The PFAS Module
Sorption to (distribution between soil water and) airwater interface
When evaluating the fate and transport of polyfluoroalkyl substances (PFAS) under dynamic vadose zone conditions, one needs to consider sorption to (distribution between soil water and) airwater interface [Silva et al., 2019, 2020, 2021]. In that case, the term accounting for the distribution of a solute between the liquid and gaseous phases is replaced with a new term accounting for solute sorption to the airwater interface as follows:
where Gamma is the interfacial adsorbed concentration (i.e., mass or moles per unit area of the interface, or M/L2), and Aaw is the airwater interfacial area, which has units of [L2/L3] or [1/L]. The airwater interfacial area Aaw is calculated from the pressure headsaturation relationship using equation (8) of Bradford et al. [2015] (also Bradford and Leij [1997]):
where Sigma_aw is the airwater surface tension [M/T2], Paw is the capillary pressure [M/L/T2], theta_s is the saturated water content [L3/L3], ro_w is the density of water [M/L], and g is the gravitational acceleration [L/T2]. The users can specify at input an additional constant, which allows them to linearly scale the interfacial area Aaw.
The interfacial adsorbed concentration (Gamma, mass or moles per unit area of the interface, or M/L2) can be calculated directly using the FreundlichLangmuir adsorption isotherm as:
where Gamma_max [M/L2] is the maximum surface concentration (i.e., at the solubility limit), and KH [for linear sorption, L3/M1, for Freundlich sorption L^(3Beta)/M^Beta], Beta [], and KL [L^(3Beta)/M^Beta] are empirical coefficients.
Concentration Dependence of the Soil Hydraulic Functions
The scaling technique, similar to one used to describe the temperature dependence of soil hydraulic properties, is used in HYDRUS to express the concentration dependence of the soil hydraulic functions. The primary impact of surfaceactive organic solutes on unsaturated flow is through the dependence of soil water pressure head on surface tension. The dependence of surface tension on solute concentration is described according to Adamson and Gast (1997) and Henry et al. (2001).
where a [L3/M1] and b [] are constants for the compound of interest, sigma(c) the surface tension at concentration c, and sigma_0 the surface tension at the reference concentration.
A similar expression is used to consider the dependence of viscosity as a function of solute concentration (Smith and Gillham, 1999).
where d [L3/M1] and e [] are constants for the compound of interest, nu(c) the kinematic viscosity at concentration c, and nu_0 is kinematic viscosity at the reference concentration.
Test Examples
 Project Group: PFAS
 Description: Examples demonstrating the development and verification of the PFAS module
 Availability: Download HYDRUS projects now (5.0 MB)
Project

Description

01SorbLin

Test on Linear Sorption

01SorbALin

Test on Gas Linear Sorption

01SorbAWILin

Test on AWI Linear Sorption

01SorbAWILin1

Test on AWI Linear Sorption, ScalAWI=2

02SorbNonLin

Test on Nonlinear Sorption, eta=0.9, beta=1.0

02SorbNonLin1

Test on Nonlinear Sorption, eta=0.0, beta=1.2

02SorbNonLin2

Test on Nonlinear Sorption, eta=0.0, beta=0.8

03AWINonLin

Test on AWI NonLinear Sorption, eta=0.9, beta=1.0

03AWINonLin1

Test on AWI NonLinear Sorption, eta=0.0, beta=1.2

03AWINonLin2

Test on AWI NonLinear Sorption, eta=0.0, beta=0.8

03AWINonLin3

Test on AWI NonLinear Sorption: eta=0.0, beta=0.8, Sorption to solid: eta=0.2, beta=0.8

03AWINonLin_inv

Test on AWI NonLinear Sorption, eta=0.9, beta=1.0, Inverse

Paper1a

Silva et al. (2020); Vertical transport of a tracer, and a chemical sorbing to solid phase, AWI, and both solid and AWI, one rainfall event

Paper12

Silva et al. (2020); Vertical transport of a tracer, and a chemical sorbing to solid phase, AWI, and both solid and AWI, transient BCs

References
 Silva, J. A. K., J. Šimůnek, and J. E. McCray, A modified HYDRUS model for simulating PFAS transport in the vadose zone, Water, 12, 2758, 24 p., doi: 10.3390/w12102758, 2020.
 Silva, J. A. K., J. Šimůnek, and J. E. McCray, Comparison of methods to estimate airwater interfacial areas for evaluating PFAS transport in the vadose zone, Journal of Contaminant Hydrology, 247, 103984, 13 p., doi: org/10.1016/j.jconhyd.2022.103984, 2022.
 Silva, J. A. K., J. Šimůnek, J. L. Guelfo, and J. E. McCray, Simulated leaching of PFAS from landapplied municipal biosolids at agricultural sites, Journal of Contaminant Hydrology, 251, 104089, 14 p., doi: 10.1016/j.jconhyd.2022.104089, 2022.
 Šimůnek, J., M. Šejna, G. Brunetti, and M. Th. van Genuchten, The HYDRUS Software Package for Simulating the One, Two, and ThreeDimensional Movement of Water, Heat, and Multiple Solutes in Variably Saturated Media, Technical Manual I, Hydrus 1D, Version 5.0, PC Progress, Prague, Czech Republic, 334p., 2022. (PDF 3.9MB)
 Šimůnek, J., M. Th. van Genuchten, and M. Šejna, The HYDRUS Software Package for Simulating One, Two, and ThreeDimensional Movement of Water, Heat, and Multiple Solutes in VariablySaturated Porous Media, Technical Manual II, Hydrus 2D/3D. Version 5.0, PC Progress, Prague, Czech Republic, 283 p., 2022. (PDF 3.6MB)