Combine the juices, brine, and simple syrup in a Collins glass and stir to combine. Add ice and top with soda water, gently stirring once more. Garnish with the grapefruit slice or pickle.
Batch for 6: Combine 1 1⁄2 cups grapefruit juice, 1⁄2 cup plus 1 tablespoon lemon juice, 3 oz. brine, and 3 oz. simple syrup in a pitcher and stir to combine. Add ice and top with 3 3⁄4 cups soda water, gently stirring once more. Divide among 6 Collins glasses, adding more soda water if you wish. Garnish each with a grapefruit slice or pickle.
Note: You could keep a batch of the base (the juices, syrup, and brine) in the fridge for a couple weeks without losing much integrity. Just top with soda water to serve.