ARLINGTON — Arlington school district will spend $200,000 replacing wood chips with pea gravel at 35 playgrounds in response to a fire blamed on spontaneous combustion of the wood fiber, the superintendent said.

Surveillance video showed that nobody was around to start the fire at an elementary school playground last week, officials said. The fire melted the plastic and metal equipment, causing $35,000 in damage.

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Officials said the fire was like the combustion that can occur with organic material in a compost pile if it is not turned regularly.