1. INTRODUCTION
An estimated 3% of global anthropogenic carbon emissions can be attributed to the aviation sector, with this percentage expected to increase substantially by 2050 with growth in the aviation industry (Mezher, Rathbun, Wang, & Ahmad, 2013). Although fossil fuel alternatives such as plant-derived biofuels have been tested in commercial aircrafts (Rahmes et al., 2009), these fuels often fail to meet the requirement of sustainability, as they compete for land and water that could otherwise be channelled into the production of food for humans and livestock (Warshay, Pan, & Sgouridis, 2011). Recently, salt tolerant plants called halophytes have been identified as a more sustainable biofuel feedstock source, owing to the fact that these plants can be cultivated in highly saline environments, using marginal soils, and with seawater irrigation (Khan & Duke, 2001; Warshay et al., 2011).
Successful cultivation of halophytes like Salicornia in Mexico and Eritrea using coastal soils under seawater flood irrigation conditions (Glenn et al., 2013) has fostered the possibility of a further demonstration of cultivatingSalicornia in coastal soils of Abu Dhabi in the Arabian Peninsula, this time to generate feedstock for aviation biofuels. However, the arid climate, high average seawater salinity of 45-46 g/L of the Arabian/Persian Gulf (Coles, 2003), and the prevalence of gypsiferous coastal soils containing evaporitic minerals (Environment Agency - Abu Dhabi, 2009) pose a significant challenge to the success of such a demonstration. The dissolution/precipitation dynamics of such saline water agriculture systems in arid coastal environments is largely unknown and can potentially cause many problems, including soil salinization, groundwater impact, and loss in yield over time owing to poor drainage resulting from mineral deposition (Breckle, 2009). Hence, there is a pressing need for anticipating some of these problems by developing a modelling framework that considers soil type and feedwater quality.
Gypsum and anhydrite, which are prevalent in the coastal soils of Abu Dhabi, are known to show unusual dissolution characteristics when coming in contact with seawater. Generally, the dissolution of evaporitic minerals occurs when they come into contact with porewater that is undersaturated for the mineral. The undersaturation of gypsum and anhydrite holds true not only for freshwaters, but also for seawater where both of these minerals are significantly undersaturated when compared to carbonate minerals like calcite and aragonite (Hoareau, Monnin, & Odonne, 2011). Additionally, the presence of dissolved halite (NaCl) in seawater is reported to enhance the solubility of both gypsum and anhydrite (Bethke, 2011; Serafeimidis & Anagnostou, 2015). Hence, the dissolution/precipitation dynamics of gypsum are likely to play a role in sustained operation of saline water farming of halophytes in Abu Dhabi. Modelling the precipitation/dissolution reaction term for gypsum and other minerals couples their interfacial mass transfer term with their saturation index (the ratio of their ion activity product and their pure phase solubility product). In turn, ion activities in saline water are a function of ionic strength and are estimated using ion activity coefficient models such as the B-dot model and the Pitzer ion interaction model (Bethke, 2011; Prausnitz, Lichtenthaler, & de Azevedo, 1999). Such geochemical complexities must be taken into account when modelling even the most simplified systems representing seawater farming.
In this study, evaporation modelling was conducted to evaluate the available coastal feedwater sources, i.e., the Arabian/Persian Gulf seawater and coastal shallow groundwater. It should be noted that significant evaporation from coastal sabkhas, playas, and saline lakes is known to bring about rapid (i.e. in a few days) onset of gypsum precipitation (Reznik, Ganor, Gruber, & Gavrieli, 2012; Yechieli & Wood, 2002). Following this exercise, a 1-D saturated-flow advection-precipitation/dissolution model was applied to regional site-specific soil mineralogies obtained from the Abu Dhabi Soil Survey while applying an evaporative load to the upper soil layers. The model was then used as a predictive tool to: (1) determine the change in agricultural return water salinity with different seawater irrigation rates, and (2) assess the effect of soil characteristics on mineral deposition in the soil column (e.g., change in mineral content and porosity loss), over a one-year period of continuous seawater irrigation. The study points to careful selection of land and saline water resources for the sustainable farming of halophytes, especially in arid coastal regions where evaporitic soils and high evapotranspiration conditions are likely to bring the onset of land degradation over long-term operation.