BEMIDJI CRUDE OIL SPILL

Darren Cartwright

Stephen Toone

Background• In 1979, crude oil from a pipeline rupture

contaminated a shallow aquifer in Bemidji, Minnesota

• After initial clean-up, 400’000 litres of oil remained in the unsaturated zone and near the water table

• This continues to be a source of contaminants to a shallow outwash aquifer

• Results of research collected on this site have provided fundamental knowledge that has been used to remediate similar sites worldwide

Aerial View of Site

Description and History of Site• Land surface and shallow sub-surface were contaminated

when the pipeline burst spilling 1’700’000 litres of crude oil onto a glacial outwash deposit

• Crude oil also sprayed southwest covering 7’500 m2 of land• After initial clean-up some crude oil had percolated through

the unsaturated zone to the water table near the rupture site

• Some of this oil flowed over the surface towards a small wetland forming a second area of significant oil infiltration

Features of the Research Site

The Project• A long-term, interdisciplinary research project sponsored by

the USGS Toxic Substances Hydrology Program began in 1983

• The research is directed towards understanding the physical, chemical and biological processes controlling the mitigation and fate of hydrocarbon contaminants in the subsurface

• The goal is to provide information and methods to help evaluate the potential for enhanced bioremediation of hydrocarbon contamination worldwide

• As of 1998, over 370 wells and test holes have been installed

Research Results• The fate, transport and multiphase flow of hydrocarbons

depends on geochemical processes and on processes of volatilisation, dissolution, biodegradation, transport and sorption, which occur as shown on the next slide

• Multi-phase flow modelling was used to study oil flow after the spill

• Transport and biodegradation modelling was used to simulate the evolution of the plume, evaluate factors limiting biodegradation and develop a mass-balance for contaminants

• This could be used to evaluate the amount and rate of removal of hydrocarbons by biodegradation

Geochemical Processes

Research Results – Oil Phase• As of 1996, leading edge of oil plume had

moved 40m downstream• Degradation of oil product has resulted in

selective loss of soluble and volatile compounds through dissolution and volatilisation

• Total loss of oil estimated at 11% between 1979-89

Research Results – Aqueous Phase

• Although geochemical processes that were predicted have occurred, the plume has not moved as far as predicted considering groundwater flow velocities and sorption constants

• As of 1996, the contaminant plume had moved 200m compared with the groundwater that had moved 500m

• The primary reason for this was that the hydrocarbons have degraded under aerobic/anaerobic conditions

• This lead to the view that the rate of removal of organic contaminants by natural attenuation and the factors that affect rates of biodegradation are important considerations in making decisions concerning clean-up of contaminated groundwater

Research Results – Vapour Phase• Distribution of gases in the North Oil Pool

has changed considerably:• 1985 – leading plume 150m downstream• 1997 – leading plume 75m downstream

• This is also due to aerobic degradation

Geochemical Zones in North Oil Pool

• Zone 1 – Oxygenated uncontaminated native groundwater• Zone 2 – Low oxygen concentrations, high concentrations of DOC• Zone 3 – Anaerobic plume of groundwater• Zone 4 – Low concentrations of hydrocarbons due to aerobic degradation• Zone 5 – Oxygenated water

Research Results• Biodegradation of petroleum derived hydrocarbons in

aerobic conditions is generally considered to be more efficient than anaerobic conditions

• Although anaerobic conditions can still remove substantial amounts of contaminant

• The next slide shows the microbial geochemistry of the oil plume

Contaminant Plume’s Microbial Geochemistry

Contributions of Research• Provided a comprehensive documentation of natural

attenuation of hydrocarbons in a contaminated aquifer under both aerobic and anaerobic conditions, promoting a widespread adoption of this procedure to remediate other contaminated sites

• This research may result in less expensive remedial actions on similar sites

• New methods were developed in testing saturated soils such as the ‘Freezing Drive Shoe’

• A two-dimensional, multispecies solute-transport model code

with biodegradation (BIOMOC) was developed to quantify natural rates of biodegradation; this code can be readily applied to other (similar) sites

Conclusion• This was the first and best example of intrinsic

bioremediation – where contaminated ground is remediated without human intervention and is now widely used

• Toxic chemicals leaching from crude oil can be rapidly degraded by natural microbial populations

• It was shown that plumes of contaminated groundwater stop enlarging as microbial degradation came into balance with rates of contaminant leaching

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