(FORTUNE Magazine) – The wandering bargeload of Long Island garbage that took an unexpected Caribbean cruise recently was no anomaly. The U.S. is rapidly running out of safe dump sites, and stiff government regulations to control ground water contamination plus public opposition to proposed sites make it difficult to build new ones. As a result, many cities are toting their trash great distances. Ports in North Carolina, Louisiana, Mexico, and Belize shut out the Mobro 4,000, filled with 3,168 tons of commercial refuse from Islip, New York. At the end of April it was reported to be schlepping its rancid cargo back to New York. One study estimates that half of U.S. cities will exhaust their landfill capacity by 1990. The crisis is most acute in the crowded Northeast, where the cost of disposing garbage has roughly doubled in the past two years. Some towns on Long Island regularly ship waste to Pennsylvania at costs as high as $150 a ton. Gordon Boyd, executive director of the New York State Legislative Commission on Solid Waste Management, fears companies will start to consider waste-disposal costs in deciding where to build plants. He says: ''They're getting to be nearly as important as electricity rates in the cost of doing business.'' The solution to the problem is more waste recycling and incineration. In mid-April, New Jersey became the first state to make recycling -- turning over glass, paper, and other materials for reuse -- mandatory. Compliance will not be strictly enforced at first, but by 1989 the state hopes to recycle 25% of its municipal solid waste. Resource-recovery plants now burn about 5% of America's trash to create electricity or steam. Combustion Engineering, one of several companies that build such plants, predicts the proportion will rise to as much as 40% by the year 2000. But while the process reduces the volume of waste 90%, the remaining ash can be tough to get rid of. Philadelphia will soon ship its ash to Panama.