Sumac, a citrusy spice, is used to make a simple syrup that combines with lime, mint, and seltzer in this effervescent ade.
Bring sugar and 3/4 cup water to a boil in a small saucepan, stirring until sugar is dissolved. Stir in sumac. Remove from heat; let cool completely. Strain mixture, discarding solids. Refrigerate, covered, until cold, at least 1 hour and up to 1 week.
Muddle limes and mint in the bottom of a large pitcher with a muddling tool or wooden spoon. Stir in sumac syrup. Add ice and top with seltzer. Serve in ice-filled glasses, garnished with lime wedges.