MACKINAC COUNTY, MI -- In 1990, a helicopter patrol spotted a patch of oil on the ground about a mile south of Millecoquins Lake near Engadine. The oil was from Enbridge Line 5, which had spilled 630 gallons through a pinhole leak.

That spill is among almost 30 spills along the pipeline -- many of them previously unknown or largely forgotten incidents -- unearthed in federal records by National Wildlife Federation (NWF) pipeline safety specialist and researcher Beth Wallace.

The organization released the results of Wallace's research this week, estimating that Line 5, which runs from Superior, Wis., to Sarnia, Ontario by way of Michigan, has spilled at least 1.13 million gallons of oil in 29 incidents since 1968.

The data comes from inspection records obtained by the NWF through the Freedom of Information Act and others put online recently by the Pipeline Hazardous Materials Safety Administration (PHMSA), which Congress has required to make more information accessible to the public.

The NWF considers its estimate conservative because the research identified known spills, like a 1980 spill in Hiawatha National Forest, that weren't in federal records. Regulatory requirements during much of the 1980s stipulated Enbridge only had to report spills to the state, not the federal government.

Many incidents were related to construction mishaps. Others were caused by manufacturing defects in the pipe, such as stress cracking along a seam.

The most common theme was the method of spill discovery.

"Only one spill I could find was discovered by leak detection systems," said Wallace, a Pipeline Safety Trust board member who co-authored the NWF's 2012 "Sunken Hazard" report that helped galvanize scrutiny on the Line 5 section under the Straits of Mackinac.

Many of the spills incident records do not say how the leak was initially detected. The remainder were found by the public or Enbridge staff on the ground.

The NWF says that's a troubling reminder of the colossal failure that caused the Line 6B spill into the Kalamazoo River in 2010, when the company didn't notice the pipeline had ruptured until being alerted by an outside caller 17 hours later.

The NWF has taken an aggressive stance against the pipeline, fighting the government and Enbridge in court to shut down the controversial submerged Line 5 segment. The group says the amount of manufacturing and construction defects and weld failures revealed by Wallace's research call into question the overall integrity of the Line 5 system.

"This newly-released data shows a worse history of spill and detection systems failing over time," said Mike Shriberg, NWF Great Lakes regional director, who also sits on the state of Michigan's Pipeline Safety Advisory Board.

Enbridge spokesperson Ryan Duffy dismissed the findings.

"This is not new information and we have addressed this issue many times in the past," Duffy said via email. "Over the past fifteen years, there have been three incidents on Line 5 that have resulted in a total of approximately 21 barrels of product being released off the mainline. All of the product released during these three incidents was recovered."

"There has never been an incident on Line 5 at the Straits."

Duffy did not respond to questions about whether Enbridge disputed any of the estimates reached by the NWF or why only the last 15 years of spill history is a relevant benchmark for a pipeline that began operating in 1953.

Although the line is 64 years old, 1968 is the earliest year of data available

The most recent spill is a small one, about 8 gallons, caused by an equipment failure on March 5, 2015 near Marenisco that Enbridge staff discovered while conducting a station review. The cause was recorded as a seal that failed due to "normal wear and tear."

According to the data, the largest spills happened earlier in Line 5's history -- apart from a fairly well known 222,600-gallon oil and natural gas liquid spill near Crystal Falls in 1999, caused by the line lying on a rock, which forced the evacuation of about 500 people after responders ignited a vapor cloud that sparked a 36-hour long fire.

In 1968 near Lake Gogebic, a 285,600-gallon spill was caused by a weld failure, but PHMSA records lack more detail beyond "equipment rupturing pipe."

In 1972 near Iron River, a 252,000-gallon spill was caused by a longitudinal weld failure. A resulting fire injured two people.

Two people were also injured a few years later in 1976 when another 210,000-gallon spill near Lake Gogebic was caused by a "pipe failure."

The 1990 spill spotted by chopper patrol is actually one of two along the pipeline near Engadine. The other occurred in 1972 when a defective weld caused a 4,200-gallon spill just south of the Krause Road and Hiawatha Trail intersection, near the headwaters of O'Niel Creek, which flows south a few miles into Lake Michigan.

Wallace's familiarity with the pipeline and regulatory jargon were key in ferreting out Line 5 spills in the PHMSA records, which in some cases required knowing "what kind of mile markers Enbridge uses to read notes in the database."

Wallace said mapping the data reveals incident clusters that may indicate potential chronic problems with certain sections of pipe, which is not "seamless" through most of its path through Michigan -- the exception being under the straits -- but rather rolled like a cigarette and welded together with a seam.

On its 645-mile journey from Wisconsin to Ontario, Line 5 crosses 23 counties and 360 Michigan waterways.

"The Kalamazoo River spill didn't happen in the river," she said. "The oil made its way there though a major tributary."

Wallace said the number of spills caused by stress cracking along the longitudinal pipe seam is a major red flag. That kind of failure caused the Line 6B disaster.

Such stress cracking along the seam isn't limited to the Upper Peninsula section. The NWF released 2011 inspection photos obtained through FOIA that show stress cracking along the longitudinal seam between Bay City and Sarnia.

A FOIA photo from a March 13, 2011 Enbridge inspection report shows stress corrosion cracking the length of the longitudinal seam on a section of Line 5 located between Bay City and Sarnia.

"That can be very dangerous," she said. "That's a big problem for the overall pipeline if it's system-wide."

"We should be talking about this as we consider long term planning for this pipeline."

The NWF research comes ahead of a pair of state-ordered studies on Line 5 commissioned by the pipeline safety board that are expected out in June.

Meanwhile, Michigan legislators on both sides of the aisle are targeting the pipeline. Sen. Rick Jones, R-Grand Ledge, has reintroduced a bill from last session that would require all existing submerged pipelines undergo a third-party review and be shut down if the risk is too great.

Michigan House Democrats say they are preparing a bill package that would tighten pipeline regulation and give the Michigan Department of Environmental Quality regulatory oversight of pipelines crossing the state.

In March, Enbridge admitted that parts of Line 5 under the straits had lost its outer wrap anticorrosion coating, although there's no bare metal exposed.

Enbridge says it plans to test Line 5 under the straits at its original 1953 pressure, install more anchor supports and take a closer look at the defective coating.