An improved residential countertop water treatment unit for producing potable water suitable for human ingestion such as by drinking or cooking that comprises a vertical arrangement of a base, a cartridge, and a spigot at the top. The base includes an elongated threaded stub pipe extended downward for fastening the base to a sink surface or countertop and for receiving water from a potable cold water supply line. The base includes an internal valve for controlling water flow between the elongated threaded stub pipe and the inlet of the water filter cartridge. With the valve closed, the cartridge may be readily exchanged by simply unscrewing the spent cartridge from the base and then screwing in a new cartridge.