The Soil Profile

The soil profile is the manifestation of the soilforming or pedogenic processes on the geological substratum or parent material (Fig. 7-1). The soil profile is an assemblage of horizons displayed by a vertical cut at the surface. Horizons are layer-like, more or less parallel to the surface, and differ from each other in morphology, composition, and consistency (Table 7-1). 

Soil profiles form because of the endless migration of ions, molecules, and particles into the soil material. Meteoric inputs include H2O, CO2, O2, nitrogenous compounds, pollutants, salts, and dust. These molecules and compounds come from space, from the atmosphere and the ocean, and other terrestrial ecosystems. 

Soil profile a vertical cut at the surface of the continental earth crust displaying an assemblage of genetic horizons 

Horizons layers, more or less parallel to the smface, that differ from each other in morphology, composition, and consistence (Fig. 7-1) 

How the soil profile develops continuous or sporadic migration of ions, molecules, particles and aerosols. Water is the vehicle for transportational processes 

---

Water leaves the field either as surface mnoff, carrying pesticides dissolved in the water or sorbed to soil particles suspended in water, or as water draining through the soil profile, carrying dissolved pesticides to deeper depths. The distribution of water between drainage and mnoff is dependent on the amount of water appHed to the field, the physical and chemical properties of the soil, and the cultural practices imposed on the field. These factors also impact the retention and transformation processes affecting the pesticide. 

Mention should be made of the soil profile (section through soil showing various layers) because it is important to recognise that the soil s surface... 

There was a discrepancy between water salinity limits for the three locations, which may be attributed to factors related to difference in soil texture and stmcture. This affects soil infiltration capacity and water retention. These soil hydrologic characteristics influence salt development in the soil profile, which affects plant... 

Plant uptake is one of several routes by which an organic contaminant can enter man s food chain. The amount of uptake depends on plant species, concentration, depth of placement, soil type, temperature, moisture, and many other parameters. Translocation of the absorbed material into various plant parts will determine the degree of man s exposure—i.e., whether the material moves to an edible portion of the plant. Past experience with nonpolar chlorinated pesticides suggested optimal uptake conditions are achieved when the chemical is placed in a soil with low adsorptive capacity e.g., a sand), evenly distributed throughout the soil profile, and with oil producing plants. Plant experiments were conducted with one set of parameters that would be optimal for uptake and translocation. The uptake of two dioxins and one phenol (2,4-dichlorophenol (DCP)) from one soil was measured in soybean and oats (7). The application rates were DCP = 0.07 ppm, DCDD 0.10 ppm, and TCDD = 0.06 ppm. The specific activity of the com-... 

Several facts have emerged from our studies with 2,7-DCDD and 2,3,7,8-TCDD. They are not biosynthesized by condensation of chloro-phenols in soils, and they are not photoproducts of 2,4-dichlorophenol. They do not leach into the soil profile and consequently pose no threat to groundwater, and they are not taken up by plants from minute residues likely to occur in soils. Photodecomposition is insignificant on dry soil surfaces but is probably important in water. Dichlorodibenzo-p-dioxin is lost by volatilization, but TCDD is probably involatile. These compounds are not translocated within the plant from foliar application, and they are degraded in the soil. 

If we assume that the TCDD is contained in the surface 6 inches of the soil profile since it is relatively immobile (5), then the 2,4,5-T at the 947 lbs of active ingredient/acre treatment would have had to contain 2.1 ppm TCDD to be observed. At the lower application rates of 584 and 160 lbs/acre, the 2,4,5-T would have had to contain 3.5 and 12.5 ppm TCDD in the technical materials to have 1 ppb in the top 6 inches of soil. Since the soil is sandy and high rainfall occurred in the area, maximum movement of materials in soil may occur causing TCDD to be present deeper in the profile. If the TCDD moved uniformly throughout the 36 inch soil profile, then six times more TCDD would have had to be present in the original 2,4,5-T for detection. This would have required the presence of 12.6, 21.0, and 75.0 ppm TCDD in the 2,4,5-T applied in the three treatments. These calculations are based on the assumption that no degradation occurred in or on the soil. 

The inner probe (with liner) is removed, leaving the outer retainer sleeve in the soil profile. While the liner Is still in the inner probe, a red cap is carefully placed on the top of the liner. Next, the probe is inverted, the A liner removed from the inner probe, and the bottom of the liner... 

The time to cleanup may actually be somewhat less than 9 years if Pb migrates down in the soil profile with the addition of EDTA, or if tillage practices serve to smooth out the hot spots. Regulatory cleanup levels are usually based on a limit that cannot be exceeded, such as 400 mg/kg, and soil concentrations would need to be analyzed to ensure compliance at the end of each year. 

UNSAT-H simulates plant transpiration with a PET concept. The model partitions plants removal of soil-water between soil layers based on (1) distribution of plant roots within the soil profile for cheatgrass (an invading and weedy grass species found in dry regions of Washington State) or (2) the user may supply other functions. The user must enter soil-water parameters that describe the limits for plant extraction of water from each layer of soil. The model also uses the same daily value pattern for the LAI for each year. 

Table 25.3 compares the characteristics of these four models.14 UNSAT-H and HYDRUS are the most widely known Richards equation models that use modem soil physics principles to estimate water movement within the soil profile. HELP and EPIC are widely known engineering models. 

Examination of Table 25.3 and the comments above clearly demonstrate that both HYDRUS and UNSAT-H are likely to produce very good estimates of water movement within the soil profile. However, they do not estimate snowmelt, model mixed plant communities, directly estimate surface runoff, or consider the effect of soil density on root growth and water use.14... 

Transport rates for dissolved material are based on the internal and external fluxes (flows) computed in the hydrology section of the module. Soluble chemicals are transported down through the soil profile and are washed out into streams with surface runoff, interflow and groundwater flow. Sediment... 

Simple models are used to Identify the dominant fate or transport path of a material near the terrestrial-atmospheric Interface. The models are based on partitioning and fugacity concepts as well as first-order transformation kinetics and second-order transport kinetics. Along with a consideration of the chemical and biological transformations, this approach determines if the material is likely to volatilize rapidly, leach downward, or move up and down in the soil profile in response to precipitation and evapotranspiration. This determination can be useful for preliminary risk assessments or for choosing the appropriate more complete terrestrial and atmospheric models for a study of environmental fate. The models are illustrated using a set of pesticides with widely different behavior patterns. 

These two conclusions are the important results from the model because they enable us to say how deep into the soil profile the majority of an organic chemical penetrates due to water inputs. If V centimeters of water are applied to a soil surface, then the water penetrates the soil to a depth of V/9. If V is sufficient to dissolve all the organic chemical present, then depth where the maximum concentration of chemical in the soil will be found is V/pKp. 

Table III illustrates the impact of adsorption on the leaching of organic chemicals in the soil. A water input of 305 cm was used, which is equivalent to a full year of precipitation in the eastern United States. In a soil with a field capacity of 30%, the water would penetrate 1017 cm. Mirex with a very large Kqc is practically immobile after a full year of precipitation, it is still on the surface. It is likely that any compound adsorbed this strongly would be carried off the land surface by soil erosion instead of being leached into the soil. In contrast, DBCP, which is very weakly adsorbed, penetrates the soil profile almost as far as the water does.

DBCP. The predictions suggest that DBCP is volatile and diffuses rapidly into the atmosphere and that it is also readily leached into the soil profile. In the model soil, its volatilization half-life was only 1.2 days when it was assumed to be evenly distributed into the top 10 cm of soil. However, DBCP could be leached as much as 50 cm deep by only 25 cm of water, and at this depth diffusion to the surface would be slow. From the literature study of transformation processes, we found no clear evidence for rapid oxidation or hydrolysis. Photolysis would not occur below the soil surface. No useable data for estimating biodegradation rates were found although Castro and Belser (28) showed that biodegradation did occur. The rate was assumed to be slow because all halogenated hydrocarbons degrade slowly. DBCP was therefore assumed to be persistent. 

Mlrex. Mirex does not leach into the soil profile and is predicted to volatilize only slowly. There Is no evidence for any rapid transformation so it should be considered persistent. Because It is so strongly adsorbed to the soil and stays on the surface, a major loss from terrestrial systems would probably be erosion and transport Into surface waters. 

Toxaphene. Toxaphene is apparently strongly adsorbed and should not move in the soil profile. Because of its strong sorptive interaction with soils, some material may erode into surface waters during irrigation or precipitation events. There is no evidence for oxidation, hydrolysis, or biodegradation (29). Photolysis probably would not occur. However, volatilization is... 

---