Combine 1 1/4 cups water and sugar in a small saucepan; cook over medium heat, stirring constantly, until sugar dissolves. Remove from heat, and let cool completely.
Combine sugar mixture, 1/2 cup mint leaves, and next 3 ingredients. Cover and chill.
Pour mixture through a wire-mesh strainer into a pitcher, discarding mint and orange rind. Just before serving, stir in sparkling water. Serve over ice. Garnish with fresh mint sprigs, if desired.