This opinion seems to be the consensus of the people that know:
It is generally considered a good idea to plant your septic field, but it isn’t the ideal place for a vegetable garden. According to the Virginia Cooperative Extension (VCE) shallow-rooting plants can help the septic drain system work well by removing moisture and nutrients from the soil; they can also reduce soil erosion. As far as planting vegetables is concerned, the VCE materials say that the question of contamination depends on the soil’s ability to filter viruses and bacteria: clay soils can eliminate bacteria within a few inches of the drain trenches, but sandy soils may allow bacterial movement for several feet. The VCE explains that a properly operating septic system will not contaminate the soil but notes that it is difficult for home gardeners to determine whether a system is working as it should.
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