Weathering in a soil

Only for the intermediate cases - those with velocities in the range of about 100 m yr-1 to 1000 m yr-1 - does silica concentration and reaction rate vary greatly across the main part of the domain. Significantly, only these cases benefit from the extra effort of calculating a reactive transport model. For more rapid flows, the same result is given by a lumped parameter simulation, or box model, as we could construct in REACT. And for slower flow, a local equilibrium model suffices. 

Perhaps the most notable observation about these results is how poorly they reflect field observations. Even in clean orthoquartize aquifers undergoing rapid recharge, groundwater below about 60 °C is not observed to approach equilibrium with quartz, or even necessarily follow a clear trend along the direction of flow. Instead, silica concentration is highly variable and commonly in excess of quartz saturation, rather than below it. 

A number of factors contribute to the disparity between the predictions of kinetic theory and conditions observed in the field, as discussed in Section 16.2. In this case, we might infer the dissolution and precipitation of minerals such as opal CT (cristobalite and tridymite, Si02), smectite and other clay minerals, and zeolites help control silica concentration. The minerals may be of minor significance in the aquifer volumetrically, but their high rate constants and specific surface areas allow them to react rapidly. 

As a second example, we construct a simple model of how minerals might dissolve and precipitate as rainwater percolates through a soil (Bethke, 1997). The soil, 1 m thick, is composed initially of 50% quartz by volume, 5% potassium feldspar (KSiAEOfO, and 5% albite (sodium feldspar, NaSiAEOx). The remaining 40% of the soil s volume is taken up by soil gas (15% of the bulk) and water (25%). 

Rainwater recharges the top of the profile and reacted water drains from the bottom. We take discharge through the soil to be 4 m yr-1 and assume the dispersivity or (see Chapter 20) is 1 cm. The rainwater is dilute and in equilibrium with the CO2 fugacity of the atmosphere, 10-3 5. Within the soil, however, the soil gas is taken to contain additional CO2 as a result of the decay of organic matter, and root respiration. The pore fluid is assumed to maintain equilibrium with the soil gas and CO2 fugacity within the soil is held constant over the simulation, at 10-2. 

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Track the possible fate of a ion from the moment it is released by weathering in a soil to its burial at sea. 

Fig. 27.3. Saturation states (top) and reaction rates (bottom) for minerals in a simulation of weathering in a soil profile, at the calculation s stationary state. Rainwater in the simulation recharges the top of the profile (left side of plots) at 4 m yr 1, and reacted fluid drains from the bottom (right).

Wilson, M.J. Farmer,V.C. (1970) A study of weathering in a soil derived from a biotite-... 

RICH (C.I.), 1958. Muscovite weathering in a soil developed in the Virginia Piedmont. Clays and Clay Min. 5, 203-213. 

Figure 7.1 represents an idealized course of weathering in a soil profile. The basicity and acidity are emphasized because pH is an easily measured indicator of the state of weathering. The zones of basicity and acidity leach down through the soil profile during development. The alkali cations released during breakdown of the parent material accumulate in a narrow, rarely noticeable zone. This is followed closely by a much more obvious band of CaCC>3 accumulation. 

The total petroleum hydrocarbons represents a summation of all the hydrocarbon compounds that may be present (and detected) in a soil sample. Because of differences in product composition between, for example, gasoline and diesel, or fresh versus weathered fuels, the types of compounds present at one site may be completely different from those present at another. 

Over millions of years, rocks are weathered down into small particles, which form the basic ingredient of almost all soil. The size and chemical composition of the particles depends on the rock they came from—that is, the physical location and geological profile of your area—and determines the type of soil you have. There are three types of weathered rock particles that make up soil sand, silt, and clay. The proportion of the different particles found In a soil determines Its type—what name it is given—and how it behaves and should be managed. Most soil contains a mixture of all... 

Thus the "stability sequence" is equivalent to increasing the intensity of weathering or to completing the process of equilibration under conditions of high water content, low concentration of alkali and silica ions and oxidation of iron in a soil profile. In fact the weathering process in a soil horizon sequence is much the same for pelitic rocks in all environments the dominant sequence is repeated with minor variations due to local conditions of pH or efficiency of the oxidation process. 

JOHNSON (L.J.), 1964. Occurrence of regularly interstratifled chlorite-vermiculite as a weathering product of chlorite in a soil. Amer. 

Bornyasz, M. A., Graham, R. Allen, M. F. (2005). Ectomycorrhizas in a soil-weathered granitic bedrock regolith linking matrix resources to plants. Geoderma, 126, 141-60. 

Egerton-Warburton, L. M., Graham, R. C. Hubbert, K. R. (2003). Spatial variability in mycorrhizal hyphae and nutrient and water availability in a soil-weathered bedrock profile. Plant and Soil, 249, 331 2. 

Weathering in older soils produces decreasing permeabilities due to in situ secondary mineral formation and the development of hard pans and argillic horizons. Such zones of secondary clay and iron-oxides are clearly evident in the increased aluminum and iron at a depth of a meter in the Panola regolith (Figure 3). Low permeabilities in such features are commonly related to the absence of continuous pores due to the formation of thick cutans of clay (O Brien and Buol, 1984). This process is enhanced by physical translocation and collapse of saprolite structures (Torrent and Nettleton, 1978). 

White A. F., Blum A. E., Schulz M. S., BuIIen T. D., Harden J. W., and Peterson M. L. (1996) Chemical weathering of a soil chronosequnece on granitic alluvium I. Reaction rates based on changes in soil mineralogy. Geochim. Cosmochim. Acta 60, 2533-2550. 

Andrews J. H. and Schlesinger W. H. (2001) Soil CO2 dynamics, acidification, and chemical weathering in a temperate forest with experimental CO2 enrichment. Global Bigeochem. Cycles 15(1), 149-162. 

Figure 9 Early Paleozoic changes in (a) soil differentiation as indicated by clay content (volume percent) and alumina/bases (molar ratio) of the most weathered horizon of calcareous red paleosols (b) soil bioturbation as indicated by proportion of transect in paleosols occupied by roots or burrows (percent) and by measured rooting depth (m) (c) atmospheric CO2 levels (PAL) calculated from a sedimentary mass balance model (d) maximum coal seam thickness and average thickness of at least 10 consecutive seams (m) (e) diameter of fossil plant stems and roots (m) (f) diversity of fossil land plants (number of species) (g) diversity of soil animals (number of families) (Retallack, 1997c) (reproduced from Dinofest, 1997, pp. 345-359).

Loveland P. J. (1981) Weathering of a soil glauconite in Southern England. Geoderma 40, 40-42. 

Chlorinated phenolic compounds in air-dried sediments collected downstream of chlorine-bleaching mills were treated with acetic anhydride in the presence of triethylamine. The acetylated derivatives were removed from the matrix by supercritical fluid extraction (SEE) using carbon dioxide. The best overall recovery for the phenolics was obtained at 110°C and 37 MPa pressure. Two SEE steps had to be carried out on the same sample for quantitative recovery of the phenolics in weathered sediments. The SEE unit was coupled downstream with a GC for end analysis . Off-line SEE followed by capillary GC was applied in the determination of phenol in polymeric matrices . The sonication method recommended by EPA for extraction of pollutants from soil is inferior to both MAP and SEE techniques in the case of phenol, o-cresol, m-cresol and p-cresol spiked on soil containing various proportions of activated charcoal. MAP afforded the highest recoveries (>80%), except for o-cresol in a soil containing more than 5% of activated carbon. The SEE method was inefficient for the four phenols tested however, in situ derivatization of the analytes significantly improved the performance . 

It is necessary to conclude that many of the mineral stability diagrams that are commonly constructed to explain primary mineral weathering (such as the one for feldspar in Figure 6.12) have no quantitative value—they are useful only to the extent that they gauge the tendency of the weathering reaction to proceed in a forward direction. If, for example, feldspar and kaolinite coexist in a soil, overall equilibrium between the two minerals is not possible when a realistic temperature and time frame is considered. The reaction is irreversible if it proceeds, feldspar must decompose and kaolinite must precipitate. It is true that back reactions such as the... 

The Mo concentration measured in a soil also is affected by the influence of geochemical processes on its mobility, transport, and deposition. The extent of its mobilization and transport from source rocks is determined by mineral stability, which in turn is affected by the weathering environment. Once it is mobilized from the mineral source, the transport, deposition, and availability of Mo to organisms are dependent on its interactions with other soil components (such as clays, organic matter, microbes, and Fe and Mn oxyhydroxides) and the chemistry (pH, Eh, and other ion concentrations) of the soil solution. Molybdenum associated with clay minerals, oxyhydroxides, and organic matter represents the available fraction. 

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