In March 1950, four Alberta “pipeline walkers” spoke with a reporter from Canadian Press about their tireless work. Each worker walked twelve to fifteen miles per day, checking on pipeline facilities in the Edmonton district and looking for leaks, a consistent problem for Alberta’s booming oil industry in the mid-twentieth century. A day’s work was long, exhausting, and ultimately fruitless. The reporter noted that 18 separate leaks occurred in the district in 1949 with one substantial oil spill near Leduc where “several thousand barrels of oil escaped before the leak was discovered.” Considering the number of leaks to occur in the district in spite of the walking inspections, the report captured the futility of the men’s labour: “Resembling northern trappers walking their trap lines, the men walk miles over their designated routes the year around. It’s a monotonous job â€” few leaks are found.” After walking more than 800 miles on the job, Dick Caws, one of the “pipeline walkers,” confessed, “The funny thing about my job is that I’m supposed to be looking for oil leaks, but since I started last October 1 I haven’t found one.”  Sixty-five years later, a contractor discovered one of the largest leaks on an oil pipeline in Canadian history, using the same method of detection.
Following Imperial Oil’s discoveries of substantial oil and gas resources in Leduc and Redwater in the late 1940s, oil companies and their pipeline subsidiaries began to build the province’s modern oil and gas pipeline systems, an extensive transportation network that now consists of nearly 400,000 km of energy pipelines.  Those companies also built Canada’s first long-distance pipelines for the delivery of Alberta crude and natural gas to urban industrial markets in Vancouver, central Canada, and parts of the US. These conduits for the movement of oil and natural gas unlocked Canada’s transition to a new energy regime in the second half of the twentieth century that saw the country become one of the world’s highest per capita consumers of high-energy fossil fuels. According to Richard Unger and John Thistle, “Canada and Canadians have long used relatively large quantities of energy and that total energy consumption in the country has risen dramatically over what is historically, and compared to many other countries, a short period of time.”  This would not have been possible without the development of large-scale pipeline infrastructure.
In the past two decades, most of the oil production activity in Canada has taken place in the northern Athabasca region of Alberta where corporations extract and transport enormous quantities of bitumen for processing into synthetic crude oil. As Canadians learned late last week with the news that a feeder pipeline operated by Nexen released more than five million litres of bitumen emulsion, the problem of oil pipeline spills is endemic to the industry and continues into the present. According to early media reports, the massive spill, one of the largest in Canadian history, was first spotted by a contractor who was walking along the pipeline.
As evidence in my previous articles on this topic has shown, oil pipeline spills have been a consistent environmental hazard in Canada since the mid-twentieth century. They are a symptom of the broader transition in Canada from an economy based on traditional fuels (wood, animal, coal) to high-energy fossil fuels (oil, natural gas) that took place in the years after 1947. Several sources of contemporary and historical evidence reveal that such spills in Canada have occurred frequently over time. For example, the annual spill rates on two of the longest oil pipelines regulated by the federal National Energy Board (NEB) illustrate the regularity of such incidents:
- Trans Mountain Pipeline, 1961-2013: 81 oil spills reported to NEB; 1.53 spills per year
- Interprovincial Pipeline, 1962-1996: 190 oil spills reported to NEB; 5.43 spills per year 
Data on incidents along oil pipelines regulated by the province of Alberta were obtained by a Global News investigation in 2013, showing an astonishing rate of spills for the period from 1975 to 2012. Investigative reporters found 28,666 crude oil spills for an average of nearly 775 per year or 2 oil spills every day for a period of 37 years.
When considering pipeline incidents in the recent past, the evidence is still sobering. In 2013, Alberta Energy Regulator (AER) — the provincial regulator of oil pipelines within the borders of the province — published an analysis of pipeline performance from 1990 to 2012. It found that over this period, there were 6,488 separate spill events involving the release of liquid hydrocarbons into the environment, an annual average of 282 spills per year. In 2012 alone, there were 234 liquid hydrocarbon releases.
But as the Nexen spill last week illustrates, size also matters when it comes to assessing the risks associated with transporting oil via pipelines. Across different parts of Canada, oil pipelines have ruptured in several massive on-shore spills in the past. Numerous media reports accurately identified this as “one of the worst leaks in Canadian history.”Â By volume, this might be the third largest on-shore liquid hydrocarbon spill in Canadian history (at least according to AER and NEB records). In 1980, a pipeline operated by Pembina Pipeline Corporation ruptured and spilled an estimated 6.5 million litres of crude oil east of Valleyview, Alberta. The spill drained into a nearby stream or river, according to provincial records.  The second largest on-shore oil spill by volume occurred in Manitoba in October 1967 along Line 3 of the Interprovincial Pipeline near Glenboro. The pipeline bulged and burst during a pressure test, spewing 5,247,000 litres. Less than one month later, a section of Line 3 failed again during a pressure test, spilling 2,544,000 litres of crude oil. 
Large-scale oil spills make up a small percentage of the total number of oil spills on pipelines regulated by AER and the NEB. However, they have the potential to result in the most significant adverse environmental effects. As the Auditor General of Alberta noted in a report on pipeline safety and regulation last March, “Not all incidents have equal impact.”
Still, numerous large-scale oil spills have occurred in the past with considerable frequency. For example, the Interprovincial Pipeline reported 16 separate incidents that resulted in uncontrolled spillage of more than 1 million litres of liquid hydrocarbons between 1962 and 1996, almost one such incident every two years. And in Alberta, on pipelines regulated by AER, there were 25 crude oil and crude bitumen spills between 1975 and 2012, nearly one large-scale spill every one and a half years.
As I told Bill Tieleman in a interview in The Tyee nearly three years ago, there is no leak-proof oil pipeline system. Oil is an environmentally hazardous material and, as such, its transportation creates risk. Technological improvements have helped mitigate that risk, but as the Nexen spill and many others in the past have shown, technology cannot prevent all circumstances that lead to on-shore oil spills. Ultimately, the environmental damage associated with oil pipeline spills is a product and function of the mass consumption of oil and the energy regime transition that began in Canada (and other parts of the world) in the second half of the twentieth century and continues into the present.
 “These ‘Oil Men’ Hope They Never Strike a Gusher” Winnipeg Free Press, 6 March 1950, 9.
 Energy Resources Conservation Board, “ST57-2013 Field Surveillance and Operations Branch â€“Â Field Operations Provincial Summary 2012” (Calgary: June 2013), 9.
 Richard W. Unger and John Thistle, Energy Consumption in Canada in the 19th and 20th Centuries (2013), 13.
 These figures were derived from statistical analysis of a pipeline incident database from the National Energy Board library with data for the period 1950 to 1996. It also includes analysis of a second database obtained and published by the Canadian Broadcasting Corporation via an access-to-information request covering a period from 2000 to 2012. It made the database available here: http://www.cbc.ca/newsblogs/community/editorsblog/2013/10/digging-for-data-on-pipelines.html. Finally, data was also drawn from spill data released by Kinder Morgan for the Trans Mountain pipeline available here:Â http://www.transmountain.com/uploads/pages/1422040950-Dec-2014-Spill-Chart-for-posting-FINAL.pdf
 For details, see the pipeline incident database acquired by Global News in 2013 here: http://globalnews.ca/news/622513/open-data-alberta-oil-spills-1975-2013/
 â€œPipeline related incidents under N.E.B. jurisdiction,â€ Interprovincial Pipe Line Company, National Energy Board Library, TN 879.5/C16, p. 1 of 30.