Soils solute transport

Stonestrom D. A., White A. F., and Akstin K. C. (1998) Determining rates of chemical weathering in soils-solute transport versus profile evolution. J. Hydrol. 209, 331-345. 

Soil reactions are generally classified according to the nature of the main chemical process involved adsorption, ion exchange, dissolution, etc. However, in order to assess the kinetics one should consider the nature and the rate of the transport processes associated with the chemical reaction flow and diffusion in the soil solution, transport across the solid-liquid interface, diffusion in liquid-filled pores and micropores, and surface diffusion penetration into the solid. An expression for the kinetics of soil reactions can be devised by assigning rate equations to transport and chemical processes and combining these equations. The expression finally obtained has to be validated by comparison to experimental results. 

Keywords soil solute transport diffuse pollution modelling groundwater protection... 

Under experimental conditions, most rock-forming silicate minerals are sufficiently nonreactive that transport of solutes toward, or away from, the dissolving surface does not control the rate. The rate of the overall reaction is controlled by reactions at the interfaces, that is, the second sequential step. In soil, solute transport is sufficiently rapid that silicate weathering is controlled by the reaction kinetics at the mineral-solution interface (Berner 1978). 

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

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). 

Depending on the ability of specific transport systems to utilize the predominant metal chelates present in the soil solution, competition may occur between plants and microorganisms and between different types of microorganisms for available iron. This has been particularly well studied for Pseudomonas sp., which produce highly unique iron chelators that are utilized in a strain specific manner but which also retain the ability to use more generic siderophores pro-... 

Soil solution is the aqueous phase of soil. It is in the pore space of soils and includes soil water and soluble constituents, such as dissolved inorganic ions and dissolved organic solutes. Soil solution accommodates and nourishes many surface and solution reactions and soil processes, such as soil formation and decomposition of organic matter. Soil solution provides the source and a channel for movement and transport of nutrients and trace elements and regulates their bioavailability in soils to plants. Trace element uptake by organisms and transport in natural systems typically occurs through the solution phase (Traina and Laperche, 1999). 

Valocchi, A.J., 1985, Validity of the local equilibrium assumption for modeling sorbing solute transport through homogeneous soils. Water Resources Research 21, 808-820. 

PROFILE is a biogeochemical model developed specially to calculate the influence of acid depositions on soil as a part of an ecosystem. The sets of chemical and biogeochemical reactions implemented in this model are (1) soil solution equilibrium, (2) mineral weathering, (3) nitrification and (4) nutrient uptake. Other biogeochemical processes affect soil chemistry via boundary conditions. However, there are many important physical soil processes and site conditions such as convective transport of solutes through the soil profile, the almost total absence of radial water flux (down through the soil profile) in mountain soils, the absence of radial runoff from the profile in soils with permafrost, etc., which are not implemented in the model and have to be taken into account in other ways. 

SOIL AND SOIL SOLUTION SAMPLING, SAMPLE TRANSPORT, AND STORAGE... 

The root time method of data analysis for diffusion coefficient determination was developed by Mohamed and Yong [142] and Mohamed et al. [153]. The procedure used for computing the diffusion coefficient utilizes the analytical solution of the differential equation of solute transport in soil-solids (i.e., the diffusion-dispersion equation) ... 

Transport-related non-equilibrium behavior (e. g., physical non-equilibrium) is excluded, which plays an important role in non-ideal solute transport in the field and in some experimental column systems. Physical non-equilibrium is due to slow exchange of solute between mobile and less mobile water, such as may exist between particles or between zones of different hydraulic conductivities in the subsurface soil column, and occurs for sorbing and non-sorbing molecules alike. 

Yong RN, Samani HMV (1989) Analysis of two-dimensional solute transport in clay soils using irreversible thermodynamics. Proc CANCAM 23 54... 

This chapter is concerned with how ions and uncharged solutes in the water and soil solution in submerged soils interchange between the solid, liquid and gas phases present. This is a large topic. 1 give here the bare essentials needed to understand the transport and transformation processes discussed elsewhere in the book, and 1 give references to more detailed treatments where appropriate. The water and atmosphere overlying the soil are dealt with first and then the additional complexities in the soil. 

The main transport processes involved are shown in Figure 6.7. In essence these are the same as in a non-flooded soil there is a dynamic equilibrium between solutes in the soil solution and those sorbed on the immediately adjacent... 

This is somewhat surprising in dryland crops the total root length is generally far larger than necessary to account for N uptake. An important difference is that, as a result of NH4+ adsorption on the soil solid, unlike for NOs , the concentration of NH4+ in the soil solution is less than for high affinity NH4+ transporters in the root and so V < Vmax and a larger root length is required. 

Transport of NH4+ to the roots in Kirk and Solivas experiment was mainly by diffusion. The additional transport resulting from mass flow of soil solution in the transpiration stream would have increased the influx across the roots by about QQaVa/0.5bD% where Va is the water flux (Tinker and Nye, 2000, pp. 146-148), or about 4% in Kirk and Solivas experiment. A sensitivity analysis showed that rates of diffusion will generally not limit uptake in well-puddled soils, but they may greatly limit uptake in puddled soils that have been drained and re-flooded and in unpuddled flooded soils. 

The extent of NO3 absorption by soil-grown plants will depend on its rate of formation and loss in the rhizosphere (this is considered in Section 6.5). Transporters for amino acids have also been found in plant roots, and concentrations of amino acids in the soil solution in flooded soils can be appreciable. Therefore it seems likely that some N is also absorbed as amino acids, but as yet we do not have the necessary data to quantify this. 

Transport of contaminants by surface runoff is illustrated in the experimental results of Turner et al. (2004), which deal with the colloid-mediated transfer of phosphorus (P) from a calcareous agricultural land to watercourses. Colloidal molybdate-reactive phosphorus (MRP) was identified by ultrafiltration associated with particles between l am and Inm in diameter. Colloidal P compounds can constitute a substantial component of the filterable MRP in soil solution and include primary and secondary P minerals, P occluded or adsorbed on or within mineral or organic particles, and biocolloids (Kretzschmar et al. 1999). 

Rouen D, Scher H, Blunt M (1997) On the structure and flow processes in the capillary fringe of phreatic aquifers. Transp Porous Media 28 159-180 Rose CW (1993) The transport of adsorbed chemicals in eroded sediments. In Russo D, Dagan G (eds) Water flow and solute transport in soils. Springer, Heidelberg, pp 180-199 Rosenberry DO, Winter TC (1997) Dynamics of water-table fluctuations in an upland between two prairie-pothole wetlands in North Dakota. J Hydrol 191 266-289 Russo D (1997) On the estimation of parameters of log-unsaturated conductivity covariance from solute transport data. Adv Water Resour 20 191-205 Russo D, Toiber-Yasur 1, Laufer A, Yaron B (1998) Numerical analysis of field scale transport of bromacU. Adv Water Resour 21 637-647... 

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