Zest the oranges, making sure not to get any of the white pith. Juice one of the oranges to make 1/2 cup juice, then strain out the pulp. Add zest and juice to a saucepan along with sugar, water, and citric acid. Bring to a boil, then reduce heat to a bare simmer and cook for 15 minutes. Remove from heat and let cool. Bottle and refrigerate the cooled syrup. For each serving, mix 1 part syrup with 3 parts seltzer, adjusting to your personal taste.
