A tanker truck carrying liquid herbicide overturned and caused a spill over a field. The liquid herbicide remained on the soil for 30 minutes before evaporating into the air. The mass diffusivity of the liquid herbicide in the soil is 2 × 10^–8 m²/s. If a concentration of 0.1% by weight will destroy most life, determine the depth of the soil at which plant and insect life is likely to be affected by the spill.