Over the years, pesticides have been repeatedly applied to agricultural farmlands to promote crop growth. (The term “pesticide” is used herein to include both insecticides and fungicides.) The agricultural pesticides and/or their toxic residue, by their nature, tend to become tightly bound to soil particles, particularly fine soil (clays), and to organic matter in the soil. Many are not water-soluble. These properties inhibit downward migration from the ground surface (where the pesticides were generally applied) into the soil column. The result is that, even years after the pesticides were applied to the land, the concentration profile of the pesticides or the toxic residue of the pesticides still decreases substantially with depth. Areas with high concentrations of the pesticide contaminant within several inches of the ground surface may have only natural background concentration levels within 1.5 to 2 feet below ground surface.
Historically, the pesticides of choice have evolved from arsenical pesticides such as lead arsenate and calcium arsenate, or through organochlorine pesticides such as aldrin, dieldrin and most commonly DDT and its metabolites. In recent years, the negative impact of these persistent chemicals on human health has been recognized. To address these risks, regulatory limits have been set. Each of these chemicals, by itself or broken down into its components (i.e. arsenic, lead), typically has an associated residential soil cleanup criteria expressed in parts per million to quantify the human health risks. Lands with concentrations of pesticides exceeding these regulatory limits are deemed unsuitable for residential building and the like, unless the pesticides are removed or their concentrations are reduced.
There are at least six (6) recognized remediation alternatives for remediating sites with persistent contaminates such as pesticides: (1) consolidating and covering contaminated soil (e.g., under roads and structures); (2) capping with clean soil; (3) blending with clean soil from on-site; (4) blending with clean soil from off-site; (5) excavating and removing contaminated soil; and (6) using innovative soil treatment technologies.
Each of the alternatives carries disadvantages and problems. Remediation that involves merely covering areas that would exceed clean-up thresholds if uncovered [alternatives (1) and (2)] will normally require use restrictions on the property and deed notices of the restrictions to prevent later exposure of the covered contamination. Excavation and removal [alternative (5)], importation of clean soil to the site [alternative (4)] and most innovative treatment technologies [alternative (6)], are much more expensive than alternatives that do not require offsite movement and/or treatment of soil. The most economical alternative, where it can be accomplished, is blending the soil on-site [alternative (3)].
Although economical, on-site soil blending is not without problems. The main problems with blending to reduce pesticide contamination are locating enough clean soil in reasonable proximity to the contaminated soil, and finding an effective way to blend the clean and contaminated soil to sufficiently lower contaminate concentrations to below the cleanup criteria.
In many instances, the most proximate source of clean soil is in the vertical profile below the surface layer of concentrated pesticide contamination. However, mixing the contaminated soil layer with clean soil below it is not effectively accomplished with conventional mobile machinery. For example, conventional surface earthmovers such as bulldozers merely scrape or carry the concentrated surface contamination to another location without significant mixing. Augers can create deep vertical wells, but with little mixing. Conventional farm machinery such as plows and harrows merely turn over soil in the contaminated surface layers, not effectively blending the contaminated layer with the subsurface clean soil. In the same respect, power shovels, which bite deeply into soil, merely lifting and moving the soil, also do not effectively mix and blend the clean soil with the contaminated soil.
The inefficiencies of the traditional machinery in vertically mixing and blending contaminated soil with clean soil in close vertical proximity are exemplified at sites where there are non-uniform distributions of contaminate residue throughout the site. Irregular distributions are commonly found on former orchards where pesticide residue is concentrated in patterns corresponding to individual trees of the orchard. The areas under the canopy of individual trees in an orchard will generally show a higher pesticide contamination level than the areas between trees and rows of trees. If the trees are removed for residential building, these areas will show up as local “hot-spots” of pesticide contamination.
When the conventional surface earthmovers are used on sites where the contamination is concentrated in hot spots (e.g., in orchards), the mixing that occurs is predominantly limited to the top layer of the hot spot with the top layer of the surrounding area (i.e., horizontal mixing). This horizontal mixing approach may result in a spreading of the contaminant along the surface without sufficient blending with clean soil to bring the contaminant concentration below clean up levels. Instead of remediating the site, this horizontal mixing approach merely creates a wider area having a contaminant concentration that may still exceed the cleanup criteria levels.