California’s storms do plenty of good, including replenishing the state’s water supply by filling its reservoirs and dampening the risk of wildfires. But the rainwater runoff also carries heavy pollutants that wash directly onto the shore, creating a toxic mix that’s unsafe for beachgoers. Anyone who’s visited a beach after it rains has encountered such stormwater pollution, the unfiltered trash that piles onto the sand after flowing from rooftops, sidewalks and streets, picking up a trail of pesticides, bacteria, oil and grease before traveling through the storm drains.