Combine water and granulated sugar in a medium saucepan.
Heat over medium heat, stirring occasionally, until the sugar is dissolved. Reduce the heat to low and cook for 4-5 minutes. Remove the saucepan from the heat and let the sugar syrup cool down to room temperature.
Add ground sumac to the saucepan, stir to combine, and allow to infuse for 15 minutes.
Strain the syrup through a fine-mesh strainer or a coffee filter. This syrup is good enough to make 8-10 glasses of lemonade.
Add 4-5 ice cubes to a tall serving glass.
Pour ¼ cup of sumac syrup into the glass.
Top with freshly squeezed lemon juice and top the glass with club soda or seltzer. Garnish with a slice of lime and a few mint leaves and serve immediately.