150' is plenty far away. Here in Va the minimum seperation requirement between septic and a body of water is 50', although I am aware of at least one county that adopted a code to increase that to 100' and I'm sure there are many others. But keep in mind that those seperation distances are for protection in the event the septic system fails and untreated effluent surfaces and then is able to be washed downslope by runoff towards a body of water. Generally, waste water from a septic system that is functioning properly will recieve treatment from the soil immediatley around and beneath the system and if it somehow migrated 150' through soil and made it to the pond, which is very unlikely, it theoretically should have recieved more than enough treatment to no longer be a health risk.

I say build away, only post pictures here so we can check it out!