An additive delivery system may incorporate a cartridge system, including a container cap and a reservoir assembly that provides for storage of an additive. The container cap includes a mixing nozzle for mixing of the additive with a base fluid as the base fluid flows from the base fluid container through the cartridge. A one-way valve prevents backflow of base fluid and/or mixed base fluid/additive from an area downstream of the mixing nozzle such that the base fluid supply remains in a pure state.