It's not 100%. Suppose a plant absorbs 5% of the lead. Well in the first year you get 5% of the lead in all water used for irrigation. But, 95% is left in the soil. Next year you get 5% of 195%. Until, the soil has 20 years worth of lead and all new lead added in a year ends up in the plant. Drop that to 1% and it would take ~100 years to get the same effect.

Note, you would really get a differential equation with several terms, but more or less the same idea just after more time and balanced with water that overflows the field etc.