OLD GREENWICH - Authorities say that 1.6 million gallons of raw sewage from the Riverside Shopping Center has spilled and contaminated a nearby waterway and neighborhood.Officials say plumbing work in the basement of the A&P done over the summer could be a cause of the leak, which has been going on since July.A cleanup effort is being led by the shopping center and is scheduled to begin tomorrow. Town officials say this type of cleanup usually takes about a week, but may be delayed by heavy rain.