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.