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.