aerate over drain field, or not?

I've never dealt with a septic system before but my hubby and I just bought a house with one. We're just South of Seattle, Wa. and the land the house sits on used to be marsh land.
We have just over 1/3 of an acre ( all in the back yard) that has never been maintained. The soil is very weedy,sandy,rocky, and mossy but the grass that IS growing looks pretty good.
I'd like to encourage new grass growth thus "choking out" the numerous weed species. I don't care too much about the pretty little flowery weeds that are native and kinda pretty. They need to be managed though. Its the THOUSANDS of dandelions and broad leafed weeds that gotta go.
I treated the lawn with about 50 lbs of epsom salts the other day, was thinking about getting some sweetener and throwing that on there (no idea what will happen but I've read it's good for sandy soil?) Aerating the heck out of it and tossing some new seed.
Couple of problems (maybe?) my drain field takes up about half of the back yard. Is aeration a good idea? Or will I be likely to puncture something that's expensive to fix? I know NOTHING about septics or drain fields other than we need em and I have no idea how deep the lines (or whatever they're called) are in the ground.


Also, we have a dog. She likes to eat grass. A lot (we're working with the vet to figure out why--acid reflux is what we're looking at presently...that's a whole different story.) So traditional "weed and feed" seems like a bad idea. I'm trying to figure out how to repair this yard to a point of manageability as naturally as possible.

HELP!!!!!! PLEASE!!!! ps....we bought a riding mower yesterday so I was thinking of a tow able aerator. why work harder if I can work smarter...

Thanks in advance.