Beyond a chain-link fence topped with spiraled barbed wire, swaying coastal grasses conceal a cache of buried radioactive waste and toxic pesticides from a bygone chemical plant. Warning signs along the Richmond, Calif., site’s perimeter attempt to discourage trespassers from breaching the locked gates, where soil testing has detected cancer-causing gamma radiation more than 60 times higher than background levels in some places. For most of the 20th century, the former Stauffer Chemical Co. disposed of thousands of tons of industrial waste near its factory grounds along Richmond’s southeast shoreline. … In a January letter to Albany and Berkeley city officials, [the State Water Board] wrote that the landfills “may have accepted industrial waste materials that could present a risk to water quality, human health, and the environment.”