# Presentation: 070484

## Sorption of a hydrophilic pesticide: Effects of soil water content and matric potential

- Tyson E. Ochsner*1, Brandon M. Stephens2, William C. Koskinen1, and Rai S. Kookana3
*1** USDA-ARS, Soil and Water Management Research Unit, St. Paul, MN 55108 *
*2** Dep. Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN  55108 *
*3** CSIRO Land and Water, PM B2, Glen Osmond, SA  5064, Australia *

## Introduction

- The leaching risk of a pesticide in soil is characterized primarily by the sorption coefficient (Kd).
- Lower Kd values indicate greater potential for leaching.
- The effects of soil water content and matric potential on Kd are not well understood.
- This is a serious gap because pesticides are often applied on or near the soil surface where the water content varies dramatically.

## Pesticide -- Dicamba

- Dicamba is a widely used broadleaf herbicide sold under the trade names Banvel and Clarity.
- During 2002, approximately 11% of US corn was treated with dicamba at a rate of 0.22 kg active ingredient ha-1 yielding a total of 512 Mg of active ingredient applied.

**Fig. 1.  Molecular structure of dicamba, a hydrophilic, weakly-sorbing herbicide with pKa of 1.9.**

## Soils

- Sorption of dicamba was measured in three soils (Table 1), each at two initial water contents.

- Soils were moistened to the desired initial water content using a solution containing 14C-labeled dicamba; the resulting dicamba concentration was ~1 g g-1.

## Unsaturated transient flow experiments

- After equilibration, the soil was packed into a column with 20 sections (0.9 cm thick). The column was positioned horizontally and connected to a Mariotte bottle (Fig. 2).
- The bottle supplied 5 mM CaCl2 solution (without dicamba) to the column inlet at a constant pressure.
- Infiltration terminated when the wetting front reached 3⁄4 of the way through the column.

**Fig. 2.  Soil column and Mariotte bottle set-up used for horizontal infiltration experiments.  **

## Water and pesticide distributions

-  Infiltrating water displaced the antecedent solution, creating a plane of separation (vertical line, Fig. 3).
- For positions beyond this plane, the total pesticide content per unit mass of dry soil was regressed against the solution volume per unit mass of dry soil.

**Fig. 3.  Distributions of water and dicamba after horizontal infiltration.**

## Determination of Kd

- For positions beyond this plane, the total pesticide content per unit mass of dry soil was regressed against the solution volume per unit mass of dry soil (Fig. 4).
- The intercept divided by the slope gave the Kd.

**Fig. 4. ****Dicamba concentration versus solution volume. **

**Determination of K****d**

## Sorption coefficients

**Sorption coefficients**

## Regression analysis

**Regression analysis**

- No significant relationship existed between Kd and matric potential.

- We found a strong linear relationship between Kd and the product of soil water content and organic C content (r2 = 0.86, Table 3).

## Conclusions

- The number of dicamba sorption sites increases with soil organic C content, while the accessibility of these sites increases with soil water content.
- This may be caused by the decreasing hydrophobicity of soil organic matter with increasing water content.
- The effects of water content on pesticide sorption require further research and may ultimately have implications for the methods used to determine sorption and for managing pesticide application.
- ochsner@umn.edu