The effluent will vary in composition based on what is running down the drain. Salt levels are likely to be high no matter what the pH is, so plan to use salt-tolerant plants. Some plants that are both salt and moisture tolerant include hollyhock, bee balm, violet, arborvitae, redosier dogwood, inkberry holly, and blueberry. If your field is well-drained, you might try including yarrow, columbine, chrysanthemum, delphinium, daylily, peony, clematis, wisteria, and rose. Sometimes the only place to put a vegetable garden is over a leach field, raising the question of bacterial and viral contamination from the effluent.
Soils vary a great deal in their ability to filter viruses and bacteria from the effluent.
Clay soils work best, eliminating bacteria within a few inches of the drain lines, but sandy soils may allow bacterial movement for several feet. Do not plant root crops over drain lines. Leafy vegetables could be contaminated by rain splashing soil onto the plant, so either mulch them to eliminate splashing or don't grow them. Dwarf tree fruits would be ideal for a leach field because their roots are not vigorous and are unlikely to cause problems in the drain lines and because the fruit is borne well off the ground. Thoroughly wash any produce from the garden before eating it.