Facilitated Transport of Dioxins in Soil Following Unintentional Release of Pesticide-Surfactant Formulations

Abstract

Colloids such as surfactant micelles can act as transport facilitators for highly lipophilic, generally immobile contaminants in soil. Following a fire at a pesticide facility, this study investigated vertical and lateral migration of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in heterogeneous soil beneath bunded ponds, where contaminated wastewater containing high surfactant loads was stored until remediation. Initially, surface and subsurface soil was obtained during excavation, and subsequently intact cores to 5.7 m were collected. ΣPCDD/F concentrations were elevated in the wastewater (15-81 ng/L) and correspondingly in pond surface soils (6.1-61 ng/g). Maximum ΣPCDD/F concentrations were, however, observed at 2-2.5 m depth (68-130 ng/g), far below their expected mobility range based on physicochemical properties. Congener specific analysis further indicated that PCDD/F mobility was reversed, with the least water-soluble congener migrating to the greatest extent. The presence of higher chlorinated PCDD/Fs throughout a core collected in the direction of groundwater flow indicated subsequent lateral transport. These results provide field evidence for rapid vertical migration (2.4 m in <4 months) of highly lipophilic PCDD/Fs and suggest surfactant facilitated transport as the dominant transport mechanism. Quantification and evaluation of such fundamental changes in contaminant transport and fate in the presence of surfactants is required to identify areas at risk of groundwater contamination.

Isomer-specific investigation of PCDD/F mobility and other fate processes in deep soil cores

“…Of these, cores A1 and Q1 were analysed at the National Measurement Institute, Sydney following methods detailed previously (Grant et al, 2011). Cores B1-B4 were analysed at Empa, Dübendorf and the analytical procedure as well as the OCDD concentrations have been published previously (Camenzuli et al, 2014).…”

“…The remaining 3 cores (A2, M1 and M2) were analysed for OCDD based on a screening method described previously (Gaus et al, 2001) which is also further described in Section S1-SI. For soils throughout Queensland, a strong correlation (r 2 = 0.98) has been established between levels of OCDD (the dominant PCDD/F congener which also has the highest migration potential with facilitators (Schramm et al, 1995;Grant et al, 2011)) and RPCDD/F concentrations (Gaus et al, 2009). RPCDD/F concentrations in Queensland soils can therefore be estimated with low uncertainty using the measured OCDD levels.…”

“…These include hydrology, climate, soil properties, as well as the type and ubiquity of facilitators present (de Jonge et al, 2004;Paria, 2008). The majority of field evidence from remote (Brzuzy and Hites, 1995;Krauss et al, 2000;Moeckel et al, 2008;Bergknut et al, 2010Bergknut et al, , 2011, agricultural (Prange, 2003;Wang et al, 2004;Cheng et al, 2014) or contaminated (Persson, 2007;Yang et al, 2010;Grant et al, 2011) soils report HOC migration typically to depths only up to 0.5-1 m. At most sites, DOC or particulate organic matter (POM) has been proposed as the responsible facilitators. Although scarce, more extensive migration has also been attributed to facilitated processes with surfactants (Grant et al, 2011).…”

“…for historical contamination). Characterising complex natural soil systems and subsurface hydrology is costly and often unfeasible, and responsible facilitators can seldom be identified in retrospect (Grant et al, 2011). However, previous field studies have demonstrated the utility of assessing congener-specific subsurface distributions of HOCs to further understanding of various post-depositional fate and mobility processes (Guggenburger et al, 1996;Krauss et al, 2000;Moeckel et al, 2008;Bergknut et al, 2010).…”

“…Fate and transport processes are generally governed by contaminant physico-chemical properties and thus result in compound-selective changes. For example, preferential partitioning and associated transport of more hydrophobic HOC congeners with facilitators has been shown to increase their relative abundance with soil depth (Schramm et al, 1995;Kim et al, 2002;Grant et al, 2011). For PCDD/Fs, reported profiles are typically limited to 2,3,7,8-chlorinated congeners.…”