The Tujunga Spreading Grounds may look like a vast, barren plot of dirt. But it’s what’s beneath the dirt that matters. Earlier this week, officials brandished shiny shovels to break ground on a project there that they say will play a key role in bolstering the region’s water supply and protecting against future droughts. The spreading grounds, a 150-acre tract of porous soil in the northeast San Fernando Valley, capture stormwater that falls from the sky or runs off from nearby mountains and hills, and allows the water to filter into a vast aquifer that can be drawn down when the resource is in short supply.