Tens of millions of Americans are drinking water with unsafe levels of arsenic, a known toxin and carcinogen, according to a report released yesterday by the Natural Resources Defense Council. NRDC is threatening to sue the EPA unless it immediately strengthens its 58-year-old standard for arsenic levels in drinking water. EPA officials say a more stringent standard is expected to take effect next year, though not as stringent as NRDC would like. Arsenic, which is lethal at extremely high levels, is a naturally occurring substance commonly found in rocks; it makes its way into drinking water during well-drilling or as a byproduct of industrial solvents that seep below ground.