"At a Bee Wellness workshop I taught at last year, there was an agricultural chemist among the participants who told me that the government uses math to calculate the rate at which chemicals "leach" out of the soil, or are otherwise rendered "harmless"."

EPA does very little outside of office type work with respect to Ag products.  They require registrants to supply experimental data.  Generation of such data is not and never has been EPAs responsibility.  For instance metabolic rates in soils are done by taking a variety of typical soils, spiking with known amounts of test substances and developing kinetic data on depletion as well as determination of degradation products produced and in appropriate cases the degradation rates of those metabolites.  We did such tests in soils incubated under controlled lab or green house conditions such that reproducible data could be expected unlike any such test done outside.  At bare minimum such data would give half lives and some indication of the kinetic order of the degradation.  ie first or second or some fractional kinetic order.  Ordinarily you expect first order kinetics unless some incubation time is required to develop some specific microbes that degrade
 the product.  Incubation conditions are picked to reasonably replicate field conditions.

Likewise with leaching we ordinarily ran columns containing soils of various types to determine rates of migration.  With some pesticide types aimed at root uptake but not seed applied we worked very hard to increase migration rates by formulation tricks.  Many such pesticide candidates moved too slowly without formulation help to be of benefit.  Such formulation tricks can help move the pesticide a foot or so.  I doubt if any would be effective beyond this distance.

I have no clue what your participant was talking about with respect to the EPA using calculations to get such data.  Why calculate when experimental data is supplied by the registrant as part of the required registration data package?

I should point out that such studies are not aimed at determining the fate in massive spills.  Under such conditions you saturate the metabolic pathways with product and are likely to see much longer half lives.  You also flood all soil receptors which bind the substance and likely would see faster movement.  These conditions are not relevant to normal use per the label.  Such flooding studies have been used by environmental activists to damn pesticides in at least one case I know of.  I am not aware that EPA paid the slightest attention to this stunt.  The EPA people are not stupid by a long ways.

I suspect the person was not actually talking about either soil degradation rates nor migration rates.  Rather, he likely was talking about potential bioaccumulation rates in animals or birds.  The EPA also requires a great deal of physical data on any substance submitted including things like octanol-water distribution coefficients, solubility and pKa.  Such data is sometimes looked at early on to decide what further studies are required.  You can calculate approximate guesses on uptakes and where the compound might like to go in an organism.  For instance things like intestinal absorption rates or distribution in fat versus water phases after absorption.  EPA does do such calculations.  But, only to give themselves guidance on what other questions they want experimental answers to.

I have been pretty much out of the industry now for 20 years and regulations have gotten much, much harder during that 20 years.  It is entirely possible that during that time EPA has learned to do better calculations than they did when I was actively involved.  But, I find it hard to believe they would use such calculations for anything beyond guidance on further experimental tests.  On the other hand I could easy believe FDA would rely on such calculations, even for soil degradation or movement for drug  products instead of requiring experimental data.

Dick


" Any discovery made by the human mind can be explained in its essentials to the curious learner."  Professor Benjamin Schumacher talking about teaching quantum mechanics to non scientists.   "For every complex problem there is a solution which is simple, neat and wrong."  H. L. Mencken

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