Empirical model approach for the evaluation of pH and conductivity on pollutant diffusion in soil environment

ISSN: 2410-9649 Ukpaka et al / Chemistry International 2(4) (2016) 267-278 iscientic.org. 268 www.bosaljournals/chemint/ editorci@bosaljournals.com and diffusion of pollutants in soil is very key and important for a number of reasons. Some chemicals are important as they are required for soil and plant nutrient, other chemicals may be highly toxic particularly if they are present in high concentration. A chemical becomes a pollutant if its concentration exceeds some prescribed water and or soil required quantity/quality standard or if a beneficial use has been impaired, and if the cause is induced by human activities (Parlange et al., 1995). The investigation of pollutant diffusion in, and through the soil environment majorly centers on this zone. Mathematical models are developed based on the flow mechanism in this zone (Jirka et al., 2002). The geological media between the land surface and the regional water table below is known as the vadose zone (Stephens, 1996). The zone includes the crop root layer; it therefore, plays an integral role in the global hydrological circle controlling surface water infiltration, runoff, Hence, the availability of soil water and nutrient for plant (Ukpaka, 2014a – i; Amadi and Ukpaka, 2014a – d; Ikenyiri et al., 2012, 2012a). Initially, investigation of this zone was focused on water availability for crops and optimal management of the root zone. However, in recent years much of increased use of agrochemicals such as Pesticides, Fertilizers and increased demands to store and dispose of industrial and municipal waste such as sewage has made this unsaturated zone to be contaminated(Thomas, 2007; Ukpaka 2013a–i; Amadi and Ukpaka, 2015, 2015a; Ikenyiri and Ukpaka, 2014, 2014a). This zone is typically the first subsurface soil environment to encounter surface applied agrochemicals and industrial contaminants and hence all surface and subsurface chemical concentrations and subsequent environmental impact are inextricably linked to the Physical, Biological and Chemical dynamics within the zone; which include SorptionDesorption, Volatilization, Photolysis, Degradation etc (Looney and Falta, 2000; Ukpaka and Otabiri, 2013; Ukpaka and Pele, 2012, Ukpaka and Amdai, 2014). Unlike the aquifer of the saturated zone below, the unsaturated zone is not a source of readily available water for human consumption but it is of a great importance in providing water and nutrients that are vital to the biosphere, and intensively used for the cultivation of plans, construction of building and disposal of waste as earlier mentioned. The zone is majorly or often the main factor controlling water movement to the aquifer. Thus, it strongly affects the rate and management of groundwater (Ukpaka, 2015; Ukpaka and Ogoni, 2015; Theophilus et al., 2012; Prommer et al., 1999; Ruhdi and Mohammend, 2010; Daichao and David, 1999; Eluozo et al., 2012; Ewing et al., 1999). It is often regarded as a filter that removes undesired substances from groundwater. To some extent this is true but a more general fact is that the flow rate and chemical reactions in the unsaturated zone controls whether, where and how fast contaminants enters groundwater supplies (Looney and Falta, 2000). Understanding the physical, biological and chemical processes of this zone is therefore crucial in determining the quality of ground water for human consumption. Modeling the spread of pollutant in soil is complicated due to non-random spatial and temporal variations of physical, biological and chemical component of the soil environment (Stagnitti et al., 1995; Jack et al., 1998; Jacquas et al., 2008; Looney et al., 2000; Maddalera et al., 2007). The two main flow mechanisms in this zone are the Matrix (uniform) flow and the Preferential (funnel, macro pores) flow mechanism. Preferential flow is the uneven and often rapid movement of contaminate through porous media, typically soil characterized by regions of enhanced flux that a small fraction of media such as worm holes, root holes, cracks etc., participates in most of the flow, allowing much faster transport of a range of contaminants through soil and create significant consequence for groundwater quality. While Matrix flow is a relatively slow an even movement of water and solute (contaminants) through the soil while sampling all pore spaces, obeying the convective-dispersive theory which assumes that water flow an average flow path through soil. Both types of flow occur often simultaneously but have considerably different consequence for water flow and chemical leaching (discharge, escape). The chemical composition of preferential flow reflects the concentration of water near the surface while matrix flow represents the concentration of water around the drain (Mckone and Bennatt, 2003; Mirbagheri, 2004; Muibat and Jimoh, 2013; Okoromah and Ukpaka, 2013; Parlenge and Hill, 1995; Witterson and Nicholson, 1996). The spread of pollutant in soil is controlled by the flow of fluid (liquid or gaseous phase) in the soil environment. Model describing the spread of pollutants through the soil is discussed in most cases by the convectivedispersive equation also called the Richard’s equation. The convective-dispersive equation is a combination of the diffusion and convection (advection) equation which describes Physical Phenomena where particles, energy or other physical quantities are transported inside a physical or chemical, and or biological system due to the following processes: Diffusion, Convection, Chemical reactions/degradation (Van and Alves, 1982; Ukpaka et al., 2015; Ukpaka et al., 2011; Ukpaka et al., 2012; Simunek et al., 2013, 2001). Contaminants transported by water in the soil can follow a variety of paths. In the simplest case, a onedimensional convection takes place which can be accompanied by dispersion, adsorption and chemical reaction and probably other factors. Description of contaminants transport in terms of average water velocity v, by the convective-dispersive equation remains the first and fundamental transport equation to be considered (Stagnitti, et al., 1995; Amadi and Ukpaka, 2013, 2014, 2015; Schmidt and Gier, 1990; Schwarz et al., 2009). ISSN: 2410-9649 Ukpaka et al / Chemistry International 2(4) (2016) 267-278 iscientic.org. 269 www.bosaljournals/chemint/ editorci@bosaljournals.com Empirical modeling depends on the availability of representative data for model development and validation of already existing models. Also it seeks to approximate the unknown functional relationship by some mathematical functions based on experimentally gathered data from the physical system (Ukpaka, 2012; Singh et al., 2007). In this Fig. 1a: Soil samples obtained from the vicinity of the Asphalt plant Company, Enito 3, Ahoada west LGA, ready for analysis in the Chemical Sciences Research Laboratory of the Niger Delta University, Wilberforce Island, Bayelsa State Fig. 1b: systematic distribution of contaminants in subsurface soil environment Top Soil