You might want to think twice before you head out with your sunscreen and towel this weekend. The number of sewage-tainted beaches is on the rise, jumping 19 percent between 2000 to 2001, according to a report released yesterday by the Natural Resources Defense Council. The group said that 13,410 beach closings and water advisories were issued by government agencies in 2001, compared to 11,270 in 2000. The culprit? High levels of bacteria associated with fecal contamination, in 87 percent of the cases. That type of pollution most often comes from sewage discharges or street runoff, but the NRDC said local authorities couldn’t identify the exact source of contamination in 54 percent of the cases. But before you wax nostalgic for the pollution-free beaches of yore, the group had this to say: Much of the rise in beach warnings resulted from increased monitoring and better testing standards for bacteria.