Logic prevails that wastewater recharges the groundwater after being processed by the septic tank. Well, yes and no according to Dr. Jack Watson Extension Water Quality Specialist with The University of Arizona. In general, individual septic tanks can contribute to groundwater recharge. However, the amount from a single system is so small that the potential contribution is insignificant. If you have a cluster of septic systems, the wastewater could impact groundwater.
Some systems use plants to take up water and utilize the nutrients.
A leach field can also become plugged and downward percolation of effluent can be retarded so that the soil around it also contains excess water and nitrogen, giving plants above it a lush green look. There are many reasons. Not all the leachate water is recharged. Water doesn't just seep down. It can move up and out of the soil from evaporation, and can get taken up by plant roots.