# Comment on Nonlinear and Nonequilibrium Sorption and Sorbed - Phase Degradation in PRZM5 Dirk F. Young Office of Pesticide Programs US Environmental Protection Agency Washington, DC

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# Background • Requests by registrants to incorporate nonlinear & nonequilibrium sorption into modeling (primarily groundwater)
• Submitted modeling studies incorporating nonlinearity and nonequilibrium always show a decrease in aquifer concentrations

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# NAFTA Groundwater Conceptual Model

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# Nonlinearity, Equilibrium, & Nonequilibrium Definitions 

# ( ) N F 

# C K f S k t S ) 1 ( 2 2 2


# − − = ∂ ∂ 

# S 2 

# At equilibrium: S1 

# = fK F

# C N


# At Equilibrium: S2 

# = (1 - f)K F

# C N 

# S 1 

# Nonequilibrium:

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# FOCUS Degradation Conceptualization
S 2 

# S 1 Degradation Occurs in Aqueous and Equilibrium Sorbed Phase (S1)

Degradation Does Not Occur in Nonequilibrium Region (S2) 5

# EPA does not distinguish between sorbed and aqueous- phase degradation • Submitted studies are typically inadequate to make such a determination • Sorption properties are not measured for soil used in degradation study
• EPA models total system degradation to correspond to available laboratory

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# Literature Heavily Weighted To Aqueous - Phase Only Degradation Ainsworth et al. (1993); Basagaoglu et al. (2002); Bengtsson & Carlsson (2001); Bertelkamp et al. (2014); Bosma et al (1997); Brusseau et al., (1999 ); Chen et al. (2011 ); Fry and Istok (1994 ); Gou et al. (2000); Heitkamp et al. ( 1984); Haws et al. (2006); Hu and Brusseau , (1998); Huixian et al (1992); Johnsen et al., (2005); Kim et al. (2003); Karapanagioti et al. (2001 ); Manilal & Alexander (1991); Mihelcic and Luthy (1991); Ogram et al. (1985 ); Pignatello et al. (1990); Ramaswami and Luthy (1997); Rao et al. (1993); Rijnaarts et al . (1990); Sabbah et al. (2004); Scow (1993); Scow & Alexander (1992); Smith et al. (1992); Steen et al. (1980) Volkering et al. (1992); Scow et al. (1995); Weber and Colbe (1968 ); Stevens , et al. (1990); Weissenfels et al. (1992); Wodzinski and Bertolini (1972); Wszolek and Alexander (1979 ); Zhang et al. (1998 ); Zhao et al. (2000) Feng et al. (2000); Woo et al. (2001); Poeton et al. (1999); Tang et al. (1998); Guerin & Boyd (1992, 1997) Remberger et al. (1986)

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# Residence Time and Degradation Two Concepts: Aqueous - Phase Residence Aqueous /S1 Residence Time FLOW IN

FLOW OUT 8Low concentration would result in higher retardation. Aq/S1 residence time increases Aqueous - phase residence is the same

Nonlinear Isotherm Nonlinear & Nonequilibrium Current Standard

Simple conceptualization. Implied sorbed - phase degradation. Aq/S1 Residence time is constant with conc Aqueous - phase residence is the same Nonequilibrium sites further lower the concentration. Aq/S1 residence time increases even more Aqueous - phase residence is the same 9

# Impact of Nonequilibrium & Nonlinearity Aqueous - Phase Only Degradation: Nonlinearity: No Effect Nonequilibrium: No Effect Nonequilibrium & Nonlinearity: No Effect Aqueous - & Sorbed - Phase Degradation: Nonequilibrium: No Effect Nonlinearity: More Degradation Nonequilibrium & Nonlinearity: Even More Degradation

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# Incorporation of Nonlinearity into SWCC &PRZM5 • Added GW to SWCC • Added Freundlich Isotherm Option • S olved with predictor- corrector procedure • Included sub - daily time step option for testing • Included option for Aqueous - Phase Only degradation
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# SWCC Nonlinear Isotherm Inputs

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# Test effect of Nonlinearity with SWCC and NAFTA Groundwater Scenario Hypothetical Example: Isotherm Half Life Aquifer Conc. Total System Degradation: Linear Isotherm Koc = 50 ml/g 60 day 68 ppb Nonlinear Isotherm K Foc = 50; N = 0.88 60 day 3.5 ppb Aqueous Only Degradation: Linear Isotherm Koc = 50 ml/g 26 day 66 ppb Nonlinear Isotherm K Foc = 50; N = 0.88 26 day 69 ppb
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# Final Points • Although EPA’s current conceptualization implies sorbed - phase degradation, it should not be construed as mechanistic . • Sorbed - phase degradation implies degradation is not retarded by increased sorption, which seems to be contrary to the bulk literature.
• Use of nonlinearity should be explored. However, a sensible and transparent conceptual model regarding degradation would be needed prior to its use.
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