Abstract

Conventional clean-up approaches to contaminated wetlands are aggressive and expensive, and may harm the wetlands ecosystem. Current findings suggest that wetlands may be capable of attenuating contamination through natural processes including absorption, adsorption, biodegradation, and volatilization. The objective of the study was to conduct a field evaluation of the role of these attenuation mechanisms at four contaminated natural wetlands in Alberta. A second focus of the study was to evaluate contaminant impact on the wetlands vegetation.

The results indicate that contaminant attenuation in wetlands are influenced by several parameters, including depth of impact, contaminant composition, presence of peat, flow rates, and seasonal effects. Overall, natural processes appear to be completely attenuating hydrocarbon contamination at two of the four wetlands study sites. At the other two sites, minor concentrations were reaching down gradient receptors. Impact on vegetation was observed at three of the four sites, primarily through mortality of black spruce. The impact on shallow vegetation was generally negligible.

Introduction

Wetlands contamination by hydrocarbons as a result of upstream oil and gas activity is well known, both in Alberta and worldwide. Conventional clean-up approaches (e.g. excavation, trenching, and bell holes) are aggressive and expensive, and may harm the wetlands ecosystem. A more passive, longer term solution may be to focus on containing or removing the source, and allow natural processes to remediate the remaining contamination. This approach, known as intrinsic remediation or natural attenuation, has been shown to be effective in various scenarios, from fuel-contaminated groundwater at Air Force Bases to oil spill-impacted shorelines.

There has been little investigative work on the subject of intrinsic remediation in hydrocarbon-impacted wetlands. Current findings suggest that wetlands may be capable of retarding and potentially depleting contaminant plumes through natural remediation processes including: absorption, adsorption, biodegradation, and volatilization.

Wetlands are typically water-saturated environments with a high percentage of peat. Peat is an organic-rich material containing partially to highly decomposed plant fibers, roots, and mosses (Mitsch and Gosselink, 1993)1. Wetlands containing peat have several unique attributes, which enhance contaminant attenuation in the following ways:

  • absorption: The peat matrix is highly porous, (up to 70%) and has been found to be capable of absorbing up to eight times its weight in oil (D' Hennezel and Coupal, 1972)2.

  • adsorption: Peat contains approximately 40% organic carbon by weight. Laboratory testing completed by Tinh et al. (1971)3 indicates that peat's adsorptive capacity is between 30 to 50 % of granular activated carbon. Peat is therefore an excellent filter for organics.

  • biodegradation: Due to their high organic content and biodiversity, wetlands exhibit microbial activity exceeding most natural environments (Hammer, 1992)4. The enhanced microbial activity results in greater potential for biodegradation, whereby microbes stimulate the breakdown of organics such as hydrocarbons to form CO2, water, and biomass.

  • volatilization: Volatilization describes the partitioning of hydrocarbons from the soil phase, dissolved phase, and/or liquid phase into the vapor phase. The rate of volatilization is controlled in part by the surface area per unit volume of the soil media (Arthurs et al., 1995)5.

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