Officials looking into last week's Burns Bog fire in British Columbia's Lower Mainland have determined it was most likely caused by human activity.

Police working with the B.C. Forest Service and the Delta Fire Department have pinpointed the ignition point to a grassy area near the 6100 block of 104 Street.

Investigators say that while the specific cause may never be known, a carelessly discarded cigarette butt or a spark from an all terrain vehicle seems the most likely source of the blaze.

The fire then spread to a treed area of the bog.

Eliza Olson is president of the Burns Bog Conservation Society, a group that styles itself the unofficial guardians of the 4000 hectare natural area.

She says the area that burned, over 200 hectares of the largest undeveloped urban landmass in North America, was drier than usual.

She attributes this to the Municipality of Delta lowering a beaver dam within the bog at the request of local farmers.

Olson claims the farmers expressed concerns earlier this summer that water from the bog might intrude into their fields causing problems for agricultural operations.

Olson says the dam helped to retain water within the bog and lowering it allowed the area that burned to dry out.

She points to experience gained from a similar fire in 1996 which indicates most of the burned area will start to regenerate within six weeks.

However, she expects it will take much longer for the bog to recover to its state before the fire started on September 11.

Olson says, “One of the important things about this bog is that it's a huge carbon sink. It's probably the largest carbon sink next to any major city in North America. And with Canada honoring the Kyoto protocol this is one of the reasons to protect the bog.”

She adds, “Less than three per cent of the earth's surface is covered by peat bogs. Yet they store three times as much carbon and produce more oxygen than a tropical rain forest.”

According to the Municipality of Delta's website, the fight to extinguish the fire has moved into the mop-up stage.

Officials anticipated that a pull-out plan would be in place by Wednesday and that ongoing monitoring of the site will occur after that time.