Glyphosate Transport in Two Louisiana Agricultural Soils: Miscible Displacement Studies and Numerical Modeling

Abstract

Glyphosate (N-(phosphonomethyl) glycine) (GPS) is currently the most commonly used herbicide worldwide, and is generally considered as immobile in soils. However, numerous reports of the environmental occurrence of the herbicide coupled with recent evidence of human toxicity necessitate further investigation as to the behavior of GPS in the soil environment. Batch sorption studies along with miscible displacement experiments were carried out in order to assess the mobility of GPS in two Louisiana agricultural soils; Commerce silt loam and Sharkey clay. Batch results indicated a high affinity of both soils for solvated GPS, with greater affinity observed by the Sharkey soil. GPS sorption in the Commerce soil was most likely facilitated by the presence of amorphous Fe and Al oxides, whereas the high cation exchange capacity of the Sharkey soil likely allows for GPS complexation with surface exchangeable poly-valent cations. Miscible displacement studies indicate that GPS mobility is highly limited in both soils, with 3% and 2% of the applied herbicide mass recovered in the effluent solution from the Commerce and Sharkey soils, respectively. A two-site multi-reaction transport model (MRTM) adequately described GPS breakthrough from both soils and outperformed linear modeling efforts using CXTFIT. Analysis of extracted herbicide residues suggests that the primary metabolite of GPS, aminomethylphosphonic acid (AMPA), is more mobile in both soils, although both compounds are strongly retained.

The Influence of Fine Fractions Content in Non-Cohesive Soils on Their Compactibility and the CBR Value

“…This indicates, that the volume of the soil sample during the CBR test increases, thus the soil loosening is observed. This phenomenon is the result of the water flow inside the soil sample (Roy, 2013;Habasimibi & Nishimura, 2018). In all of the testes soil samples, regardless of the fine fraction content, a significant increase of the moisture content at points number 9 and 10 (com-pare to Figure 2) were notable.…”

“…Th e increase of the fine fraction content (on the following graphs, called as f π + f i ) results Figure 2. The distribution of moisture measurement points in the cross-section of soil samples in maximum dry density (MDD) enhancement, while the amount of water, needed to obtain optimum moisture content, decreases (Habasimibi & Nishimura, 2018;Gadzama, Yohanna, Wadai, Nwaiwu, & Malachy, 2018). Laboratory tests, that were carried out have shown that further enhancement of the content of fine fractions, when the maximum soil compaction have been reached, contribute to the increase of the amount of water in the soil sample, that is necessary to obtain the optimal moisture content (OMC), while the dry density decreases (MDD) -see Figures 5 and 6.…”

“…Based on the results of the measurements, regularities of the water flow patterns in the soil depending on the content of fine fraction were observed (Thakur & Yadav, 2018;Habasimibi & Nishimura, 2018;Yin, Zhang, Jiang, & Huang, 2018;Salam, Osouli, & Tutumluer, 2018). The first of analyzed dependences is the correlation between the increase of the moisture in the middle of the sample (moisture in measurement point no.…”

Capturing In Situ Glyphosate (De)sorption Kinetics in Floodplain Aquifer Sediment Columns: Geophysical Measurements and Reactive Transport Modeling

“…We set up a one-dimensional (1D) advective–dispersive reactive transport model to quantify the mobility and retention of glyphosate in the Tufa–sand packed columns. Sorption in the model was described via a combination of kinetic reversible sorption, accounting for Freundlich-type nonlinearity in the adsorption term and an irreversible sorption term after the multireaction model (MRM) for glyphosate sorption of Padilla and Selim. , We considered Freundlich-type sorption based on results from preliminary equilibrium sorption batch experiments, detailed in the SI, section 2. Equation is the advection–dispersion equation for a sorbing compound, in which C [mg L –1 ] is the aqueous phase concentration of glyphosate and S [mg g –1 ] is the concentration of solid-phase glyphosate, the sum of reversibly ( S r ) and irreversibly ( S ir ) bound fractions, n e [−] is the flow-effective porosity, ρ s [g L –1 ] is the mass density of the solids, v [m s –1 ] is the flow velocity, D [m 2 s –1 ] is the dispersion coefficient ( D = v α + D m , where α [m] is the dispersivity and D m [m 2 s –1 ] is the molecular diffusion coefficient in water), and t and x are the temporal and spatial coordinates. The rate of change of solid-phase glyphosate is the sum of the temporal derivatives of S r and S ir , which are given by the following differential equations where k att [mg (1–n) L –(1–n) s –1 ], k det [s –1 ], and k irr [s –1 ] are the rate constants for reversible sorption and desorption, and irreversible sorption, respectively.…”

“…Sorption in the model was described via a combination of kinetic reversible sorption, accounting for Freundlich-type nonlinearity in the adsorption term and an irreversible sorption term after the multireaction model (MRM) for glyphosate sorption of Padilla and Selim. 50,51 We considered Freundlich-type sorption based on results from preliminary equilibrium sorption batch experiments, detailed in the SI, section 2.…”