San Diego’s top water quality official worries that a new desalination plant in Mexico could worsen the decades-long problem of sewage spilling across the border into the United States. The connection between that plant and sewage spills may not be obvious. But there are three connections: wastewater management, money and politics. For years, Mexican officials have been working to build a desalination plant in Rosarito Beach that would eventually make 100 million gallons a day of Pacific Ocean water drinkable.