Abstract

Inadvertent release of petroleum products into the subsurface often results in contamination of groundwater. The contaminants of particular concern are the common, water soluble and mobile aromatic hydrocarbons: benzene, toluene, ethlybenzene, and xylene isomers, as a group termed BTEX. When dissolved in groundwater, the migration and persistence of these organics can be described in terms of the processes of advection, dispersion, sorptive retardation, and biodegration.

The migration of BTEX in groundwater has been studied in a highly instrumented, well characterized, shallow, sand aquifer at Canada Forces Base, Borden, Ontario. Contaminated water was injected below the water table and the migration and persistence of BTEX and a chloride tracer under natural flow conditions were followed by detailed sampling and analysis of groundwater samples along the natural migration route. The organics migrated slightly slower than the groundwater, reflecting sorptive retardation in which the organics reversibly sorbed onto aquifer organic matter. While the zone of groundwater containing chloride tracer expanded over time due to dispersion, the zone containing the organics shrunk and the total mass of organics decreased to below detection in about 700 days. The major process limiting organic contaminant migration was aerobic biodegradation. This process was in turn controlled to a large extent by the hydrogeological processes of advection and dispersion, which limited the rate at which oxygenated groundwater mixed with the BTEX contaminated groundwater.

Mathematical models which correctly represent these processes were used to demonstrate the likely behavior of BTEX in other hypothetical, shallow groundwater systems. A very permeable sand/gravel aquifer and a less permeable silty sand aquifer were simulated along with the Borden sand aquifer. A continuous source of BTEX was assumed near the water table. This could represent a stationary pool of gasoline resting on the water table. In the sand/gravel aquifer, the contaminated zone quickly spread horizontally. The sand aquifer and the silty sand aquifers produced contaminated zones extending just over 100m and about 20m, respectively. An apparent steady state was attained in the sand aquifer, in which biodegradation removed the organic contaminant as quickly as it was introduced. In all cases the mixing of oxygenated groundwaters with the BTEX contaminated waters was the most significant factor limiting the spread of BTEX contamination.

In aquifers where considerable mixing of oxygenated groundwaters with contaminated groundwaters is likely, the natural attenuation processes documented in these studies may be sufficient to limit the extent of contamination. They may also be sufficient to passively remediate relatively small dissolved contaminant plumes. More persistent contamination will usually require more active remediation. Considerable effort may be required to characterize a contaminated aquifer sufficiently to confidently evaluate the natural processes that limit BTEX contamination.