SUBSTANTIAL MIGRATION OF DIOXINS IN AGROCHEMICAL FORMULATIONS

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1 SUBSTANTIAL MIGRATION OF DIOXINS IN AGROCHEMICAL FORMULATIONS Grant, Sharon, Mortimer, Munro, Stevenson, Gavin, Malcolm, Don and Gaus, Caroline The University of Queensland (National Research Centre for Environmental Toxicology (EnTox)), 9 Kessels Road, Coopers Plains 8, Australia; Queensland Environmental Protection Agency, 8 Meiers Road, Indooropilly 8, Australia; Dioxin Analysis Unit, National Measurement Institute, Suakin Street, Pymble, Australia ; Department of Natural Resources and Water, 8 Meiers Road, Indooroopilly 8, Australia Abstract Contaminants with low water solubility and high carbon-water partitioning are generally considered to have low mobility in soils. However, the presence of co-contaminants can act as transport facilitators for otherwise low mobility organic compounds (LMOCs). Little is known about facilitated contaminant migration and this process is rarely considered in evaluations of off-site transport and groundwater contamination potential for LMOCs. This study investigated the vertical migration of dioxins in soil, released together with high volumes of pesticides and adjuvants following an accidental fire at a pesticide production facility. Two intact cores (to. m) were obtained below clay-lined ponds where contaminated run-off water was contained. PCDD/Fs were found throughout both cores, with maximum concentrations ( and 9 ng/g) at -. meters. A reversed mobility was observed throughout the core depths with the least mobile congener OCDD transported further than the relatively more mobile TCDD. Such a reversal is consistent with surfactant facilitated transport, which is suggested to be the primary pathway for the observed migration. These results highlight that the paradigm of LMOCs being non-mobile in soils should be considered carefully together with application-specific and environmental factors which may have the ability to considerably change the predicted environmental fate of these chemicals. Introduction The fate of chemicals in the environment, including their transport in soils, is largely governed by compounds physico-chemical properties. As a consequence, these properties can be used to predict the environmental fate of contaminants, such as their potential for migration to groundwater, associated off-site transport and subsequent environmental and human exposure. Organic compounds which are considered to have low mobility in the environment, herein referred to as low mobility organic compounds (LMOCs), are defined by low water solubility, high carbon-water partitioning coefficient (K OC ) and low to intermediate volatility. Often, these compounds are also relatively persistent, and examples include dioxins, PCBs, OCs and many other pesticides. LMOCs physico-chemical properties result in strong/irreversible sorption to soils and sediments and negligible migration potential. Hence, soils and sediments represent a sink where persistent LMOCs are expected to accumulate in the top - cm -. In addition to physico-chemical properties, it is well recognised that local environmental conditions (e.g. temperature, soil organic carbon) as well as application-specific parameters (e.g. emission to air, application to soil or in mixtures) can influence chemical fate. However, the extent and specific impacts of such factors on contaminant transport and distribution are less well understood. An example where environmental and application-specific factors can have profound influences on chemical fate is the facilitated migration of LMOCs in the presence of natural colloids (e.g. dissolved organic matter, humic substances) or co-contaminants (e.g. surfactants, oils, solvents). Currently, few fate assessments incorporate the potential for facilitated transport of LMOCs, potentially because this phenomenon has to date been considered a rare exception to the rule, combined with the lack of monitoring data at depth, and/or the extent of facilitated migration is not known. However, there are an increasing number of reports in recent years that report detection of elevated levels of LMOCs in soils at depths of up to several meters, with the authors specifically or potentially attributing these findings to facilitated migration -8. Vol., 9 / Organohalogen Compounds page 8

2 This study investigated the vertical migration of LMOCs in soils, sourced from contaminated run-off water following an accidental fire at a pesticide production facility and the associated release of high volumes of pesticide formulations. The run-off water contained elevated concentrations of pesticide mixtures and dioxins, as well as a range of adjuvants, including anionic and non-ionic surfactants. Materials and Methods The study site is located km north of Brisbane, Australia. The fire occurred in August and resulted in the release of fire extinguishing water (not foam) mixed with pesticides, adjuvants, combustion products and impurities to a nearby wetland and creek. Four bunded containment ponds were constructed in the original creek bed to prevent wastewater run-off and an artificial bypass for the creek was established adjacent to these containment ponds. The run-off water was retained in the ponds until contaminated water and soil (up to meters depth) were removed for remediation over the following years. Two years after the event, two intact cores up to. meters depth were collected using a Geoprobe operated by an experienced team from the Queensland Department of Natural Resources and Water in July. Coring sites were located in containment Ponds and (P-). Prior to the coring, Pond was excavated to a depth of approximately meters and backfilled with clean soil. The top meters of core P was therefore discarded and only soil below this depth was analysed. In addition, composite soil samples were obtained approximately one year prior to the cores from the surface of the four containment Ponds (P-, P-, P- and P-surface), and samples at depth were obtained during excavation of three of the Ponds (Ponds -) between - months prior to the coring, at approximately and m depth in Pond, at m depth in Pond and m depth in Pond. Meticulous care was taken to avoid cross contamination between cores and within core slices throughout sample preparation and analysis. After careful removal from the coring tube, soils were devided into cm slices and homogenized using a solvent-cleaned mortar and pestle. A sub sample of approximately g was taken from each slice, air dried and sieved to remove particles larger than mm. PCDD/Fs were extracted from soils by accelerated soxhlet extraction with toluene. All extracts were pre-cleaned with pure concentrated sulphuric acid and then purified using Powerprep TM automated system (Fluid Management Systems, Waltman, MA, USA) (REF). In summary, sample extracts were applied to multi-layed silica - basic alumina column in sequence and eluted with hexane, dichloromethane(dcm):hexane (:98) and DCM:hexane (:). The : DCM:hexane solvent mix was transferred to a carbon column which was then eluted by ethyl acetate:toluene (:) in the forward direction and toluene in the reverse direction. The toluene fraction was collected for all samples and concentrated for high resolution gas chromatography/high resolution mass spectrometry (HRGC/HRMS). HRGC/HRMS was conducted on a MAT9XL HRMS (ThermoFinnigan MAT GmbH, Bremen, Germany) and an Agilent GC (Palo Alto, USA) equipped with a CTC autosampler. The DB-MS capillary column was operated under temperature programmed conditions of - C at C min - followed by - C at C min - and C at C min -. The injection port temperature was maintained at 9 C. GC carrier gas was helium maintained at a flow rate of. ml/min. Mass spectra were recorded with an electron impact ionisation source operated at ev. PCDD/Fs were quantified according to isotope dilution techniques using surrogate (internal) standard that included C labelled PCDD/Fs. Total organic carbon (TOC) was determined for all soil samples at QHSS laboratories using LECO induction furnace with subsequent detection of CO according to a standardised procedure (Method 89). Total anionic surfactant concentrations were determined for a sub-set of the soil samples as methylene blue active substances (MBAS) at QHSS laboratories according to standardized procedures (Method ). Results and Discussion In contrast to the expected retainment of LMOCs in the top - cm, PCDD/Fs were present below the surface layers in all four pond cores, and concentrations remained elevated (at ppb levels) throughout the top meters (Fig.). The total PCDD/F concentrations even increased with core depth, peaking between. meters in Ponds, and ( ng g - dw at. m depth, 8 ng g - dw at m and 9 ng g - dw at m, respectively). Below. meters, concentrations decreased relatively rapidly to ppt levels at approximately meters depth. In Vol., 9 / Organohalogen Compounds page 9

3 Pond, PCDD/Fs were also elevated below the surface ( ng g - dw at m), however core samples were only available from the surface and at meters depth, hence it was not possible to determine where the maximum concentrations occurred within this core. No correlation was observed between the TOC content of the core soils and the concentration of PCDD/Fs with depth, suggesting that the elevated PCDD/F concentrations in the deeper soil core layers compared to the surface cannot be attributed to an increased capacity to sorb LMOCs. PCDD/Fs contained in the wastewater run-off would be expected to be retained in the surface layers of the pond soils, particularly given a higher organic carbon loading in these surface soils compared to deeper layers (> meters depth); however, the present results suggest substantial migration of PCDD/Fs occurred following the contamination event. PCDD/F concentration (ng g - dw) Depth (m) Pond Pond Fig.. PCDD/F concentrations throughout the cores collected from the containment ponds (Ponds to ). Red squares represent the concentrations in samples collected one year prior to the cores (black squares). The PCDD/F congener profiles in all core samples were similar, dominated by OCDD (range 8 99 % of PCDD) with decreasing concentrations towards lower chlorinated,,,8-substituted congeners and homologues (Hp>Hx>Pn Te). Similarly, on an isomer basis almost identical patterns were present for PCDDs throughout the pond cores, and were comparable to isomer patterns in the wastewater run-off. In contrast to PCDDs, PCDF concentrations of most congeners were generally low or below the limit of detection (LOD) with the exception of TCDFs, which were elevated in surface samples (maximum concentrations. - ng g - dw) and present above LOD in some of the core samples up to approximately meters depth. Due to the generally low concentrations of PCDFs, congener profiles or trends with depth are difficult to identify accurately or to interpret. However, in samples where TCDFs were above the LOD, isomer patterns were clearly similar between the wastewater run-off and the pond core samples, characterised by an unusual dominance of a single TCDF (,,,8-TCDF ) isomer. Despite the overall and striking similarity of PCDD/F patterns in all samples, congener and homologue profiles were observed to shift slightly, but progressively, towards higher chlorinated congeners with increasing depth, up to the depth where maximum concentrations occurred. In particular the contribution of OCDD PCDDs to Pond Pond Vol., 9 / Organohalogen Compounds page

4 showed a clear increase with depth (Fig. A), whereas the contribution of TCDD to PCDDs (Fig. B) decreased. This pattern is only observed in the top. meters, and below this level no trends are apparent. It is suggested that the groundwater level at the time of the fire may have been at approximately. meters depth, which resulted in significant dilution of the surfactant-loaded run-off waters at this depth and thus disaggregation of any surfactant micelles. This would explain the highest concentrations of PCDD/Fs occurring at. meters across all the ponds and also explain why the progressive trend towards higher chlorinated congeners with depth did not continue below this point. In contrast, samples collected one year prior to the cores contained slightly higher contributions of OCDD (and correspondingly lower contributions of lower chlorinated PCDD/Fs) (Fig. A and B). These samples were collected after the ponds had been drained and exposure to subtropical air temperatures may have resulted in some loss of more volatile, lower chlorinated congeners. The observed vertical concentration gradients and gradual PCDD/F profile shifts in the top. m indicate a fractionation of dioxin homologue groups through the pond cores, and suggest a higher mobility for higher chlorinated (i.e. more hydrophobic, less water soluble) PCDD/Fs. A) OCDD as a % of total PCDDs B) 9% 9% 9% 9% 98% % TCDD as a % of total PCDDs.%.%.% Depth (m).... Pond Pond Pond.. Fig.. Percent contributions of different homologues to sum PCDD concentrations in Ponds, and. Pond data are not included as insufficient core samples were available for trend analysis. Data are shown for core samples between surface layer and the depth where maximum concentrations occur. The remaining deeper samples are not shown as concentrations for most congeners were close to the LOD. Red symbols represent samples collected one year prior to the remaining core samples (black symbols). An increased mobility of chemicals with increasing hydrophobicity (and decreasing water solubility) contradicts the expected behaviour based on these physico-chemical properties. In the present study, the generally nonmobile PCDD/Fs migrated to substantial depths and further compared to relatively mobile pesticides (remediation monitoring data for pesticides by EPA not shown). Similarly, the least water soluble and most lipophilic PCDD/F congener, i.e. OCDD (log K OW = 8. 9 ) appears to have a greater migration potential compared to the slightly more water soluble and less lipophilic TCDD (log K OW,,,8-TCDD =.8 9 ). Such a reversal of mobility (and associated substantial migration of LMOCs) cannot be explained via leaching or movement with preferential water flow through root channels or clay fissures, but can occur in the presence of facilitators onto or into which the PCDDs can sorb/partition and undergo transport in water. Compounds that are known for their potential to facilitate migration of LMOCs include colloids (e.g. via sorption to DOM or humic substances) and non-ionic or anionic surfactants (via partitioning into amphiphilic micelles). Laboratory studies have shown that migration of PCDDs with anionic or non-ionic surfactants can result in a reversed mobility of the PCDD congeners, as observed in the present study -. While this phenomena has been clearly demonstrated in column experiments, relevant field data are lacking for PCDD/Fs. Stock records from the pesticide production facility in the present study showed that high volumes (>8, L) of different anionic and non-ionic surfactants (commonly used as adjuvants in pesticide formulations) were stored on-site prior to the spill event. In general, if surfactants are present in a soil-water system it is expected that concentrations above the CMC (critical micelle concentration; generally in the order of, ppm for a soilwater system ) have to be reached to facilitate transport of LMOCs via partitioning into the lipophilic surfactant micelle layers. Considering that the four containment ponds had a capacity of approximately megalitres, the Vol., 9 / Organohalogen Compounds page

5 release of surfactants stored prior to the spill were sufficient to result in concentrations above CMC (i.e. approximately, kg surfactants would be required to achieve CMC). The presence of surfactants near or above CMC is also supported by observations of extensive foaming of the wastewater run-off immediately after the event. In addition, pond soil subsamples analysed years after the contamination event for this study were found to still contain elevated anionic surfactant levels (.. μg/g dw), despite their relatively short half lives in water or soil (e.g. the anionic surfactant stored with the highest volumes at the site, calcium dodecyl benzene sulfonate, has a reported half life in soil in the order of days ). Hence surfactants represent the likely facilitators of LMOC transport at this site. The present study highlights that LMOCs, and in particular highly persistent organic compounds generally considered least mobile (e.g. PCDD/Fs), have the potential to migrate to substantial depths. The facilitators in this case are suggested to be amphiphilic surfactants, via partitioning to their lipophilic interiors and transported as contaminant-surfactant complex in water. Interestingly, this concept has found commercial application in engineering fields, where surfactants are introduced to contaminated sites to solubilise and remediate LMOCs under controlled conditions. In contrast, however, facilitated transport of LMOCs in surfactant mixtures at chemical spill and pesticide application sites is poorly understood or investigated, but may be more common than currently acknowledged. The consequence of neglecting the potential for this transport pathway may be severe, resulting in unrecognised or unexpected groundwater contamination and off-site transport of contaminants. Given this, the simple paradigm that LMOCs are non-mobile based on physico-chemical properties should be considered carefully together with LMOC application-specific and environmental factors that may have the ability to considerably change, or in the present case even reverse the predicted environmental fate of these chemicals. Acknowledgements We thank the Queensland Environmental Protection Agency for access to the study site and assistance with sample collection. Our thanks also to the staff at the Dioxin Analysis Unit, National Measurement Institute for invaluable advice and access to their laboratory space and equipment, and to the staff at Natural Resources and Water (Acid Sulphate Soil team, in particular Jeremy Manders and Ian Hall) for collection of the cores and ongoing support. This research was funded through an Australian Research Council Discovery grant (DP) with seed funding for development obtained through the University of Queensland. EnTox at The University of Queensland is co-funded by Queensland Health Forensic Scientific Services. References. Freeman, R. and Schroy, J. Chemosphere 989; 8:.. Hagenmaier, H., She, J. and Lindig, C. Chemosphere 99; :.. Pereira, W., Rostad, C. and Sisak, M. Environ Toxicol Chem 98; :9.. Brzuzy, L. and Hites, R. Environ Sci Technol 99; 9:9.. Prange, J., Gaus, C., Papke, O., Weber, R. and Muller, J. Organohalogen Compounds 9:.. Pavlov, D., Klyuev, N., Shelepchikov, A., Feshin, D., Brodskii, E. and Rumak, V. Doklady Earth Sciences ; :.. Hofmann, T. and Wendelborn, A. Env Sci Pollut Res ; :. 8. Persson, Y. PhD Thesis, Umeå University, Umeå.. 9. USEPA. Flury, M. J Environ Qual 99; :.. Carsch, S., Thoma, H. and Hutzinger, O. Chemosphere 98; :9.. Hsi, H.-C., and Yu, T.-H. Chemosphere ; :.. Schramm, K.W., Merk, M., Henkelmann, B. and Kettrup, A. Chemosphere 99; :9.. Haigh, S. Science of the Total Environment 99; 8:.. Ying, G.-G. Environment International ; :.. Paria, S. Advances in Colloid and Interface Science 8; 8:. Vol., 9 / Organohalogen Compounds page