Land subsurface

Detonations which produce particle populations of the first category are land surface bursts, land subsurface bursts, vented underground bursts, and tower bursts. 

Land Subsurface Burst. Everything which was said above about land surface burst applies exactly to the aerial cloud particle population produced by a land subsurface burst in which an aboveground fireball appears. However, a third component of the particle population is found. This component appears to result from soil material which interacted with the fireball at high temperature but which was separated from the fireball early, before the temperature had fallen below the melting point of the soil materials. The particles in this component have diameters ranging from tens of microns to several centimeters and have densities which are apt to be quite low compared with those of the original soil components. The relative abundance of radionuclides in this component is quite constant from sample to sample and is independent of particle size. If we indicate by subscript 1 this third component and by 2,3 the aerial cloud components, radionuclide partitioning can be described by a series of equations of the forms... 

The following discussion gives an example of the data treatment required to characterize the population for three cases—a land surface burst, a land subsurface detonation, and an airburst detonation. These three examples cover the complete range of types of solutions to the characterization problem. 

Figure 4. Land subsurface detonation. Aerial filter samples.

Table V. Fission Product Distribution Parameters (Land Subsurface Detonation)...

Table VI. Parameters of Distribution Functions (Land Subsurface Burst)...

The excess of evaporation from the oceans is made up for by runoff from the land. Although this flux is much smaller than precipitation and ET, it is a major link in many cycles and is of particular importance to humans in terms of water supply. Runoff can be broadly categorized into subsurface, or groundwater, flow and surface flow, consisting of overland runoff and river discharge. 

Subsurface runoff. When precipitation hits the land surface, the vast majority does not go directly into the network of streams and rivers in fact, it may be cycled several times before ever reaching a river and the ocean. Instead, most precipitation that is not intercepted by the vegetation canopy and re-evaporated infiltrates into the soil, where it may reside as soil moisture, percolate down to ground-water, or be transpired by plants. 

Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration. 

Ward CH, Tomson MB, Bedient PB, et al. 1986. Transport and fate processes in the subsurface. Water Resources Symposium 13(Land Treatment Hazardardous Waste Management Alternatives) 19-39. 

As illustrated in Figure 11.3, when hydraulic conditions are favorable, ffeshwaters flow below the land surface toward the ocean. Subsurface flows are only possible if the water has voids to move through, such as cracks in crustal rocks or the spaces (pores) between sediment particles. Subsurfece deposits with a high degree of pore spaces are... 

Reversible and irreversible retention of contaminants on the subsurface solid phase is a major process in determining pollutant concentrations and controlling their redistribution from the land surface to groundwater. After being retained in the solid, contaminants may be released into the subsurface liquid phase, displaced as water-immiscible liquids, or transported into the subsurface gaseous phase or from the near surface into the atmosphere. The form and the rate of release are governed by the properties of both contaminant and solid phase, as well as by the subsurface environmental conditions. We consider here contaminants adsorbed on the solid phase. 

One way that contaminants are retained in the subsurface is in the form of a dissolved fraction in the subsurface aqueous solution. As described in Chapter 1, the subsurface aqueous phase includes retained water, near the solid surface, and free water. If the retained water has an apparently static character, the subsurface free water is in a continuous feedback system with any incoming source of water. The amount and composition of incoming water are controlled by natural or human-induced factors. Contaminants may reach the subsurface liquid phase directly from a polluted gaseous phase, from point and nonpoint contamination sources on the land surface, from already polluted groundwater, or from the release of toxic compounds adsorbed on suspended particles. Moreover, disposal of an aqueous liquid that contains an amount of contaminant greater than its solubility in water may lead to the formation of a type of emulsion containing very small droplets. Under such conditions, one must deal with apparent solubility, which is greater than handbook contaminant solubility values. 

Under natural conditions, contaminants often reach the earth s surface as a mixture of (potentially) toxic chemicals, having a range of physicochemical properties that affect their partitioning among the gaseous, liquid, and solid phases. As a consequence, contaminant retention properties in the subsurface are highly diverse. Contaminants may reach the subsurface from the air, water, or land surface. 

Table 8.10 shows the concentration range of potential toxic trace elements in U.S. sewage sludges, as summarized by Chaney (1989). In this table, data on maximum concentration of toxic trace elements in dry, digested sewage sludges are compared to concentrations of the elements in median sludges and in soils. The subsurface contamination that may result from uncontrolled disposal on land surfaces... 

It should be emphasized that, to date, the ability to quantify the complex chemical reaction phenomena that occur in the subsurface and also integrate the variability in flow behavior caused by natural heterogeneity and fluctuating boundary (land surface) conditions remains very limited. As a consequence, developing and improving the predictive capabilities of models is an area of active research. 

When water flows over a contaminated land surface, pollutants released from higher elevations are transported, as dissolved solute or adsorbed on suspended particles, and accumulate at lower elevations. This behavior is reflected in the spatial variability of contaminant concentration, which affects contaminant redistribution with depth following leaching. If a sorbed contaminant is not of uniform concentration across all soil-size ranges but is higher in the fine sediment fraction, the deposition of this soil fraction controls contaminant redistribution in the subsurface. 

The behavior of nonaqueous phase liquids (NAPLs) as they enter the partially saturated subsurface from a land surface source follows two well-defined scenarios in one case, the physical properties of the NAPL remain unchanged, while in the second case, NAPL properties are altered during transport. In the case of dense NAPLs, the contaminant plume reaches the aquifer and is subject to longterm, continuous, slow local redistribution due to groundwater flushing-dissolution processes. These plumes become contamination source zones that evolve over time, often with major negative impacts on groundwater quality. 

Contaminants retained on colloid surfaces may be transported by flowing water through the vadose zone and reach the groundwater or be relocated on the land surface by runoff processes. Operationally defined as particles between 1 j,m and 1 nm in size, colloids in the subsurface may be of mineral, organic, or biological origin. 

---