Organochlorine pesticides in soils and air of southern Mexico: Chemical profiles and potential for soil emissions

The extent of organochlorine pesticides (OCs) contamination in southern Mexico was investigated in this study. Biweekly air samplings were carried out in two sites in the state of Chiapas (during 2002-2003), and one in each state of Veracruz and Tabasco (during 2003-2004). Corresponding to the air sampling locations, soil samples were also collected to gauge the soil-air exchange of OCs in the region. Sigma DDTs in soils ranged from 0.057 to 360 ng g(-1) whereas those in air ranged from 240 to 2400 pg m(-3). DDT and metabolite DDE were expressed as fractional values, F-DDTe = p,p'-DDT/(p,p'-DDT + p,p'-DDE) and F-DDTo = p,p'-DDT/(p,p'-DDT + o,p'-DDT). F-DDTe in soils ranged from 0.30 to 0.69 while those in air ranged from 0.45 to 0.84. F-DDTe in air at a farm in Chiapas (0.84) was closer to that of technical DDT (0.95) which is suggestive of fresh DDT input. Enantiomer fractions (EF) of o,p'-DDT in air were racemic at all locations (0.500-0.504). However, nonracemic o,p'-DDT was seen in the soils (EFs = 0.456-0.647). Fugacities of OCs in soil (f(s)) and air (f(a)) were calculated, and the fugacity fraction, ff = f(s)/(f(s) + f(a)) of DDTs ranged from 0.013 to 0.97 which indicated a mix of net deposition (ff < 0.5) and volatilization (ff > 0.5) from soil among the sites. It is suggested that DDTs in Mexico air are due to a combination of ongoing regional usage and re-emission of old DDT residues from soils. Total toxaphene in soils ranged from 0.066 to 69 ng g(-1) while levels in air ranged from 6.2 to 230 pg m(-3). Chromatographic profiles of toxaphenes in both air and soil showed depletion of Parlar congeners 39 and 42. Fugacity fractions of toxaphene were within the equilibrium range or above the upper equilibrium threshold boundary. These findings suggested that soil emission of old residues is the main source of toxaphenes to the atmosphere. Results from this study provide baseline data for establishing a long-term OC monitoring program in Mexico. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.