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    Migration of Pesticide from Soil to Groundwater

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    The following percentages of three pesticides added to a soil remained after increasing times.
    Time (days) Diazinon % Benefin % Terbutol %
    0 100 100 100
    15 18.7 no data no data
    30 1.50 57.5 94.3
    60 0.30 29.0 80.2
    90 0.01 14.5 73.7
    180 0.01 7.50 51.3

    1) Fit the decomposition data for each of these pesticides to a standard exponential decay function (linear form) and show results graphically.
    Nt = N0(1/2)t/t1/2
    2) Calculate the decomposition half-life of each pesticide.
    3) Draw the chemical structures of each pesticide and suggest a chemical explanation for the relative persistence of these chemicals in soil.
    4) Calculate diazinon Kd for a soil with 5% organic matter assuming soil minerals have no tendency to absorb this pesticide.
    5) Use this Kd to determine how many years it would take for diazinon to migrate from soil surface into shallow groundwater at 2 m depth (bulk density 1.3 g/cm3, porosity 0.45). Rainfall is 90 cm and evapotranspiration is 40 cm.

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    Solution Summary

    This solution explains:

    1) How to fit decomposition data to a linear graph.

    2) How to calculate half-life of pesticide decomposition.

    3) How to to draw chemical structures and explain persistence in soil of diazinon, benefin, and terbutol.

    4) How to calculate Kd for diazinon gives 5% organic matter.

    5) How to use Kd to calculate the soil transfer rate of diazinon over time.