(Asher Ben-Dashan for CommonWealth Beacon) WHEN IT RAINS IN Fitchburg, water races down Spring Street, the steep hill next to Longsjo Middle School. As it flows, it picks up oils, animal feces, and other pollutants that lay atop the asphalt and funnels them straight into a storm drain, and then the canals of the city. There’s little vegetation to slow, absorb, and clean the water. The area is densely developed with houses on both sides and mostl…