Answer: If there were seed heads in the bales of hay/straw, then as they broke open they seeded the area. Whether the grass will come back next year depends on what type of grass is in the hay. Most likely it's a perennial grass (hay usually is), so it looks like you've reseeded that area to grass! If the bales were indeed straw, rather than hay, then there should have been few if any seed heads. Straw is made up of the stems of grasses after the grain has been harvested, while hay is made from freshly-mown grassy fields and contains seed heads.
If the growing grass has already formed seed heads, then it has already gone ahead and reseeded itself. I would probably cut it down and let it compost in place. However, before you make any plans for the area, I would check with the contractor that installed the leach field. The general practice is to grow only lawns over leach fields--not flower beds, shrubs, or deep-rooted perennials. Check to get their OK before investing in any plants for the area.
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