The Fate and Behaviour of MTBE and BTEX in the Vadose Zone

Ivonne Bravo-Lopez, Imperial College, Exhibition Rd., South Kensington, London, SW7 2BP, Tel (+44) 207 594 4289, Email: a.bravo-lopex@imperial.ac.uk
Chris D. Collins, Imperial College, Exhibition Rd., South Kensington, London, SW7 2BP, Tel (+44) 207 594 7378, Email: c.collins@imperial.ac.uk

There is wide interest in the environmental fate and behaviour of MTBE and the BTEX compounds because they are major constituents of petrol and hence frequently discharged into the environment as a consequence of accidental spillages.  The vadose zone is often the first barrier to these contaminants before they reach the groundwater, where they can become more recalcitrant and subsequently create problems with drinking water supplies. In a range of column studies we observed that the downwards transport, interaction with the soil and evaporation of these compounds could be correlated with their physico-chemical properties. In degradation studies it was found that MTBE was metabolised more rapidly in a mixture with the BTEX than alone (t_0.5 = 1.7 day alone and 1.1 in mixture). It was also seen that TBA was produced as a direct consequence of this degradation.  These data were subsequently used to parameterise the HAZCHEM model and the model predictions compared with the laboratory data.

Mobility and Persistence of Petroleum Hydrocarbons in Peat Soils of Southeastern Mexico

Silke Cram, Instituto de Geografía, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F. C.P 04510, México, Tel: 0052-55-56224336, Email: silre@servidor.unam.mx
Christina Siebe, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F. C.P 04510, México,  Tel: 0052-55-56224286, Fax: 0052-55-56224317, Email: siebe@servidor.unam.mx
Rutilio Ortíz-Salinas, Instituto de Geografía, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F. C.P 04510, México, Tel: 0052-55-56224336
Andrea Herre, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F. C.P 04510, México, Tel: 0052-55-56224286, Fax: 0052-55-56224317

Spilled crude petroleum from oil wells contains numerous hydrocarbons, some of which are toxic and threaten life. We have studied the mobility and persistence of hydrocarbons in waterlogged soils that contain large proportions of fermented organic matter (Histosols) and large concentrations of dissolved organic carbon (DOC) in the State of Tabasco, Mexico. We sampled soil and phreatic water at sites polluted by oil spills for several decades, as well as at sites that had only recently (few weeks) been polluted, and compared their hydrocarbon contents with those of unaffected sites in the same area. Samples were analyzed for 16 non-alkylated polyaromatic hydrocarbons (PAHs) and n-alkanes from nC9 to nC34.

The spilled hydrocarbons had remained predominantly in the organic surface horizons of the soil where spillage occurred; there was little evidence of movement within the soil. The fraction of low molecular weight compounds was larger at sites of recent spills than where spills happened several decades ago. Nevertheless, sites of old spills still contained large concentrations of hydrocarbons, among which those of low molecular weight represented from 30 to 49% of total PAHs and from 50 to 84% of total n-alkanes, indicating that volatilization or microbial degradation is slow in these soils. In the peat horizons the measured organic carbon partition coefficients (K_oc) for the higher molecular weight PAHs were consistently smaller than those estimated by empirical equations by up to two orders of magnitude. The dissolved organic carbon of these peat soils seems to influence this behavior. At sites of old spills, partition coefficients for the PAHs were larger than at sites of recent spills.

Effects of Surfactants on Gas/Solid Partitioning of Herbicides

Wenli Yang, University of Connecticut, Environmental Engineering Program, 261 Glenbrook Rd, Unit 2037, Storrs, CT 06269-2037, Tel: 860-486-0356, Email: wenli@engr.uconn.edu
Britt A Holmén, Ph.D., University of Connecticut, Environmental Engineering Program, 261 Glenbrook Rd, Unit 2037, Storrs, CT 06269-2037, Tel: 860-486-3941, Email: baholmen@engr.uconn.edu

Many pesticides have been detected in air samples collected downwind of agricultural regions. However, little is known about the gas vs. particle transport behavior or the influence of surfactants commonly found in pesticide formulations on pesticide volatilization and partitioning with airborne particulate matter.  This study addresses these important soil-air exchange processes that can lead to widespread dispersal of pesticides in the environment.

Most herbicides are applied as formulation mixtures that consist of adjuvants (surfactants and solvents) in addition to the active ingredients. The presence of surfactants has been shown to affect pesticide partitioning in soil-water systems, therefore the addition of surfactant is anticipated to influence the partitioning of herbicide in soil-air systems such as in unsaturated soils and airborne soil-derived particulate matter. This study is the first to investigate the effects of surfactants on herbicide partitioning between the soil and air compartments. Seven herbicides from two families — chloroacetanilide and dinitroaniline — and alcohol ethoxylate (AEO) surfactants are being examined.

Desorption kinetics experiments were conducted in soil-gas flow systems at ambient temperature and relative humidity. Herbicide-spiked soil was allowed to dry prior to loading onto a filter in a stainless steel filter holder. A vacuum pump pulled clean and particle-free air through the spiked soil and the polyurethane foam (PUF) plugs that were used to capture gaseous herbicides desorbed from the soil. The PUF and soil samples were extracted by supercritical fluid extraction (SFE) and analyzed by GC/MS to quantify herbicide concentration in the gas and solid phases, respectively.

Preliminary results for metolachlor and pendimethalin showed more rapid release to the gas phase from the soil spiked with the formulated mixture compared to pure herbicides, and also greater overall desorption losses in the presence of the adjuvants. The surfactant effects on partitioning of the other five herbicides are being explored in our on-going research as a function of soil type, relative humidity and temperature.

Slow Desorption of Phenanthrene from Silica Particles: Influence of Pore Size, Pore Water, and Aging Time

Michael H. Huesemann, Pacific Northwest National Laboratory, Marine Sciences Laboratory, 1529 West Sequim Bay Road, Sequim, WA 98382, Tel: 360-681-3618, Fax: 360-681-3699, Email: michael.huesemann@pnl.gov
Timothy J. Fortman, Tom Hausmann, Robert G. Riley, Ph.D., Christopher J. Thompson, Ph.D., Zheming Wang, Ph.D., Michael J. Truex, Pacific Northwest National Laboratory, and Brent Peyton, Ph.D., Washington State University

In an effort to better understand the environmental fate and transport of aged petroleum hydrocarbons in aquifer solids, we performed a series of sorption and desorption experiments using phenanthrene as a model hydrocarbon compound and porous silica particles as model aquifer solids. When micro-porous and meso-porous silica particles were exposed to aqueous phenanthrene solutions for various durations it was observed that sorbed-phase phenanthrene concentrations increased with aging time only for meso-porous but not micro-porous silicas. Desorption equilibrium was reached almost instantaneously for the micro-porous particles while both the rate and extent of desorption decreased with increasing aging time for the meso-porous silicas. These findings indicate that phenanthrene can be sequestered within the internal pore-space of meso-porous silicas while the internal surfaces of micro-porous silicas are not accessible to phenanthrene sorption, possibly due to the presence of physi- or chemi-sorbed water that may sterically hinder the diffusion of phenanthrene inside water-filled micro-pores. By contrast, the internal surfaces of these micro-porous silicas are accessible to phenanthrene when incorporation methods are employed which assure that pores are devoid of physi-sorbed water. Consequently, when phenanthrene was incorporated into these particles using either supercritical CO₂ or via solvent soaking, the aqueous desorption kinetics were extremely slow indicating effective sequestration of phenanthrene inside micro-porous particles. A two-compartment conceptual model is used to interpret the experimental findings and assess the bioavailability and risk of aged petroleum hydrocarbons in groundwater aquifers.

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