Benzene, Toluene, Ethyl Benzene and Xylene (BTEX) are the volatile components commonly associated with petroleum products. Most petroleum contaminated sites are located within modified environments, and the primary requirements for ecological protection relate to the protection of off-site environment quality and to the associated ecosystems. 

In New Zealand, soil acceptance criteria have been developed for BTEX, representative polyaromatic hydrocarbons (PAHs); pyrene and benzo-a-pyrene, and total petroleum hydrocarbons (TPH), based on the protection of human health and have been derived for a number of land uses, soil types and depth of contamination (MfE, 1999). 

Asthetic impact, protection of terrestrial ecosystems and protection of groundwater quality have not been considered in the derivation of these criteria and need to be addressed on a site-specific basis. Thresholds for the protection for groundwater quality are given as separate soil acceptance criteria in these guidelines to help evaluate future off-site impacts.

Efforts to establish soil criteria for petroleum hydrocarbons in soil are complicated by a number of variables such as the:

• altered toxicity of aged constituents

• role of the oil phase in altering bioavailability

• lack of information on bioaccumulation of particular constituents

• dependence on uptake by biota and toxicity on soil characteristics

• variability and uncertainty in ecotoxicity of different constituents of petroleum mixtures

• presence of co-contaminants at field locations where adverse ecological impacts may be observed (Efroymson, 2000). 

Countries like Canada are developing a nation-wide standard for petroleum hydrocarbons which will contain action levels for soil contaminants. This standard was not available at the time of publication.

There are very few studies reported in the international literature on the ecological impacts of BTEX compounds to soil flora and fauna. Mostly, BTEX is volatilised from the surface of the soil after a spill, and these monoaromatic hydrocarbons are thought to be primarily a human health concern. Risks to groundwater is considered the next most important due to the high relative mobility of BTEX through soils.

We conducted some pilot studies to determine ecotoxicoity values for BTEX to soil invertebrates and plants using standardised methodologies. However under the standardised conditions prescribed by the testing methodologies we were unable to obtain consistent soil concentrations, as these compounds were rapidly volatilised from the test soil. For example, soil samples individually spiked with 2000 ppm of xylene,ethylbenzene or toluene, showed the following losses with time:

• Xylene: after 30 minutes - up to 50% loss, after 6 hrs in an open test vessel up to 89% loss

• Ethylbenzene: after 30 minutes - up tp 65% losss, after 6 hours in an open test vessel up to 98% loss

• Toluene: after 30 minutes - up to 83% loss, after 6 hours in an open test vessel up to 99% loss.

These results are in accordance with Salanitro and colleagues who report that 40 - 95% of BTEX hydrocarbons applied to a soil sample were volatilised during the soil preparation process (Salanitro et al., 1997). Therefore, unless volatilisation can be contained or prevented, it is difficult to define consistent toxicity relationships between BTE and X and the fauna and flora inhabiting the topsoil. Currrently, such conditions contravene the prescribed moisture conditions for invertebrate or plant culture. Until more sophisticated standards and systems for testing monoaromatics are designed, we beleive that the derivation of ecological tolerance levels for BTEX as stand alone compounds is largely unachievable.

References

Efroymson, E 2000 Contaminated Soils Advisory Group. SETAC Globe 1 (2):57.

MfE, 1999: Ministry for the Environment - Guidelines for assessing and managing petroleum hydrocarbon contaminated sites in New Zealand. Module 4 - Tier 1 Soil screening criteria.

Salanitro, J. P., Dorn, P. B., Huesemann, M. H., Moore, K. O., Rhodes, I. A., Rice J., L.M., Vipond,T. E., Western, M. M., Wisniewski, H. L. 1997: Crude oil hydrocarbon bioremediation and soil ecotoxicity assessment. Environ.Sci.Technol. 31:1769-1776.