Crop nutrients are obviously a concern, but if you're only trying to survive for a couple of cycles seems totally feasible and could be extended artificially.
Potatoes are one of the most calorie-dense vegetables by weight and by growing space. They have 350 calories per pound, so you need 5700 pounds of potatoes to feed a family of four.
A good potato yield is about 25,000 pounds per acre, so you need nearly a quarter acre of potatoes (1000 square metres) to feed your family for a year.
I grow a lot of stuff and have for years and it’s an excessive amount of labor, you only get certain food during certain times of the year, sometimes for only for a few weeks, and that’s assuming you don’t have losses due to pests.
Agriculture is best left to the professionals for feeding societies.
If your municipal water supply is compromise, the last thing you should be doing with water is pouring it on the ground.
In the real world, people doing this for a living find that they need literally 100 to 1000 times that much land to feed a family of four.