5 cm depth. Samples were sieved througha 2-mm mesh and stored at �20 �C until analysis. Supplementary information 1 (SI-1) shows a map of the sampling sites and description of each site is detailed in SI-2.Soil samples (30 g, wet weight) were fortified with surrogates prior to extraction.The surrogates were 10 ng each of [2H6]-a-hexachlorocyclohexane, [13C10]-hepta-chlor exo-epoxide, [13C10]-trans-nonachlor, [13C12]-dieldrin, and 50 ng [2H8]-p,p0-DDT. The spiked soils were ground in a glass mortar, mixed with 15 g granularanhydrous sodium sulfate (baked at 450�), placed in precleaned ceramic thimblesand extracted using Soxhlet apparatus with 400 mL of dichloromethane (DCM) for18–22 h. The extracts then were concentrated to 1 mL and solvent exchanged to

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F. Wong et al. / Environmental Pollution 158 (2010) 749–755750

isooctane, passed through a column packed with 3 g neutral alumina (0.063–0.30 mm grain size, baked at 450 �C and deactivated by adding 6% deionized water,and eluted with 35 mL of 20% DCM in hexane). The extracts were finally concen-trated to 1 mL and solvent exchange to isooctane for analysis. Target pesticides weremeasured using capillary gas chromatography–electron capture negative ion massspectrometry (GC–ECNI-MS), on an Agilent 6890 GC–5973 MSD. Target OCs werehexachlorocyclohexanes (a-HCH, b-HCH, g-HCH, d-HCH), trans-chlordane (TC), cis-chlordane (CC), trans-nonachlor (TN), aldrin, dieldrin (DIEL), heptachlor (HEPT),heptachlor exo-epoxide (HEPX), endosulfans (ENDO I, ENDO II, endosulfan sulfate-ESUL), DDTs (p,p0-DDE, o,p0-DDE, o,p0-DDD, p,p0-DDD, o,p0-DDT, p,p0-DDT) andtoxaphenes. For toxaphene analysis, extracts were further cleanup by vortex mixingwith 1 mL of 15% fuming sulfuric acid and 2 mL of petroleum ether, centrifuging andwashing the solvent layer with deionized water. The extracts were finally blowndown to 150 mL. Toxaphenes were quantified as technical toxaphenes, i.e. sum of 7-Cl, 8-Cl and 9-Cl homologs. Ten individual peaks were quantified vs. Parlar conge-ners 21, 26, 32, 39, 40, 41, 42, 44þ, 50, 63 (44 þ refers to Parlar 44 and unidentifiedoctachlorobornanes which co-elute with 44). Details of the instrumental operatingconditions, ions monitored and sources of chemical standards are reported in Ale-gria et al. (2008).

Chiral analysis was performed after the extracts had undergone sulfuric acidcleanup. Enantiomer separations for TC and CC were done on a primary column,BetaDEX-120 (20% permethylated b-cyclodextrin in polydimethylsiloxane), withconfirmation using a secondary column, BGB-172 (30% tert-butyldimethyl-silylatedb-cyclodextrin). Details of analytical procedures are given in Kurt-Karakus et al.(2005). Results of enantiomer separations were expressed as enantiomer fraction(EF), defined as the peak areas of the (þ)/[(þ)þ (�)] enantiomers. EF¼0.500 indicatesthat the chemical is racemic, whereas EF s 0.500 indicates non-racemic. The averagedifference in EFs determined by the two chiral columns was 1.5% for TC (n ¼ 24) and1.1% for CC (n ¼ 6). Similar comparisons for a larger set of soil samples are given byKurt-Karakus et al. (2005). Only the BGB-172 column was used to separate enantio-mers of o,p0-DDT. Results were also expressed as deviation from racemic, whereDEVrac ¼ the absolute value of (0.500–EF) (Kurt-Karakus et al., 2005).

A separate portion of each soil sample was dried at 105 �C for 48 h to determinethe moisture content. Organic carbon content was determined by combustion andmeasurement of evolved CO2 after acidification to remove carbonates (ChemisarLaboratories, Guelph, ON, Canada).

OCs in air were measured at colocated stations using passive air samplers whichhad been deployed for three months over one year. Annual mean air concentrationwas used to calculate the soil–air fugacity. Soil samples were taken at the beginningof the passive air sampling campaign. Description of the air sampling stations,methods and results are given in Wong et al. (2009a).

2.2. Quality control

Soxhlet thimbles containing sodium sulfate (15 g) were extracted as blanks(n¼ 7) and the extracts underwent the same procedures as the soil samples. Limit ofdetection (LOD) was defined as mean blank þ 3 times the standard deviation. Ifa chemical was not found in the blanks, LOD was defined as instrumental detectionlimit, which was estimated by injecting low concentrations of target analytes untila small peak at w3:1 signal:noise ratio was obtained. LODs were expressed in ng g�1

considering 30 g of soil extracted and a final sample volume of 1 mL, except fortoxaphenes, for which the sample volume was 0.15 mL. LOD for individual chemicalare listed in SI-3. Half of LOD was used in statistical calculations when the targetchemical was below LOD. Recovery percentages for the fortified surrogates were(n ¼ 51): [2H6]-a-HCH: 91 � 16%; [13C10]-HEPX: 104 � 36%; [13C10]-TN: 90 � 12%;[13C12]-DIEL: 99 � 28%; [2H8]-p,p0-DDT: 82 � 15%. Results are based on the dryweight of the soils.

In chiral analysis, ion ratios for each enantiomer peak were required to fallwithin the 95% confidence interval of standards for a satisfactory analysis; other-wise, the result was rejected. Decisions as to whether a particular sample containedracemic or non-racemic residues were made by determining whether its EF wassignificantly different from the mean EF of standards at p < 0.05. Standard EFs � SDwere: TC ¼ 0.501 � 0.004 (n ¼ 10), CC ¼ 0.500 � 0.002 (n ¼ 9), o,p0-DDT ¼ 0.501 � 0.002 (n ¼ 7).

3. Results and discussion

Discussion in this paper includes data collected from southernMexico (Wong et al., 2008). Combined with the previous study,there are a total of 29 soil sampling sites covering 12 states ofMexico, comprising rural (n ¼ 4), urban (n ¼ 9) and agricultural(n ¼ 16) sites. Soil concentrations at each site are listed in SI-3 anda summary of the descriptive statistics is presented in Table 1.Arithmetic means (AM) and geometric means (GM) were calculatedwith substitution of 1/2 the LOD for concentrations less than theLOD. Detection frequencies of the OC classes were: DDTs 100%,

toxaphenes 97%, endosulfans 93%, chlordanes 93%, HCHs 55%, DIEL21%, HEPX 14% and HEPT 3%. No b-HCH, d-HCH nor aldrin wasdetected. Fig. 1 shows the box–whisker plot for the most frequentlydetected OCs. The arithmetic mean is greater than the geometricmean which suggested the data are skewed. This is reflected inkurtosis and skewness coefficients (K ¼ kurtosis/standard error ofkurtosis; S ¼ skewness/standard error of skewness) of frequencydistributions (Table 1) in which most OCs have K > 10 and S > 3.Significant kurtosis and skewness is shown by K and S values > 2;HCHs have the lowest K and S, with both values < 2. These resultsindicate the wide variability of most OCs in the soils and thereforeGM is probably the best measure of central tendency.

3.1. DDT

The SDDT (sum of o,p0-DDE, p,p0-DDE, o,p0-DDD, p,p0-DDD, o,p0-DDT, p,p0-DDT) in soils ranged from <LOD to 360 ng g�1, withGM ¼ 1.6 ng g�1 and AM ¼ 19 � 67 ng g�1. The urban soils had thehighest SDDT (GM¼ 4.7 ng g�1, AM¼ 45�118 ng g�1) followed bythe agricultural soils (GM ¼ 1.5 ng g�1, AM ¼ 10 � 21 ng g�1) andthe rural soils (GM ¼ <LOD ng g�1, AM ¼ 0.17 � 0.07 ng g�1). Thereis no significant difference between the urban and agricultural-rural soils (p > 0.05) due to the large standard deviations of theurban soils. The high SDDT in the urban soils is driven by one highvalue of 360 ng g�1 at a cemetery located in Tapachula, Chiapas(Site 21). Chiapas is an endemic malarious region and it consumedthe greatest amount of DDT in Mexico during 1989–1999 in sani-tary campaigns (Gallardo Diaz et al., 2000). Herrera-Portugal et al.(2005) reported that outdoor soils from a community in Chiapaswith high DDT exposure contained SDDT levels ranging from 350 to117 000 ng g�1, with AM ¼ 4760 ng g�1. Soils from the indoorenvironment contained even higher SDDT, ranging from 2000 to683 000 and averaging 21 920 ng g�1. Outdoor soils from a lessexposed community contained an average of 190 ng g�1 SDDT. TheAM SDDT levels in the farm soils sampled by us is five times lowerthan in agricultural soils of central Mexico, sampled in 2003(AM ¼ 54 � 21 ng g�1) (Waliszewski et al., 2004). DDT levels in ourrural soils are similar to those in background soils of Costa Ricawhich had a maximum concentration of 1.7 ng g�1 and most<0.04 ng g�1 (p,p0-DDE þ p,p0-DDD) (Daly et al., 2007a). A weakpositive, but not significant correlation was found between DDTused for malaria control during 1989–1999 and SDDT in soils(p ¼ 0.07, r2 ¼ 0.11).

Congeners of DDT were expressed as fractions: 1) FDDTe ¼ p,p0-DDT/(p,p0-DDT þ p,p0-DDE) and 2) FDDTo ¼ p,p0-DDT/(p,p0-DDT þ o,p0-DDT). All soils contained detectable p,p0-DDE; 59%contained detectable o,p0-DDT, 48% p,p0-DDT, and 38% o,p0-DDE (SI-3). In calculating the above fractions, 1/2 the LOD was assumed toreplace one undetectable species, while no fractional values werecalculated in cases where both species were below the LOD.Technical DDT has FDDTe ¼ 0.95, and FDDTo ¼ 0.84 assuming theWorld Health Organization (WHO) reported composition for tech-nical DDT: 77%, p,p0-DDT, 15% o,p0-DDT, 4% p,p0-DDE (WHO, 1989).FDDTe in soils ranged from 0.004 to 0.72 with AM ¼ 0.30 � 0.21.DDTs at most sites (24 out of 29 sites) were dominated by thedegradation product, p,p0-DDE, indicative of old DDT residues. Onthe other hand, there were 5 out of 29 sites with p,p0-DDT greaterthan p,p0-DDE, which suggests recent DDT usage or slower degra-dation in these soils. In soils sampled by Waliszewski et al. (2004),p,p0-DDT was detected in 100% of these soils, while p,p0-DDE wasdetected in only 12%, despite the fact that DDT had not been appliedfor at least ten years. FDDTe in air samples from Mexico showedsimilar values to the soils with mean of 0.34 � 0.26 (Wong et al.,2009a).

Table 1Summary of OCs in rural, urban and agricultural soils of Mexico (ng g�1, dry weight).

Rural (n ¼ 4) Urban (n ¼ 9) Agricultural (n ¼ 16) All sites (n ¼ 29)

GM AM � S.D. GM AM � S.D. GM AM � S.D. GM AM � S.D. MIN MAX K S

SHCH nd nd 0.032 0.043 � 0.034 0.029 0.044 � 0.044 0.027 0.039 � 0.038 nd 0.14 1.8 1.6HEPT nd nd nd nd nd nd nd nd nd 0.028 – –HEPX nd nd nd nd nd 0.05 � 0.15 nd 0.03 � 0.11 nd 0.59 – –SCHL 0.013 0.033 � 0.044 0.13 0.21 � 0.20 0.036 0.36 � 0.83 0.047 0.27 � 0.63 nd 2.7 11 3.3

FTC 0.47 0.47 � 0.10 0.49 0.49 � 0.09 0.58 0.60 � 0.15 0.53 0.55 � 0.14 0.40 0.85Aldrin nd nd nd nd nd nd nd nd nd nd – –Dieldrin nd nd nd 0.02 � 0.02 0.01 0.22 � 0.76 nd 0.13 � 0.57 nd 3.1 – –SENDO 0.041 0.14 � 0.23 0.17 0.60 � 1.3 0.22 57 � 227 0.16 32 � 169 0.011 909 28 5.3

FENDO 0.72 0.75 � 0.22 0.59 0.66 � 0.30 0.43 0.48 � 0.24 0.51 0.57 � 0.27 0.21 0.98SDDT nd 0.17 � 0.07 4.7 45 � 118 1.5 10 � 21 1.6 19 � 67 0 360 25 4.9

FDDTe 0.36 0.40 � 0.19 0.15 0.31 � 0.21 0.17 0.27 � 0.21 0.18 0.30 � 0.21 0.004 0.72FDDTo 0.77 0.77 0.43 0.62 � 0.32 0.52 0.66 � 0.30 0.49 0.65 � 0.29 0.024 0.94

STOX 0.20 0.24 � 0.18 1.1 11 � 22 0.65 23 � 83 0.64 16 � 63 nd 334 25 5.0

Abbreviations: GM¼ geometric mean; AM¼ arithmetic mean; S.D.¼ standard deviation; MIN¼minimum; MAX¼maximum; nd¼ not detected. K¼ kurtosis/standard errorof kurtosis; S ¼ skewness/standard error of skewness; kurtosis and skewness analysis was only performed for chemicals that were detected in more than 50% of the sites.SHCH ¼ a-HCH þ g-HCH; SCHL ¼ TC þ CC þ TN; SENDO ¼ ENDO I þ ENDO II þ ESUL; SDDT ¼ p,p0-DDT þ o,p0-DDT þ p,p0-DDE þ o,p0-DDE þ p,p0-DDD þ o,p0-DDD;STOX¼ quantified as technical toxaphene. FTC¼ TC/(TCþ CC); FENDO¼ ENDO I/(ENDO Iþ ENDO II); FDDTe¼ p,p0-DDT/(p,p0-DDTþ p,p0-DDE); FDDTo¼ p,p0-DDT/(p,p0-DDTþ o,p0-DDT). Calculations were not performed when both species were not detected.

F. Wong et al. / Environmental Pollution 158 (2010) 749–755 751

Plots of FDDTe vs. latitude and DDT usage for malaria campaignsare shown in Fig. 2A and B. FDDTe was negatively correlated withlatitude (p ¼ 0.001) and positively correlated with DDT used(p¼ 0.002). This suggests that fresher DDT is associated with greaterDDT usage in southern Mexico, attributed to the more recent use ofDDT in the malaria endemic region, which is concentrated in thesouthern part of the country. In this endemic region, increasedspraying of DDTs occurred, mainly in residential areas and in homes,and this is reflected in high SDDTs in soils from the outdoor andindoor environments of highly exposed communities (Herrera-Portugal et al., 2005). Moreover, there could be atmospheric trans-port of DDT from the southern neighboring countries, e.g.Guatemala, or Belize. Alegria et al. (2000) reported that atmosphericconcentrations of DDTs in Belize were as high as those in southernMexico. Similar correlations between FDDTe and DDT used/latitudewere found for the corresponding air samples (Wong et al., 2009a).

FDDTo in soils ranged from 0.024 to 0.94 with AM ¼ 0.65 � 0.29(n ¼ 18). A similar value (0.70) was found in agricultural soils by

1915

2115

15

0.001

0.01

0.1

1

10

100

1000

ΣHCH ΣCHL ΣENDO ΣDDT ΣTOX

8

3

14

15

2

9

22

7, 11

9

17, 18

24

29

1

4

14

28

26

29

18

Fig. 1. Box–whisker plot of OCs in soils of Mexico (ng g�1, dry weight). The top end ofthe box represents the 75th percentile of the data, and the bottom box represented25th percentile. The horizontal line between the boxes is the median, the square is thegeometric mean, and the asterisk is the arithmetic mean. The whiskers on the top andbottom of the boxes indicate 10th and 90th percentile. Data which fell outside thisrange are plotted as circles with station numbers. SHCH ¼ a-HCH þ g-HCH.SCHL ¼ TC þ CC þ TN. SENDO ¼ ENDO I þ ENDO II þ ESUL. SDDT ¼ p,p0-DDT þ o,p0-DDT þ p,p0-DDE þ o,p0-DDE þ p,p0-DDD þ o,p0-DDD. STOX ¼ quantified as technicaltoxaphene.

Waliszewski et al. (2004). FDDTo in WHO technical DDT is 0.84.Interpretation of FDDTo is complicated, as the percentage of o,p0-DDTin the technical DDT mixture can be considerably varied dependingon the manufacturer, and the composition of the technical DDT usedin Mexico is unknown. Plots of FDDTo vs. latitude and DDT used areshown in Fig. 2C and D. Although these correlations are significant(p¼ 0.002 and 0.03, respectively), examination of the plots suggeststhat FDDTo is not actually related to these factors. Most soils haveFDDTo w0.8 and higher, regardless of latitude or DDT use, while foursoils have FDDTo, w0.2 and lower. In all four of these soils, p,p0-DDTwas below detection and 1/2 the LOD was used in calculating FDDTo.Another source of o,p0-DDT could be from dicofol, a pesticidesynthesized from technical DDT and containing o,p0-DDT as animpurity. According to Qiu et al. (2005), the Chinese dicofol technicalmixture has o,p0-DDT/p,p0-DDT ratio of 6.7, i.e. FDDTo¼0.13. However,o,p0-DDE is also prevalent in technical dicofol at 44 g kg�1, comparedto 17 g kg�1 for p,p0-DDT, while p,p0-DDE is generally below detec-tion. In this study, o,p0-DDE were detected in 38% of the samples andthey were all w30–100 times lower than p,p0-DDE. Given thegenerally high mean value of FDDTo observed here and the low levelsof o,p0-DDE, it is concluded that the DDT residues in Mexico areprobably not related to dicofol usage.

EF of o,p0-DDT were measurable at 19 out of 29 sites (SI-4). EFs insoils ranged from 0.456 to 0.647. The o,p0-DDT in four soils wasracemic (EFs ranged from 0.497 to 0.500). Other soils showedenantioselective degradation of either the (þ) or the (�), with 7soils showing EF < 0.500 and 8 soils EF > 0.500. The ambivalentenantioselective degradation of o,p0-DDT in soils is commonlyreported worldwide (Wong et al., 2009b; Li et al., 2006; Kurt-Kar-akus et al., 2005; Wiberg et al., 2001; Aigner et al., 1998). Accord-ingly, EFs are also expressed as deviation from racemic (DEVrac) inorder to analyse the degree of enantioselective degradationregardless of which enantiomer is depleted (Kurt-Karakus et al.,2005). SI-5 displays a positive correlation between DEVrac of o,p0-DDT in soils and DDT used for malaria control (r2 ¼ 0.21, p ¼ 0.05).This correlation is driven by Site 22, which has high DEVrac¼ 0.147,and removal of this site resulted in non-significant correlation. Nocorrelation between DEVrac and latitude (r2 ¼ 0.03, p ¼ 0.44) wasfound, with or without Site 22. DEVrac in Mexican air showedsignificant positive correlation with latitude and negative correla-tion with DDT used, both reflecting ‘‘fresher’’ DDT in the air ofsouthern Mexico (Wong et al., 2009a). It is noted that the o,p0-DDTin air was closer to racemic than in the soils as indicated by DEVracvalues in air that were generally smaller than those in soils, SI-6.

y = 0.0025x + 0.17

r2 = 0.30p = 0.002

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100 150

DDT Used (tons)

FD

DT

e

B

y = -0.031x + 0.92

r2 = 0.33p = 0.001

0.0

0.2

0.4

0.6

0.8

1.0

14 16 18 20 22 24 26 28 30

Latitude

FD

DT

e

A

NorthSouth

y = 0.0032x + 0.46

r2 = 0.27p = 0.03

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100 150

DDT Used (tons)

FD

DT

o

D

y = -0.048x + 1.6

r2 = 0.45p = 0.002

0.0

0.2

0.4

0.6

0.8

1.0

14 16 18 20 22 24 26 28 30

Latitude

FD

DT

o

C

NorthSouth

Fig. 2. Plots of FDDTe vs. A) latitude; B) DDT used; FDDTo vs. C) latitude; D) DDT used.

14+04 4

5

y = -0.67x + 5.75r = 0.52

8.4

F. Wong et al. / Environmental Pollution 158 (2010) 749–755752

This suggests that, overall the air is largely influenced by atmo-spheric transport of DDT rather than soil emission. There were onlythree sites that showed DEVrac in air about the same or greaterthan in soils. Air and soil samples were not collected at exactly thesame locations and air samples were taken several meters aboveground. Therefore DEVrac of DDTs in air does not necessarily reflectthe soil samples at that site, but could be due to emissions fromdifferent soils in the vicinity. Concentrations and chemical profilesin air reflect those in the soil immediately above the surface andchange within a few meters above the soil (Kurt-Karakus et al.,2006; Eitzer et al., 2003; Finizio et al., 1998).

ralraP ot evitaler tnuomA

Q

go

L

t

0

1

2

3

P26 P39 P42 P44+ P50 P63

Technical standard Soils Air

p=0.04

7.2

7.6

8.0

-3.5 -3.0 -2.5 -2.0Log P

L

Fig. 3. Proportion of toxaphene congeners in soils, air and technical toxaphene stan-dards normalized to the amount of Parlar 40 þ 41. Regression statistics for averagelog Q vs. log liquid vapour pressure (PL/Pa) for toxaphenes. Q ¼ CSOIL/CAIR. CAIR wasobtained from Wong et al. (2009a).

3.2. Toxaphene

Toxaphenes are reported as the sum of hepta-, octa- and non-achlorobornanes (STOX). The STOX from all sampling sites rangedfrom <LOD to 334 ng g�1, with GM ¼ 0.64 ng g�1 andAM ¼ 16 � 63 ng g�1. The highest toxaphene concentration,334 ng g�1, was found at a farm in Mazatlan (Site 15). The rural soils(GM ¼ 0.20, AM ¼ 0.24 � 0.18 ng g�1) contained the lowest STOX.Higher toxaphenes were found in the agricultural (GM ¼ 0.65,AM ¼ 23 � 83 ng g�1) and urban soils (GM ¼ 1.1,AM ¼ 11 � 22 ng g�1). The STOX concentrations in soils of Mexicoare lower compared to those in southern U.S. soils, for which AMand GM of 690 and 92 ng g�1 were reported in Alabama, Louisianaand Texas (Bidleman and Leone, 2004a). AM and GM in South

Carolina were 277 and 72 ng g�1, while 83 and 55 ng g�1 werefound in Georgia (Kannan et al., 2003). Congener profiles werenormalized to the total amount of the partly resolved Parlar40 þ 41. Fig. 3 compares the congener profiles among the soils, airand technical toxaphene standard. Compared to Parlar 40 þ 41 intechnical toxaphene, Parlar 26 is slightly depleted in soils and 39

F. Wong et al. / Environmental Pollution 158 (2010) 749–755 753

and 42 more so. The relative loss of 39 and 42 in soils of Mexico issimilar to profiles in southern U.S. soils and is likely due tomicrobial degradation of these labile congeners (Bidleman andLeone, 2004a; Harner et al., 1999). Parlar 39 and 42 are alsodepleted in air, an indication that soil residues are the main sourcerather than current toxaphene usage (Wong et al., 2009a; Bidlemanand Leone, 2004a). The enrichment of P44 in both air and soils maydue to its formation as the degradation product of P62 (Ruppe et al.,2004). The lower volatility Parlar 50 and 63 also showed enrich-ment in the soils relative to the standard.

Log soil–air concentration ratio (log Q) was plotted against thelog liquid-phase vapour pressure (log PL/Pa) of ten congeners: Parlar21, 26, 32, 39, 40, 41, 42, 44þ, 50 and 63 at all sites. Parlar 21 and 32were not determined in Chiapas, Veracruz and Tabasco soilscollected in 2002–2004 in the Mexico soil and air samples (Wonget al., 2008). Fig. 3 shows a significant negative correlation withr2 ¼ 0.52, p ¼ 0.04. This reveals that, regardless of whether toxa-phene is undergoing net deposition or volatilization, the lightercongeners accumulate in air whereas the heavier ones accumulatein the soil.

3.3. Endosulfan

Endosulfan is the only currently used pesticide reported here.The SENDO (sum of ENDO I, ENDO II and ESUL) ranged from <LODto 909 ng g�1, with overall GM ¼ 0.16 ng g�1 andAM ¼ 32 � 169 ng g�1. Agricultural soils had the highest SENDO,with GM ¼ 0.22, AM ¼ 57 � 227 ng g�1, followed by the urban soils(GM ¼ 0.17, AM ¼ 0.60 � 1.3 ng g�1), and rural soils (GM ¼ 0.041,AM ¼ 0.14 � 0.23 ng g�1). The highest SENDO concentration wasfound at a Mazatlan farm (Site 15). This is not surprising asMazatlan is an intensive agricultural area where the annual AM ofSENDO in air was 26 800 pg m�3, which is the highest measured inMexico (Wong et al., 2009a). SENDO in soils of Costa Ricanmontane forests ranged from 0.020 to 3.2 ng g�1 (Daly et al.,2007b), which is similar to the levels in Mexican urban and ruralsoils. FENDO (ENDO I/ENDO Iþ ENDO II) for all soils ranged from 0.20to 0.98, with AM ¼ 0.58 � 0.26. In air, FENDO averaged 0.82 (Wonget al., 2009a), which may reflect the higher volatility of ENDO I(Shen and Wania, 2005). Endosulfan sulfate (ESUL) was detected in93% of the soil samples. It is a major degradation product of ENDO Iand II, and ranged from 22 to 69% (AM ¼ 47%) of SENDO. There wasno correlation between the percentage of ESUL and SENDO.

3.4. Chlordane

The SCHL (sum of TC, CC and TN) ranged from <0.0033 to2.7 ng g�1, with GM ¼ 0.047 ng g�1 and AM ¼ 0.27 � 0.63 ng g�1.Highest SCHL was found at a farm in Mazatlan (Site 15, the samefarm where the highest STOX and SENDO concentrations werefound). The mean SCHL in agricultural soils (GM ¼ 0.036,AM ¼ 0.36 � 0.83 ng g�1) was not significantly different from thatin the urban soils (GM ¼ 0.13, AM ¼ 0.21 � 0.20 ng g�1); both werean order of magnitude greater than in rural soils (GM ¼ 0.013,AM ¼ 0.033 � 0.044 ng g�1). The AM concentration in the Mexicanagricultural soils was similar to the AM ¼ 0.56 ng g�1 in southernU.S. agricultural soils (Bidleman and Leone, 2004b). The GMconcentration for all Mexican soils was comparable to Costa Ricasoils, GM ¼ 0.036 ng g�1 (Daly et al., 2007a). The fraction of TC tothe sum of TC and CC was calculated for soils with 1 or 2 detectablespecies, as for the DDTs, FTC¼ TC/(TCþ CC). Technical chlordane hasFTC ¼ 0.54 (Jantunen et al., 2000). FTC of all soil samples spanneda narrow range from 0.40 to 0.85 with AM¼ 0.55� 0.14. There wereno significant differences among the FTC of urban, agricultural-rural

soils (SI-4) and this is similarly reported in the air of Mexico (Wonget al., 2009a).

EFs of chiral chlordanes were measurable at 18 sites for TC and 11sites for CC (SI-4). Enantioselective degradation of (þ)TC and (�)CCwere generally found. EFs of TC ranged from 0.380 to 0.499 withAM ¼ 0.459 � 0.028. EFs of CC ranged from 0.498 to 0.588 withAM¼ 0.529 � 0.024. These are typical degradation patterns seen inagricultural (Eitzer et al., 2003; Wiberg et al., 2001; Aigner et al.,1998; Falconer et al., 1997) and background (Wong et al., 2009b;Daly et al., 2007a; Kurt-Karakus et al., 2005) soils. No significantdifference was found between EFs in the urban and agricultural-rural soils. Heavily contaminated soils near foundations of houses inthe U.S. that were treated with technical chlordane for termitecontrol contained fresher, racemic chlordane residues (Eitzer et al.,2003). The DEVrac of TC and CC in soils were compared to the cor-responding air. Both showed the same patterns of enantioselectivedegradation, but the extent of such degradation was greater in soilsthan air (SI-6). Since the air was collected a few meters aboveground, the EFs of chlordanes in air are likely to be more influencedby regional atmospheric transport than soil emission at that site.Also, the net direction of soil–air exchange appears to be closer todeposition than volatilization which will be discussed later.

3.5. Other OCs

HCHs were above the LOD only in some urban and agriculturalsoils. The AM and GM concentrations of SHCH (sum of a-HCH andg-HCH) were 0.039 � 0.038 and 0.027 ng g�1. No b-HCH or d-HCHwere detected in any of the samples. HCHs in Mexico soils are lowerthan in those in background Costa Rican soils, for whichGM ¼ 0.14 ng g�1(Daly et al., 2007a). Waliszewski et al. (2004)reported SHCH ¼ 2.4 � 3.3 ng g�1 in agricultural soils of centralMexico which is two orders of magnitude greater than in this study.DIEL was detected in only 6 samples. Five of these samples hadconcentrations ranging from 0.028 to 0.30 ng g�1 and one soil with3.1 ng g�1 was found at MAZ (Site 15). HEPT was detected in 1sample, with very low concentration of 0.028 ng g�1. HEPX wasdetected in 6 samples at <LOD–0.59 ng g�1 with the maximum atSite 15. Mexican air also showed low concentrations of DIEL, HEPT,and HEPX (Wong et al., 2009a).

3.6. Soil–air exchange

Fugacities (f, Pa) in soil and air were estimated for OCs using themethod described in Daly et al. (2007a). Octanol–air partitioncoefficients (KOA) as functions of temperature were taken fromShoeib and Harner (2002), except for toxaphene, in which KOA wasestimated from its octanol–water partition coefficient (KOW) andHenry’s law constant (Bidleman and Leone, 2004a). The dry soliddensity of soil was assumed to be 2650 kg m�3. Annual mean airconcentrations at or near soil sampling sites were obtained fromWong et al. (2009a). Results are expressed as fugacity fraction,ff ¼ fS/(fS þ fA), where fS and fA is the fugacity of the chemical in soiland air. ff ¼ 0.500 indicates that the compound is at soil–air equi-librium, ff > 0.500 indicates net volatilization from soil to air, and<0.500 indicates net deposition from air to soil. However, a sensi-tivity analysis performed by Daly et al. (2007a) indicated thata window of 0.50 � 0.35 may not represent a significant departurefrom equilibrium. Others have used a more narrow window of0.50 � 0.20 as an equilibrium condition (Ru�zickova et al., 2008;Meijer et al., 2003; Harner et al., 2001).

Fig. 4 presents the ff of selected OCs across Mexico and valuescan be found in SI-7. The ffs cover a wide range, indicating thatsome soils are net recipients of OCs from the atmosphere, whileother soils are net sources. Similar wide ranges of ffs have been

0.00

0.50

1.00α-

HC

H

γ-H

CH TC

END

O I

p,p’-D

DE

p,p’-D

DD

o,p’-D

DT

p,p’-D

DT

ΣTO

X

Fuga

city

frac

tion

Fig. 4. Fugacity fractions (ff) of OCs in Mexico. ff ¼ fS/(fSþfA), where fS ¼ fugacity of soil;fA ¼ fugacity of air. ff ¼ 0.5 indicates soil–air equilibrium. ff > 0.5 indicates net vola-tilization from soil to air. ff < 0.5 indicates net deposition from air to soils. The top endof the box represents the 75th percentile of the data, and the bottom box represented25th percentile. The horizontal line between the boxes is the median, the square is thegeometric mean, and the asterisk is the arithmetic mean. The whiskers on the top andbottom of the boxes indicate 10th and 90th percentile. Data fell outside this range areplotted as circle. Dashed line indicates the limits over which ff may not be significantlydifferent from equilibrium (Daly et al., 2007a).

F. Wong et al. / Environmental Pollution 158 (2010) 749–755754

reported for soils in eastern and southern Europe (Ru�zickova et al.,2008). In the latter study, a trend toward lower median ffs in thecooler seasons was noted. Air concentrations in Mexico also variedseasonally, although not always with temperature (Wong et al.,2009a). For this reason, only annual mean concentrations wereused to estimate ffs. Since the ffs of most OCs showed great vari-ability, GM probably is the best representative of the overall situ-ation at the studied sites and is used in the following discussions.

The GM ff of DDTs (p,p0-DDT, p,p0-DDE, o,p0-DDT, p,p0-DDD)ranged from 0.02 to 0.05, which indicates net deposition. DDTs inair of Mexico are mostly originated from local use and/or regionalair transport rather than from revolatilization of old soil residues.This is supported by less enantioselective degradation of o,p0-DDTin air than soils. Since the use of DDT stopped in 2000 respectively,it may take more time before they can achieve soil–air equilibrium.As noted earlier, other studies have reported substantially higherconcentrations of DDTs in agricultural soils (Waliszewski et al.,2004) and in soils of communities where DDT was used for malariacontrol (Herrera-Portugal et al., 2005). DDTs have not beenmeasured in the air at these locations, however when assessing themean soil concentrations against the mean air concentrations ofDDTs from Wong et al. (2009a), these soils show ffs of >0.90,indicating a strong potential for net volatilization. DDTs in Mexico isprobably experiencing net deposition in certain areas while othersare under net volatilization, particularly in the malarious areawhere DDTs were heavily used.

The GM ff of toxaphenes is 0.17. Although below the equilibriumff ¼ 0.5, the GM ff is within the equilibrium range as suggested byDaly et al. (2007a). As noted above, depletion of Parlar congeners 39and 42 in both soil and air suggests that soil residues are the mainsource in air, and the negative correlation of the soil–air concen-tration ratio with vapour pressure (Fig. 3) indicates a close associ-ation between the two compartments. The ff of a-HCH, g-HCH andTC averaged 0.57, 0.32 and 0.17 respectively which indicates air–soilnear equilibrium. The ff of g-HCH and TC in Mexico is lower thanthose reported in Costa Rica (Daly et al., 2007a). The ff of ENDO Iwas the lowest among all OCs, with GM¼ 0.02. ffs of all sites exceptone falling below 0.15, the lower range of the equilibriumboundary. This clearly indicates that ENDO I is undergoing netdeposition from air to soils, and this is not surprising, as ENDO I isa commonly used pesticide nowadays.

4. Conclusion

Base on the results from this study, DDTs, endosulfan andtoxaphenes are the most prominent pesticides detected in air andsoils of Mexico. We have demonstrated that the soils and air reflectsthe DDT usage pattern of Mexico. It is apparent from the sitesexamined in this study that certain ‘‘hot spot’’ areas exist wheresoils may be a source of DDTs and other OCs to the atmosphere(Fig. 4). A larger survey of Mexican soils, particularly covering theendemic regions, such as Oaxaca, is needed to fully assess thecontribution of soil emissions to atmospheric levels in Mexico.

Acknowledgements

We acknowledge funding from the North American Commissionfor Environmental Cooperation (NACEC) and Environment Canadathrough the Research Affiliate Program. We thank the followingcolleagues for their assistance in soil sample collection: VıctorAlvarado (Sustancias Toxicas En Suelos Y Residuos, Centro Nacionalde Investigacion y Capacitacion Ambiental), Alfredo Avila Galarza(Facultad de Ingenierıa, Universidad Autonoma de San Luis Potosı),Erick R. Bandala and Silvia Gelover (Departamento de IngenierıaCivil y Ambiental, Universidad de Las Americas-Puebla), Juan EmilioGarcıa Cardenas, Idolina de la Cerda Hinojosa (Jefa del SistemaIntegral de Monitoreo Ambiental Agencia de Proteccion al MedioAmbiente y Recursos Naturales), Ignacio Galindo Estrada (CentroUniversitario de Investigaciones en Ciencias del Ambiente, Uni-versidad de Colima), Guillermo Galindo Reyes, Fernando EncisoCaracho (Laboratorio de Toxicologıa, Facultad de Ciencias del Mar,Univeridad Autonoma de Sinaloa), Gerardo Gold-Bouchot (Centrode Investigacion y de Estudios Avanzados del IPN), Joaquın Mur-guıa-Gonzalez and Noe Aguilar Rivera (Facultad de Ciencias Bio-logicas y Agropecuarias, Region Orizaba-Cordoba, UniversidadVeracruzana), Elias Ramirez Espinoza (Centro de Investigacion enMateriales Avanzados, Chihuahua), Alejandro Sosa Martınez (Uni-versidad Veracruzana).

Appendix. Supplementary information

Supplementary data associated with this article can be found inthe online version at doi:10.1016/j.envpol.2009.10.013.

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