2.2.4 Model Discretization and Initial and Boundary Conditions for Continuous-Flow Reactive Transport
A one-dimensional finite-difference numerical model depicting reactive transport under saturated-flow conditions was developed using the equations previously described on the GWB platform. The inlet boundary condition (x=0 at t>0) was specified by composition of the influent feedwater (Table 2 ). Porewater ionic species concentrations at equilibrium with feedwater (i.e., no flow) were taken as the initial condition at t=0 along the length of the soil column. The mineral stratigraphy data for each soil type from the Soil Survey (Tables 3(a)-(c) ) was mapped on to a one-dimensional saturated-flow finite-difference model with node-centered cell dimensions of 10 cm (Figure 2 ). An average annual evaporation of 2,000 mm was assumed for the coastal UAE region (International Center for Biosaline Agriculture, 2013), while rain precipitation was assumed to be negligible (Statistics Center Abu Dhabi (SCAD):www.scad.gov.abudhabi ). The evaporation loading was applied to only the top 3 surficial cells (cell 1 at 1 m3/m2/yr; cells 2 and 3 at 0.5 m3/m2/yr). Initial porewater conditions were assumed to be at equilibrium with the mineral content in each cell mapped from the Soil Survey. Feedwater was assumed to have the reported average ionic composition of Arabian/Persian Gulf sweater (Table 2 ). Equations (2) and (3) governed the finite difference model, which are solved in GWB using an “operator splitting method” where the system of equations are solved sequentially for transport (Figure 2 ) at each node before moving onto the next node. Default mineral-specific mass transfer coefficients for precipitation/dissolution in GWB were utilized for the various minerals present in the soil. Both, B-dot and PHRQPITZ, activity coefficient models were employed both in the agricultural return simulations and in the one-year-long continuous-flow runs. TDS of the leachate return was calculated from the ion concentration data in GWB. Three different annual irrigation water loadings, 2.5 m, 3m, and 4 m, were applied to each soil column model corresponding to leachate fractions of 0.25, 0.50, and 1.0, respectively. Leachate fraction is the ratio of water effluent to influent into the column and is a measure of the amount of water loss owing to evapotranspiration losses in comparison to total feedwater loading.