Legacy mercury contamination is problematic in California waterways due to historic gold and mercury mining.  Even today, mercury from abandoned mines still leaches into the creeks and rivers in the Sierra and Coast Ranges, flowing downstream and into the Delta, contaminating the sediments. With tens of thousands of acres of tidal wetlands planned for the Delta, the current assumption is that tidal wetlands are net exporters of methylmercury, which can result in expensive and time-consuming mitigation.