2.2.2 Evaporation Modelling
Evaporation modelling was conducted for different options of feedwater, including Arabian Gulf seawater and coastal shallow groundwater in Abu Dhabi represented by the shallow groundwater at the Masdar Institute site (Table 2 ). Equation (3) was used by itself in non-flowing evaporation modelling to evaluate two different activity coefficient models for electrolytes, namely, the B-dot model and the Pitzer ion interaction model. The B-dot model (Bethke, 2011; Sandler, 2006), a variant of the Debye-Huckel activity coefficient model, was used to determine the ion activity product in Equation 4. The B-dot model is a function of the solution ionic strength, species charge, ion size, and empirical coefficients (Plummer, Jones, & Truesdell, 1976). This model is considered to be accurate to an ionic strength between 1-2 moles•kg-1 of water (Plummer et al., 1976). Note that average seawater ionic strength is only ~ 0.7 moles•kg-1. However, in situations of strong evapotranspiration or contact with hypersaline groundwater, porewater salinity can be high enough such that measured values deviate strongly from B-dot model solubility predictions. In such cases, a more accurate approach is to incorporate ion interactions, such as those used by the Pitzer ion interaction model denoted in GWB by the PHRQPITZ virial expansion model (Ptacek & Blowes, 2000).
In GWB software, evaporation is modelled as mass of water removed and mineral oversaturation is determined using a selected activity coefficient model. Both, B-d
ot and PHRQPITZ, activity coefficient models were employed to model the removal of >99% mass of water to predict the total mass of precipitate formed as well as its mineral composition.