Simulation of Nickel in soils affected by wastewater and sludge

Introduction

Hydrus-1D software is one of the most advanced software in conjunction with one-dimensional movement of water, salts in the soil (Ghorbani et al, 2009). Many or most subsurface pollution problems at the field scale involve such simultaneous processes as water flow, multicomponent solute transport, heat transport and biogeochemical processes and reactions. 

Process-based models that integrate these various processes can be valuabl tools for investigating the mobility of a wide range of inorganic and organic contaminants subject to different hydrologic and geochemical conditions (Jacques et al, 2008). The transport of Cu, Pb and Zn in paddy soils was simulated by numerical modeling of non-equilibrium solute transport and adaptation of the Hydrus-1D model. The simulations show that leaching rates decrease in the order, Zn > Cu > Pb. This order is confirmed by the results of sequential extractions. Under constant flooded conditions at a water table of 20 cm, Cu, Pb and Zn were estimated to reach the soil depth of 1 m within 470, 495 and 370 days, respectively, emphasizing that reactive pollutants can reach groundwater in a relatively short time (Nguen et al, 2009). 

The processes of evaporation and plant transpiration also exert a major influence on water and solute distributions in near-surface environments (vrugt et al, 2001). These processes concentrate salts by decreasing the amount of water in the soil, and when combined with irrigation in arid regions can lead to highly saline conditions. Ion activities for such chemical conditions should be calculated with expressions suitable for use in brines, rather than with the more standard formulations for dilute solutions (Sarvestani and Mirbagheri, 1982). The interaction of evapotranspiration, changing soil gas composition, ion exchange and soil-water reactions may cause precipitation and dissolution (Mirbagheri, 2004). The Hydrus-1D software package is for simulating one-dimensional variably saturated water flow, heat movement, and the transport of solutes involved in sequential first-order decay reactions. 

To be able to simulate the salinization processes, we also implemented into the Hydrus software the carbon dioxide transport and production, and major ion chemistry (Simunek et al., 2009).Hydrus computer program, and the Hydrus 1D interactive graphics-based user interface.

The Hydrus program numerically solves the Richards equation for saturated-unsaturated water flow and advection-dispersion type equations for heat and solute transport (Simunek et al., 2006).The water flow equation incorporates a sink term to account forwater uptake by plant roots. Moradi research and modeling fellow cadmium transport below the root zone in arid soils amended with sludge at the arid regions of central Iran using a one-dimensional model showed Haydrus. Very little risk of groundwater pollution caused by prolonged use of sludge on soil (Moradi et al, 2005). 

The most common relationship between the van Genuchten - Moallem (Šimůnek, 2006) as follows:ϴr: residual moisture, ϴs: saturation moisture, l, m, n: experimental parameters, Ks is the saturated hydraulic conductivity and Se: relative saturation.

 Hydrus-1D model for the simulation of water movement in soil, Richards equation solve using linear finite elements. Since in the case of onedimensional, linear finite elements plan and finite difference is similar, in this respect, using an indirect plan finite difference is as follows Substituting the above equation in the original equation three diagonal equation will be resulted that can be solved by standard methods such as Gussian elimination.