For a spiked version of this drink, divide 2 cups vodka among the glasses just before serving.
Bring water, raspberries, and sugar to boil in a large saucepan, stirring occasionally, until sugar dissolves. Reduce heat, and let simmer for 15 minutes. Strain mixture through a fine sieve, pressing gently on berries to release juices; discard solids. Let syrup cool completely. Combine syrup and lemonade in a large pitcher. Divide among 16 ice-filled glasses. Top each with sparkling water, and vodka if using. Garnish with fresh raspberries.