10 ounces fresh ginger
2 cups sugar
10 cups water, divided
2 cups freshly-squeezed lemon juice (from about 15 lemons)
Peel the ginger and chop into roughly 1/4-inch pieces. Combine the ginger, sugar, and 4 cups of the water in a medium pot over high heat. Bring to a boil, reduce the heat to medium, and simmer for 45 minutes. Strain the syrup through a fine mesh sieve, pressing on the ginger with the back of a spoon. You should have about 3 cups of syrup. Cool to room temperature.
Combine the lemon juice and remaining 6 cups water in a large pitcher or pot. Stir in all of the ginger syrup. Chill before serving or serve over ice.