I just started with creeping thyme in my new landscape project and it seems durable but very low to the ground. I also have vinca minor which spreads much faster, seems very durable and has lots of lavender flowers from spring on intermittently.

Thyme needs full sun. Vinca needs shade and is invasive--escapes to the wild. Thyme will need occasional weeding. Vinca not so much. In place of vinca, I'd suggest Geranium macrorrhizum--great for shade, easy to control, never aggressive, highly weed suppressing.

But, the very experienced guy who put in my septic said--nothing over the septic field except grass. So.....

I don't think there should be a problem planting any ground cover over a leaching field as they're quite deep in the ground...or should be. I can understand not planting trees whose roots can invade the system. I can understand not over the septic tank as it has to get dug up every year or so to be pumped. But the leaching field should be fine.

How sunny/shady is the area?

The area is SW, and gets about 6-8 hours of sun (except in winter). I know many suggest that grass is the best thing over a septic leach field (and that's probably true, but they don't need to mow it), but shallow and/or loosely rooted plants are also OK (according to several academic, government, and waste disposal companies' websites). Of course, you shouldn't eat anything planted on a leach field.

I actually thought about putting some kind of marker over the tank access sites (a small circle of pea gravel or something), so they're easier to find. Right now, I'd have to measure according to the fading paper map that's 30 years old.

TV, we have imbedded a brick over each access port to the tank in the lawn. Easy to mow over.

If you like the look of moss, that too could work if it's an area that isn't walked on much. Makes it green and no mow.

I'd go with the speedwell, specifically Veronica peduncularis 'Georgia Blue', Tough, hardy to zone 5, a very long bloom period and more or less evergreen, even in colder locations. Will also take light foot traffic and thrives in sun to part shade with little need for supplemental irrigation after establishment.

I have found thyme, even creeping thyme, very hit or miss afa a substantive groundcover is concerned. Patchy at best and subject to winter dieback even in my mild climate. Thyme "lawns" here are a big fail! Much better as an accent plant that can be replaced as necessary.

Corsican mint is temperamental at best, dying back completely in winter cold or snow cover and hopefully reappearing somewhere close by in spring. Best if used in a confined and semi-protected area. Not sure it is even considered hardy below USDA zone 7.

Yes, I've used those. Thyme, is awesome. It blooms little purple flowers. Vinca is a good one, little blue flowers. I'm adding some flower beds with bling! Take a look at beautiful and cheap these are! I found yellow asiatic lilies for .17cents each! A border flower garden....maybe?