As an env. chemist I can think of several problems with this proposal. The first is transport. Any herbicides in soil will be mostly sorbed to surfaces already, and it does not seem like they will move easily into the GAC and sorb to its surfaces. Mainly because that usually occurs in saturated conditions. Perhaps the soil solution will be adquate, but transport in a reasonable amount of time seems to be a problem.

Second, assuming you have part per million levels and we're not talking about a major spill, trace levels of herbicides will have to compete with thousands of other naturally occurring chemical compounds to occupy the surface of the carbon. If the herbicide was a very water-insoluble (hydrophobic) compound it would have an advantage on that basis, but the sheer volume of competing material might swing the balance the other way.

GAC particles are a great medium for microbial growth, so they will grow a biofilm rapidly and may not act like GAC for very long.

Finally, assuming the herbicide was all neatly sequestered into the carbon, what happens next? Although it may be less bio-available, it's still in the soil.

This is all based on theory from a guy who works on the cleanup of env. contaminants and also spent a few years getting my hands black sticking toxic chemicals onto activated carbon in a lab. I haven't tried to review any literature so if there is empirical evidence that this can work, far be it from me to argue. I just didn't want you going off the deep end with a 'great idea' that may not be worth the expense.

Myself, I would opt for feeding the soil food web and letting the microbes eat the stuff. i.e., compost!

Tell me more about what you're planning, I'm very interested.

have you looked at perhaps willow or populus species to act as a filtration system?

but the first question is...how big a problem is this? do you know the levels in ppm or ppb and what the contaminant is, or is this more of, this was a farm in the past and must therefore be contaminated?

as regards cost...yeah, that be the problem. shipping adds up in a hurry. the best i've been able to do works out to about $40 cu ft. from a local yuppie "farm" supply.

from the specifity you used in the initial post...i assume you know the details. and there is a reason you want this done sooner rather than let mature take her course.

BTW, I have tomato envy from your pics.

Theoretically, activated carbon works by providing hydrophobic sites attractive to chemicals that have hydrophobic properties. Some pesticides (perhaps many) are not very water soluble. In fact, the liquid concentrates you buy often have carriers, solvents and surfactants added to make it possible to dilute the product in water and spray it. Once separated from its carriers, the relatively insoluble pesticide will prefer the carbon surface. It's similar to a water and oil situation, but the 'oil' in this case is the surface of the carbon.

With all the hundreds and thousands of naturally occurring compounds floating around in the soil solution, I just wonder how it can work at all, but if it works, it works.

Note, however, that it won't work the same for just any pesticide. For example, atrazine, which is used in mass quantities on corn and is causing groundwater problems, is much more water soluble than, say, 2,4-D, and may not respond to carbon treatment as well. Theoretically.

Very in depth, with many internal links. May help answer many of the technical questions. I know the company that sells the Biogize Soil Detox also sells granular activated charcoal. Maybe the granular would work well as a long term adsorbent.