Dispersion in Soils

Many of the factors that influence the c.xtcnt of contamination are site specific, cither climatic or hydrogeological. Other factors that influence the extent of contamintition relate to land surface features such as topography or dcNclopmcnt, which determine exposure routes. Additional important 

In the past the presence of hazardous substances in soils was not a major public concern. In spite of the large number of documented hazardous waste sites in the United States, relatively few sites liave been cleaned up widi specific redevelopment in mind. Remedial actions usually are undertaken to contain or remove chemical contaminants little or no consideration is given to the ultimate use of the site. If land reuse is decided before the cleanup there may be an opportunity to tailor the cleanup acti ities to best suit the site rcde elopment. 

The extent of cleanup that is necessary to protect human health and welfare aries with different use ctitegories. Residential development is probably the most sensiti e type of land use because of the long-term and multiple e.xposure routes and because of potential e.xposure to the most sensitive population segments (e.g., children and elderly persons). E.xcavation and removal appears to be the remedial tiction alternative selected at most sites where there is redevelopment. This is because no one can guaratitee tliat a site is stife (i.e., offers zero risk) unless all contaminants are removed. Neitlier a developer nor a municipality can accept responsibility for site safety as long as haznrdous materials remain there. In situ treatment approtiches are seldom iewed is the best option because they are unproven and because 100% detoxification or stabilization caimot be achieved. 

As piirt of each haz, irdous waste remedial action, the contamination at the site must be assessed. The options for remedial action to remo c or otherwise deal with hazardous materials will depend on the nature of the coutamiiuition lliat will be allowed to remain on site tifter cletmup. Thus there must be plans for site remediation to cover any accidental or emergency discharges to land or soils that might develop. 

To determine acceptable contaminant levels in soils, two primary exposure routes are usually considered (1) inlialation of gases, vapors, or airborne particulate emanating from the site, and (2) ingestion of contamimtted drinking water. Other routes that can contribute to e.xposure include absorption of pollutants tluough direct skin contact and uptake of wtiter or soil contantinants by plants that are part of the food chain. 

One measure of the significance of conttmiinants in soil or solid waste samples may be determined by comparing the levels ith reported naturally occurring concentrations. Provided le els are within the range that ma occur naturally, one might conclude that the sample contamiiuint levels arc of little consequence. 

---

A rigorous treatment of dispersion in soils is beyond the scope of this book. However, some qualitative discussion is warranted because of the potential and existing problems already described. Two ntain problems arise bceause dispersion in soil (or land) is anisotropic (i.e., it varies with direction) and the penneability is not only a variable but also att unknown. 

For one-dimensional dispersion in soils, llie describing equation for a conservative species and/or pollutant, c, is a cartesian (rectatigular) coordinate system moving with velocity v is... 

Abstract Unsteady liquid flow and chemical reaction characterize hydrodynamic dispersion in soils and other porous materials and flow equations are complicated by the need to account for advection of the solute with the water, and competitive adsorption of solute components. Advection of the water and adsorbed species with the solid phase in swelling systems is an additional complication. Computers facilitate solution of these equations but it is often physically more revealing when we discriminate between flow of the solute with and relative to, the water and the flow of solution with and relative to, the solid phase. Spacelike coordinates that satisfy material balance of the water, or of the solid, achieve this separation. Advection terms are implicit in the space-like coordinate and the flow equations are focused on solute movement relative to the water and water relative to soil solid. This paper illustrates some of these issues. 

Our objectives were to (i) inspect observations of the scale-dependent water movement and anomalous solute dispersion in soils, and (ii) apply and discuss fractal-based modeling approaches that would accommodate these anomalous transport phenomena. 

The mathematical model IMPACT [21 ] is for use in the near-highway environment, i.e., over a scale of meters, and consists of fate and transport analyses related to removal, reduction, and retardation (RRR) processes, plus generation of initial pollutant loadings. More specifically, the transport processes of ad-vection and dispersion (in soil) are coupled to the RRR processes of sorption, biodegradation, photolysis and volatilization. Model output consists of flows, loads (mass), concentration of surrogate chemical (surrogate for toxicity), and toxicity of tested C R materials in their appropriate reference environments. 

T6 Dispersivity in soils and the up-most groundwaters of geologic fonnatioiis 1T7 MTCs in pore-waters adjacent to particles and non-aqueous liquids... 

Part 1 Dispersivity in Soil and the Up-Most Groundwater of Geologic Formations... 

Vinogradov A.P. The Geochemistry of Rare and Dispersed Chemical Elements in Soils. New York Consultants Bureau Inc, 1959. 

The major ion composition of dispersed soil solutions and saturation extracts from selected soils in California, U.S. Concentrations of trace elements in soil solutions of the California soils that received sludge applications. Concentrations of trace elements in soil solutions extracted by saturated paste from two metal salt-spiked Israeli soils incubated at saturated regime. 

Chemical weathering of minerals during pedogenesis can be enhanced by microbial activity by a factor as high as 106 (Kurek 2002). Microorganisms can dissolve minerals by direct and indirect actions under aerobic and anaerobic conditions (Robert and Berthelin 1986 Ehrlich 2002 Kurek 2002). In some cases of attack, the microorganisms may be dispersed in the soil solution in others, they may grow in biofilms on the surface of susceptible minerals. 

The amount of chemical introduced in the Level I calculation is an arbitrary 100,000 kg or 100 tonnes. If dispersed entirely in the air, this amount yields a concentration of 1 pg/m3 which is not unusual for ubiquitous contaminants such as hydrocarbons. If dispersed entirely in the water, the concentration is a higher 500 pg/m3 or 500 ng/L, which again is reasonable for a well-used chemical of commerce. The corresponding value in soil is about 0.0046 pg/g. Clearly for restricted chemicals such as PCBs, this amount is too large, but it is preferable to adopt a common evaluative amount... 

Comparing all the groups (Tukey test) by pairs it was concluded that Brancanes, Lombador and Monte dos Mestres are comparable for Cu and Zn in soils and Fe in leaves of the rock rose plants. None of these groups is comparable with Neves Corvo in what Cu, Zn and Fe is concerned, both in soils and plants. This is due to the present mine exploitation of these metals and the dispersion of newly exploited materials in the surrounding area. 

The movement of synthetic pyrethroids in soil and sediment is basically controlled by diffusion, convection, and dispersion. When entering a water-sediment system... 

---