BOULDER, Colo. -- Boulder County Sheriff Joe Pelle said back-to-back major wildfires are putting a strain on the county's budget.

Even after fundraisers to assist victims and volunteer firefighters, fire districts and the county may struggle because of lost property taxes from the homes burned in the Four-Mile Canyon Fire in September, Pelle told 7NEWS Sunday.

Costs for the Dome Fire are approaching $500,000, he said. But because no homes were burned and because of the smaller size of the burn area, the county likely won't get any state or federal aid.

However, Pelle said the Sheriff's Office will significantly exceed its budget for overtime, fuel and firefighting.

He said they were already $180,000 over budget for overtime after the Four-Mile Canyon Fire, and the Dome Fire has put them even further in the hole.

He said because the burn area is on City of Boulder open space, the county will be asking the city to help pay the tab.

When all the bills come in, Pelle said he'll also ask county commissioners to dip into the contingency fund to cover the cost.

The 84-acre Dome Fire is 70 percent contained, authorities said Sunday.

The fire, which broke out Friday morning in Boulder Canyon west of the city, was at one point estimated at 152 acres.

But Boulder County Sheriff's Cmdr. Rick Brough said more precise mapping using global positioning technology reduced the fire burn area.

About 150 firefighters continued to fight the blaze Sunday with support from a helicopter that made a dozen water drops, he said.

The 30 percent gap in the fire line perimeter is in steep and mountainous terrain, making it difficult for fire crews to access.

Firefighters remained on-scene throughout Sunday night to patrol the fire perimeter for hot spots. But crews are not expected to actively fight the fire.

The cause of the fire has not been determined, but it is most likely the result of human activity in the area, Brough said. The investigation is continuing