A _______ check valve backflow preventer is required for a post ground hydrant.

A dual check valve backflow preventer is required for a post ground hydrant to ensure adequate protection against backflow, which could potentially contaminate the potable water supply. This type of backflow preventer employs two independent check valves to create a barrier against reverse flow, providing an added layer of security.

By having two check valves, the dual check valve system is particularly effective in preventing low-hazard backflow situations, where a potential backflow could occur due to pressure drops or other changes in the water system. The dual configuration is designed to mitigate the risk of contamination that could result from back siphonage or backpressure scenarios.

The other types of check valves mentioned do not provide the same level of protection; a single check valve may not be sufficient to prevent backflow under all conditions, while a double check valve system typically involves more complexity and is generally used for higher hazard applications. A triple check valve may be over-engineering for applications like this, and hence, the dual check valve is the most appropriate and widely accepted standard for post ground hydrants.