Abstract
Studies of the migration of organic contaminants in shallow aquifers impacted by landfill leachate at six sites in Ontario are reported. Three sites are located on very permeable sand deposits, one on less permeable sand till and two on fractured sedimentary bedrock. The migration rate and persistence of volatile, one-and-two carbon, halogenated hydrocarbons (halocarbons) and volatile aromatic hydrocarbons are emphasized. These compounds are ubiquitous in sanitary landfill leachates and are quite mobile in groundwater. They are at very low concentrations (less than 5 ppb each) at the Borden landfill site, where most waste was burned before landfilling.
At the Woolwich site, volatile halocarbons are found at very low concentrations (less than 0.5 ppb each) up to one kilometer from the site, indicating that they may be very mobile and persistent in this aquifer. Attenuation, probably due mainly to dispersion, has resulted in only sub-ppb concentrations persisting beyond two hundred meters of the site.
The contaminant plume at North Bay has been discharging to the surface about eight hundred meters from the site for a number of years. Some mobile volatile organics, therefore, are found throughout the plume. Halo-carbons do not persist and some aromatics appear to be undergoing biodegradation as well. For these and other contaminants, dramatic attenuation is observed within the eight hundred meter plume, probably as the result of dispersion.
Groundwater velocities in the less-permeable sand and sand till at the new Borden site are much lower than in the other aquifers, so contaminants have only migrated perhaps two hundred meters laterally. Volatile halocarbons may be migrating at the groundwater velocity, while some retardation of aromatics may be occurring. However, the erratic contaminant distribution complicates the consideration of contaminant migration.
Contaminant distributions are irregular in fractured bedrock at the Bay-view and Hamilton sites. The irregular and generally low concentration of halocarbons, coupled with the generally-poor background water quality in these bedrock flow systems, makes the definition of the zone of contamination at these sites very difficult. Although these low-porosity carbonate/ shale bedrock systems could distribute leachate contamination through a large volume of rock, it is encouraging to note the rather restricted zone of clearly-impacted groundwater. The major, mobile organic contaminants at the Hamilton site are the volatile aromatic hydrocarbons. Recognition of only-slightly-impacted groundwater at this site is complicated by the occurrence of these organics at ppb levels in apparently uncontaminated, background groundwater.
Temporal variations, over weeks and years, are found for all contaminants at these sites. Input from the landfill appears to be temporally variable and so is a major cause of subsequent variations within the leachate plume. The processes of dispersion, which smoothes such variations at some sites (Borden), does not appear to be effective at damping temporal variability along the plume at North Bay nor in the fractured-bedrock systems.
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