The massive water filtration plant that is being built in the Bronx 10 stories underground was expected to cost $1.3 billion in 2003, but the projected cost has risen to $3.1 billion. The agency overseeing the project, the city Department of Environmental Protection (DEP), says that costs rose because of industrywide construction cost increases, and that the portion of water users’ bills being used to pay for the plant is comparable to that of other construction projects. Critics say that the DEP underestimated costs. The plant, which has an expected completion date of 2012, will filter a quarter of New York City’s water supply and will be the first drinking-water filtration facility in the city.