Groundwater age helps us understand how water moves in big basins. Scientists use three ways to figure out groundwater age: advective age, concentration-based model age, and directly simulated age. Directly simulated age is the best for studying age distribution in basins. Groundwater age gets older as it flows from where it enters to where it leaves the basin. In the lower parts of a basin, there's a sudden change in groundwater age from shallow to deep areas, showing different flow systems. Near stagnant points, groundwater age is highest. These findings are important for making accurate models of groundwater in large basins.