A conservative tracer is applied to the ground surface and leached through the vadose zone by rainfall. The average volumetric water content is 0.40 over the upper 4 meters and 0.30 over the remaining 3 meters above the water table and the net infiltration rate (Darcy velocity) is estimated to be 22 cm/yr. If the dispersivity is estimated to be 8 cm, estimate the time between when the concentration ratios of C/Co = 0.1 and C/Co = 0.9 reach the water table (where Co is the initial tracer concentration).