Abstract: An air gap adapter for use with a water treatment faucet constructed and arranged for being mounted in a standard countertop faucet opening. The adapter includes an adapter base with an underside, defining a base throughbore for a standard faucet inlet nipple, having an RO retentate inlet port and a separate RO retentate outlet port. A cone insert has a platform covering at least a portion of the base, and a cone formation vertically projecting from the platform and being in fluid communication with the inlet port. An outlet opening is on the cone formation in fluid communication with the outlet port. An adapter cover disposed over the cone insert and the base includes a cone cover portion and a base cover portion, as well as a cover throughbore in registry with the throughbore. The adapter is constructed and arranged to be optionally mounted to a countertop with the faucet.