NORWALK — “If you help out your community, your community will help you back,” explained Jen Cunniffe to her 9-year-old daughter and the girl’s 8-year-old friend, as the three used their trash grabbers to gather litter caught in the reeds at Oyster Shell Park. Cunniffe was one of dozens of volunteers gathered at North Water Street’s Heritage Park for a community-wide cleanup effort targeting Norwalk’s coastline. In all, over 70 volunteers took to the land abutting Long Island Sound’s many tributaries Saturday as a part of the third annual “Source to Sound” river and harbor cleanup.