Groundwater sorption

There has been considerable interest recently in the migration of long-lived nuclides involving technetium. The behavior of technetium in groundwater, sorption and permeation under subterranean conditions needs to be studied for the purpose of assessing environmental safety in connection with the disposal of spent nuclear fuel. Chemical and physicochemical data on technetium under such conditions are necessary. 

Fig. 1. Schematic diagram showing the conceptual model of the groundwater pathway in the BVG. Radionuclides are transported through the fracture network by advection with the groundwater. Two mechanisms retard migration relative to the flowing groundwater sorption and rock-matrix diffusion.

An extensive pesticide properties database was compiled, which includes six physical properties, ie, solubiUty, half-life, soil sorption, vapor pressure, acid pR and base pR for about 240 compounds (4). Because not all of the properties have been measured for all pesticides, some values had to be estimated. By early 1995, the Agricultural Research Service (ARS) had developed a computerized pesticide property database containing 17 physical properties for 330 pesticide compounds. The primary user of these data has been the USDA s Natural Resources Conservation Service (formerly the Soil Conservation Service) for leaching models to advise farmers on any combination of soil and pesticide properties that could potentially lead to substantial groundwater contamination. 

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

Insecticide methomyl is a very toxic pesticide and is highly soluble in water (57.9 g/1). It has a low sorption affinity to soil and can cause groundwater and surface water contamination in agricultural areas. Solubilities of methomyl in different solvents are in methanol 1000 g/1, in aceton 730 g/1, in ethanol 420 g/1, in isopropanol 220 g/1, in toluene 30 g/1. 

However, it has been concluded from sorption and diffusion experiments that plutonium exists largely in the tetravalent state (53) and clearly not as Pu(V), in the intermediate pH-range under oxic conditions and at low carbonate concentration. This would be representative of many groundwaters and also in agreement with the calculated curves of Figure 2. 

The sorption of plutonium on a variety of the common minerals of igneous rocks under groundwater conditions is illustrated in Figure 3 (57, 58, 59). For comparison the sorption of other acti-... 

Sorption of plutonium (l.fixlO-11 M) and americium (2xl0-9 M) in artificial groundwater (salt concentration 300 mg/liter total carbonate 120 mg/liter Ref. 59) on some geologic minerals, quartz, biotite, o apatite, o attapulgite, montmorillonite. Dashed lines indicate the range for major minerals in igneous rocks. Experimental conditions room temperature, particle size 0.04-0.06 mm, solid/liquid ratio 6-10 g/1, aerated system, contact time 6 days. 

Plutonium is transported by the groundwater in fractures in the rock (usually <1 mm wide). A typical groundwater velocity (vw) at >100 m depth in Swedish bedrock is 0.1 tn/y. The fractures are filled with crushed, weathered, clayish minerals, which have a high capacity to sorb the plutonium. Assuming instantaneous and reversible reactions, the sorption will cause the plutonium to move considerably slower (with velocity vn) than the groundwater. The ratio between these two velocities is referred to as the retention factor (RF), defined by... 

Removal by sorption. It is generally assumed that adsorption is reversible. Irreversible removal directly from groundwater would be included in the term for precipitation, although slow irreversible incorporation of sorbed atoms is not considered in Equation (1). 

Reardon, E.J., Kjs—Can they be used to describe reversible ion sorption reactions in contaminant migration Groundwater 19, 279-286, 1981. 

Lesser retardation factor (slowing of migration with groundwater due to sorption to aquifer matrix). 

Since hydrogen sulfide exists as a gas at atmospheric pressure, partitioning to the air is likely to occur after environmental releases. However, the compound is also soluble in oil and water, and therefore, may partition to surface waters, groundwaters, or moist soils, and subsequently travel great distances. In addition, sorption of hydrogen sulfide from air onto soils (Cihacek and Bremner 1993) and plant foliage (DeKoketal. 1983, 1988, 1991) may occur. 

MMT (32) is a 1- or 2-dimensional solute transport numerical groundwater model, to be driven off-line by a flow transport, such as VTT (Variable Thickness Transport). MMT employs the random-walk numerical method and was originally developed for radionuclide transport. The model accounts for advection, sorption and decay. 

KoC is an important parameter which describes the potential for movement or mobility of pesticides in soil, sediment and groundwater. Because of the structural complexity of these agrochemical molecules, the above simple relationship which considers only the chemical s hydrophobicity may fail for polar and ionic compounds. The effects of pH, soil properties, mineral surfaces and other factors influencing sorption become important. Other quantities, KD (sorption partition coefficient to the whole soil on a dry weight basis) and KqM (organic matter-water partition coefficient) are also commonly used to describe the extent of sorption. K0M is often estimated as 0.56 KoC, implying that organic matter is 56% carbon. 

Schwarzenbach, R. P., Westall, J. (1981) Transport of nonpolar compounds from surface water to groundwater. Laboratory sorption studies. Environ. Sci. Technol. 11, 1360-1367. 

In simple cases, the mobility in the subsurface of a sorbing contaminant can be described by a retardation factor. Where contaminated water passes into a clean aquifer, a reaction front develops. The front separates clean, or nearly clean water downstream from fully contaminated water upstream. Along the front, the sorption reaction removes the contaminant from solution. The retardation factor describes how rapidly the front moves through the aquifer, relative to the groundwater. A retardation factor of two means the front, and hence the contamination, will take twice as long as the groundwater to traverse a given distance. 

Fig. 32.4. Chromatographic separation of metal contaminants in a groundwater flow at 25 °C, due to differential sorption. According to the surface complexation model used, Hg++ in the simulation sorbs more strongly to the ferric surface in the aquifer than Pb++, which sorbs more strongly than Zn++. Plot at top shows concentrations of the metal ions in groundwater, and bottom plot shows sorbed metal concentrations.

KEYWORDS groundwater geochemistry, arsenic mobility, redox, sorption, Quaternary aquifers... 

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