Lemongrass-lime leaf soda

I love the idea of making "custom" sodas like this (which has 8 grams of sugar per serving as opposed to about 35-40 in a canned drink), because I feel like I should at least avoid sodas loaded with sugar on top of my regular dessert habit...This drink doesn't taste like a compromise at all. It's extremely flavorful and perfectly sweet, and if you can't find lime leaves, just double the amount of lime zest.

Lemongrass-lime leaf soda
Makes 12
from Bon Appetit, May 2011

Lemongrass-infused mixture:
1/4 c. lemon zest (from about 6 lemons)
1/4 c. lime zest (from about 8 limes)
20 kaffir lime leaves
2 stalks lemongrass, roughly chopped (you usually have to peel off the tough outer layers to be able to chop it easily)
2 cups water

Combine all ingredients in a saucepan. Bring to a boil, then reduce heat and simmer about 5 minutes. Strain into a jar and let cool, then chill.

Simple syrup:
1/2 c. sugar
1/2 c. water

Heat in a saucepan over medium heat til sugar is dissolved, stirring occasionally. Pour into a jar and let cool, then cover and store in the fridge up to 1 month.

To assemble the drinks:
Fill a 12-oz glass with ice; add 2 Tbsp of the lemongrass mixture and 1 Tbsp of the simple syrup, then top off with about 1/2 cup of seltzer/soda water. Stir to combine and enjoy.