4.1 Effects of Evaporation and Activity Coefficient Models on Feedwater Options
The evaporation progress of four different potential sources of irrigation water, three coastal shallow groundwater sources and mean Arabian Gulf seawater, each having a different composition (Table 2 ), was modelled as an isothermal water depletion using two different activity coefficient models, B-dot and PHRQPITZ. The results were normalized by the total mineral mass in each sample so that the onset of precipitation could be compared between different samples and between the two activity coefficient models. The final results from this exercise are presented in Figures 3 and 4 .
In general, the difference in mineral precipitation between the two activity coefficient models became more pronounced with increasing ionic strength and salinity, with the least difference obtained for Arabian Gulf seawater, which had the lowest salinity of the four potential feedwater sources. The difference was especially pronounced for the hypersaline coastal shallow groundwater samples, including Masdar Well 4 water, which has a similar salinity and expected composition as the shallow groundwater at sites D and E (Figure 1 ). These results corroborate assertions by other researchers (Zhang, Zheng, & Wan, 2005) that the Pitzer ion interaction model is more suitable for activity calculations in strong electrolytes like hypersaline waters and brines. PHRQPITZ, the Pitzer virial expansion model, consistently showed an earlier onset of precipitation (i.e., conversely, less dissolution) for all feedwater sources. Also, since the B-dot model is a Debye-Huckel variant and depends solely on ionic strength, it fails to incorporate ion interactions between different types of ions, which can be better modelled using PHRQPITZ. For this reason, the salt composition of the precipitate showed variation between the two models (Figure 3 ), especially for the groundwater samples. The evaporation modelling exercise clearly demonstrated that the coastal groundwater is a poor candidate for use as irrigation water in saline water farming of halophyte plants because of its hypersaline characteristics, presumably owing to a significant salt flux from underlying evaporite deposits (Sanford & Wood, 2001) and up wicking of shallow groundwater resulting in rapid evaporation (Imes & Wood, 2007). The results also demonstrated that Arabian Gulf seawater is unlikely to undergo rapid precipitation until almost 90% of its water mass has been evaporated. In addition, this exercise displayed the least deviations in precipitation modelling predictions between the two activity coefficient models for seawater, confirming that B-dot could be used to model seawater evaporation with limited error. B-dot also allows for temperature correction, a feature unavailable with PHRQPITZ, which can be useful in the future to model the summer months in Abu Dhabi when the ambient temperatures can approach 50 °C.