Home furnaces and air-conditioning units are full of "low-voltage" transformers, relays, thermostats, etc. For example, the 120V to 24Vac transformer in your furnace sends a current through your wall-mounted thermostat. When the thermostat calls for heat, it closes its contacts, pulling in a relay with a 24Vac coil inside the furnace which powers (amongst other things) the 120V 1/4HP blower motor.
I cite this because this is the method I would use for your application. A 24Vac industrial "control" transformer, a 24Vac relay with contacts suitable for switching your pump are readily available at industrial electric distributors...