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.