Selecting a transceiver module for a wireless communication system
Digi's Xbee modules may be interesting. The XBee PRO ZB has a maximum range of 3.2 km, and the modules have a completely transparent Zigbee protocol implementation. Means that you use the module as a simple UART connection.  

You want to control the pumps with this, so I'm thinking reliability! Pumps should not be switched off or on by others on the network, and if you send a command to switch pump 4 on it has to be switched on for sure. 

That's what your software should take care off. I would suggest some kind of encryption, and include error correction. Let the pump station acknowledge a command, and include commands for asking a pump's state.