Figure 3. Water in the Ogallala Formation originates as precipitation or irrigation water at the land surface. Precipitation or irrigation water that percolates through the root and unsaturated zones underlying irrigated fields may transport fertilizer and pesticide residues to the formation. As more water percolates through the unsaturated zone, the ground water is driven deeper into the formation; therefore, the older water is at the bottom of the formation and the younger water is at the water table.