Effect of Hydraulic properties of porous media on solute transport under variable saturated flow conditions in repacked columns

Increasing concern of groundwater pollution associated with solutes from industrial and agricultural activities has motivated the use of solute transport experiments and mathematical models to explore and predict the transport behavior of solutes in the vadose zone. This study was comprised of two research projects designed to examine the effect of porous media on the transport behavior of solutes under variable saturated flow conditions. The objective of the first research project was to evaluate the effect of electrical conductivity (EC) of solution, duration of ponding of solution, bulk density of lining material and porous media, and wet-dry cycles on the saturated hydraulic conductivity (Ks). A natural and synthetic soil was used to examine the potential of sodium silicate (Na2SiO 3), calcium hydroxide (Ca(OH)2) and magnesium hydroxide (Mg(OH) 2) as lining materials after reacting with solutions of different ECs composed of calcium chloride (CaCl2), magnesium chloride (MgCl 2), sodium carbonate (Na2CO3) and sodium sulfate (Na2SO4). The results indicated that Ks decreased with increasing EC, duration of ponding, and bulk density in columns repacked with a Ca(OH)2 layer. This study demonstrated that Ca(OH) 2 could be used as a lining material with self-sealing capacity for desalination evaporation ponds in order to reduce the risk of groundwater pollution with saline wastes. Groundwater quality is also being negatively affected with agricultural chemicals and excessive amounts of nutrients, especially in shallow aquifers. The objectives of the second research project were to examine the individual and coupled transport of nitrate (NO3- ) and chloride (Cl-) through homogeneous and layered soil columns and determine the effect of pore water velocity on solute transport parameters under variable saturated flow conditions. Acrylic plastic columns were repacked with sand, loam and layers of both. A pulse type input of 0.1 M CaCl2, 0.1 M calcium nitrate (Ca(NO3)2), and 0.1 M of both calcium salts was individually applied under a steady flow during saturated flow experiments. The CXTFIT program was used to determine the two region non-equilibrium model parameters from breakthrough curves (BTCs) for different pore water velocities. The dispersion coefficient (D), retardation factor (R) and mobile water content (thetam) for NO3 - and Cl- increased with pore water velocity during their individual and coupled transport, except for the anion exclusion (theta ex) and mass exchange coefficient (alpha). Similar observations were made in the simultaneous transport of NO3- and Cl - in unsaturated flow experiments and experiments conducted with a layered porous medium under saturated flow conditions. Since NO3 - and Cl- exhibited similar transport behavior in soil columns under variable saturated flow conditions through a homogeneous and layered porous media, Cl- could be used to predict the transport behavior of NO3- in areas where the denitrification process is limited due to unfavorable environmental conditions. The risk of groundwater pollution in shallow aquifers with NO3- could be reduced by initially examining the downward movement of Cl - through the soil profile before application of NO3 --containing fertilizers.