Morning Alyssa, 

Thanks for reaching out to UVM Extension Master Gardener Helpline. Apologies for the delay in getting back to you. 

It's a great question. The very short answer is a minimum of 10 ft but there are many other factors to consider. I've linked a few resources below that you should check out before setting up a vege garden. 

Generally, I would be cautious about planting anything you want to eat downhill of the leech field – err on the side of more space than less and uphill if possible. I found a similar question answered by a food safety expert at UMN Extension back in 2019 and they had this to say (LINK):

"From a food safety perspective, it is a best practice to not plant edible crops like vegetables near a leach field, as they might be contaminated if the septic is not properly functioning and contaminated water is in contact with the vegetables or the roots of the plants. While there is no hard and fast rule, and it will depend on many factors, it is a best practice to have any produce or fruit trees at least 10 feet from the perimeter of the leach field to avoid having the vegetables interact with incompletely cleaned water. Trees and plants with larger root systems might need more distance from the edge of the leach field."

I hope that helps. 

Further reading: 

https://hgic.clemson.edu/factsheet/landscaping-over-septic-drain-fields/

https://extension.wsu.edu/clark/naturalresources/smallacreageprogram/landscaping-for-septic-systems/

https://extension.unh.edu/blog/2019/12/it-okay-plant-garden-over-leach-field