What you're seeing: biopiles
it looks like companies just dig the ground, leave everything in a pile for a while (few weeks?) and then bring over the dirt elsewhere.
It sounds like what you're seeing are biopiles (also known as heap pile bioremediation, bioheaps, biomounds, or static-pile composting).
From the Remediation Technologies Screening Matrix and Reference Guide, a project of the Federal Roundtable on Remediation Technology (FRTR), a group comprised of the U.S. Environmental Protection Agency, the Dept of Defense, and the Dept of Energy:
Excavated soils are mixed with soil amendments and placed on a treatment area that includes leachate collection systems and some form of aeration. It is used to reduce concentrations of petroleum constituents in excavated soils through the use of biodegradation. Moisture, heat, nutrients, oxygen, and pH can be controlled to enhance biodegradation. [...]
Soil piles and cells commonly have an air distribution system buried under the soil to pass air through the soil either by vacuum or by positive pressure. The soil piles in this case can be up to 20 feet high (generally not recommended, 2-3 meters maximum). Soil piles may be covered with plastic to control runoff, evaporation, and volatilization and to promote solar heating. [...]
Biopile is a short-term technology. Duration of operation and maintenance may last a few weeks to several months.
Biodegradation is the actual process at work to neutralize the harmful compounds. Here's a definition from the page on treatment tech for fuels in soil:
Biodegradation uses indigenous or inoculated microorganisms (e.g., fungi, bacteria, and other microbes) to degrade (i.e., metabolize) organic contaminants found in soil and/or ground water. In the presence of sufficient oxygen (aerobic conditions), microorganisms will ultimately convert many organic contaminants to carbon dioxide, water, and microbial cell mass. In the absence of oxygen (anaerobic conditions), the contaminants will be ultimately metabolized to methane. Sometimes contaminants may not be completely degraded, but only transformed to intermediate products that may be less, equally, or more hazardous than the original contaminant.
Applicability of biopiles
From the page on biopiles:
Biopile treatment has been applied to treatment of nonhalogenated VOCs and fuel hydrocarbons. Halogenated VOCs, SVOCs, and pesticides also can be treated, but the process effectiveness will vary and may be applicable only to some compounds within these contaminant groups.
Benzene, Benzo(a)anthracene, Benzo(b,j,k)fluoranthene, and Naphthalene are all listed on the fuels section of the FRTR.
A reference for Petroleum Hydrocarbon Ranges from a commercial testing laboratory indicates that petroleum hydrocarbons are all derived from crude oil. While some of these may be classified differently, it seems that remediation tech for typical fuels would also apply here.
Going back to the FRTR guide, other possible soil treatment technologies for fuels include:
- Incineration uses high temperatures, 870 to 1,200 C (1,400 to 2,200 F), to volatilize and combust (in the presence of oxygen) organic constituents in hazardous wastes.
- Soil vapor extraction is an in situ unsaturated (vadose) zone soil remediation technology in which a vacuum is applied to the soil to induce the controlled flow of air and remove volatile and some semivolatile contaminants from the soil.
- Low temperature thermal desorption systems are physical separation processes and are not designed to destroy organics. Wastes are heated to between 90 and 315 C (200 to 600o F) to volatilize water and organic contaminants. A carrier gas or vacuum system transports volatilized water and organics to the gas treatment system. Ground water treatment concentrates the collected contaminants.
What about tin and lead?
Tin and lead are inorganics, for which soil treatment technologies include:
- Solidification processes produce monolithic blocks of waste with high structural integrity. The contaminants do not necessarily interact chemically with the solidification reagents (typically cement/ash) but are mechanically locked within the solidified matrix.
- Excavation and off-site disposal of contaminated soil (with or without solidification/stabilization pretreatment).
- Acid extraction uses an acid, such as hydrochloric acid, to extract heavy metal contaminants from soils. Hydrochloric acid is then introduced into the soil in the extraction unit. [...] The soil-extractant mixture is continuously pumped out of the mixing tank, and the soil and extractant are separated using hydrocyclones.
Based on how you describe the site, it sounds like the workers may be moving the tin and lead off-site for one of these treatment methods.