I may be completely off base here, but a plateau at 6 yearsish would
probably be a function of the half life.  I plugged some numbers into an
excell spreadsheet to demonstrate.  I also made a bunch of simplifications,
which may make everything completely wrong, but it made sense to my brain
at the time.  I assumed a 1year half life, longer would make it take longer
to plateau, but basically if you planted the same thing every year you
would add the same amount of pesticide each year.  I used 100 as my
arbitrary starting number, which you can interpret to be 100% of the total,
or 100ppm or ppb or whatever. I made each horizontal line be the
degradation of that years pesticide and each column a new years planting.
You just total the columns for the amout in the soil each year.

Does this make sense to anyone?  At about 6 years you start to taper off at
about 200% the original dose.  You could change the equation to calculate
the totals for a longer half life.  I'll try to put the numbers below from
the spreadsheet for a visual.

 year 1 year 2 year 3 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year
10      100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313
0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 50 25 12.5 6.25 3.125 1.5625 0.78125 0.390625 0.195313 0.097656
100 150 175 187.5 193.75 196.875 198.4375 199.2188 199.6094 199.8047
99.90234

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