Distribution pathways of hexachlorocyclohexane isomers in a soil-plant-air system. A case study with Cynara scolymus L. and Erica sp. plants grown in a contaminated site
Introduction
Man-made organic compounds, such as pesticides and insecticides, are of great environmental concern at present. Compounds such as volatile and persistent organochlorine pesticides are found at high concentrations in water, air, soil, sediments and biota (Willet et al., 1998, Tao et al., 2005). This applies to the pesticide hexachlorocyclohexane (molecular formula 1, 2, 3, 4, 5, 6-hexachlorocyclohexane, abbreviated as HCH), and its more frequent isomers (α – of which there is a pair of enantiomers – β, γ, δ, ɛ, η, and θ). HCH is available as two formulations, technical HCH (a commercial mixture of main isomers) and lindane (γ-isomer). Their production is based on chlorination of benzene in the presence of UV light (Vijgen, 2006). Both formulations were previously used as commercial pesticides, but they are currently prohibited by international legislation. However contaminated zones still exist, particularly in areas close to the centres of production – from when commercial production was allowed – where HCH waste was disposed in an uncontrolled manner, and also in other sites, due to either: (i) direct application of HCH as a pesticide in crop management; or (ii) dispersal deposition of HCH from the atmosphere by long-range transport of the pollutants (Li, 1999, van Pul et al., 1999).
Once present in an environment, these compounds redistribute and partition into the different compartments (e.g., soil, soil biota, water, plants, air) by different processes, such as adsorption onto soil particles, adsorption onto plant root tissues, volatilization, microbial degradation, leaching, etc., which over time leads to thorough contamination of the whole ecosystem (surface water, nearby soils, fauna, etc.) (Simonich and Hites, 1995a, Walker et al., 1999). HCH isomers display a greater ability than most other organochlorine pesticides to partition into different compartments; this favours their bioaccumulation throughout trophic chains (either aquatic or terrestrial), which represents a threat to humans and to the environment. The HCH isomers most frequently found in environmental samples are α, β, and γ-HCH; the former is most commonly found in aquatic environments and in the atmosphere, whereas the β isomer, which is the most lipophilic and stable of all HCH isomers, predominates in soils and in animal tissues and fluids (Walker et al., 1999).
Plants, on the other hand, constitute an important sink for these types of organochloride compounds (Muir et al., 1993, Simonich and Hites, 1994, Barber et al., 2004), as they are able to retain them efficiently in their different compartments. Studies of phytoaccumulation of organic contaminants mainly focus on two routes of entry into the plant: (i) the soil-plant pathway; and (ii) the air-plant pathway. In the soil-plant route, the organic compounds present in the soil close to the roots can either: (i) be absorbed by the plants and, in some instances, translocated into the aerial parts through the xylem; and/or (ii) become adsorbed onto the root tissues (especially when absorption and translocation is prevented because of the high lipophilicity of the compound, as in the case of HCH isomers). The air-plant route, on the other hand, involves the air-plant partitioning of HCH through volatilization from the soil surface and further adsorption on plant leaves, but may also include the partitioning of HCH between air-suspended particles contaminated with HCH and plant surfaces. The air-plant pathway is the most common route of entry of lipophilic contaminants to the above-ground plant biomass (Paterson et al., 1991, Schreiber and Schönherr, 1992, Welsch-Pausch et al., 1995, Simonich and Hites, 1995b, Rüdel, 1997).
In the present study, we investigated the in situ partitioning and accumulation of HCH isomers (mainly α, β, γ and δ) in a soil-plant-air system in a contaminated zone, in which the two main pathways of entry of HCH into the plants (soil → plant; air → plant) are considered. We intend a qualitative evaluation of the relative importance of each of these routes, as better knowledge of such mechanisms would facilitate the use of plants for decontamination of HCH isomers from soils by means of phytoextraction (extraction of the contaminant from the soil and its accumulation in the above-ground biomass) (Pilon-Smits, 2005). We therefore undertook a field study in an area previously used as a HCH waste disposal site and planted two different species, artichoke (Cynara scolymus L.) and heather (Erica sp.) either: (i) directly in the HCH contaminated soil; or (ii) in pots filled with uncontaminated soil, which were buried in the HCH contaminated soil.
Section snippets
Area of study
The study was carried out in an experimental plot, of surface area 3560 m2 and contaminated with residues from lindane production, in Porriño (Pontevedra, NW Spain). The annual mean precipitation and temperature in the area are 1504 mm and 14.4 °C, respectively; minimum precipitation and maximum temperatures occur in July–September (Martínez Cortizas and Perez Alberti, 1999). The soils in the area (Urbic Technosols and/or Technic Regosols; WRB, 2006) derive from alluvio-colluvial sediments of
HCH in surrounding contaminated soils
The concentrations of total HCH (the sum of α-, β-, γ-, and δ-HCH isomers) in soils taken from the 48 sampling sites of the experimental area varied widely, from 14 to 34 673 mg kg−1 (Table 1). Concentrations of α-HCH ranged between 4 and 27 510 mg kg−1, and those of the β isomer between 5 and 6738 mg kg−1 (Table 1). The range of concentrations of γ and δ-HCH were much smaller, between 0.1 and 740 mg kg−1 for γ-HCH, and between 0.1 and 98 mg kg−1 for δ-HCH (Table 1).
In most of the soil samples, the
HCH distribution in soils
The concentration of total HCH in soil samples in the NP rhizoavailable soils at the end of the experiment was lower than in the surrounding soil. This was attributed to the “dilution effect” of uncontaminated soil still attached to the roots of the plants obtained from the nursery. On the other hand, the P rhizoavailable soils were contaminated with HCH at the end of the experiment, in spite of the fact that these soils were HCH-free at the beginning of the experiment (as these pots were
Acknowledgements
The authors would like to thank Carmen Perez Llaguno for sampling assistance and the staff of the Departmento de Quimica Analitica, Universidade de Coruña (Spain) for their collaboration in analysing the HCH isomers in plant tissues. The study was partially financed by the Xunta de Galicia (Ministry of Environment), the Ministry of Science and Technology of the Spanish Government (Project: REN2002-04507-C02-01), and by the A Coruña Regional Council.
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Present address: NEIKER, Berreaga kalea, 1. 48160 Derio, Bizkaia, Spain.