Assessing soil water repellency using the molarity of ethanol droplet (MED) test

The molarity of ethanol droplet (MED) test is a popular rapid method for assessing soil water repellency under field and laboratory conditions. This paper reviews the theoretical basis of the MED test, discusses controllable and uncontrollable sources of error affecting its results, and proposes a detailed protocol for its standardization. Soil water repellency is a function of soil surface chemistry. More specifically, it is a function of the free energy of the solid/gas interface in soil ( gamma(SG)). Because gamma(SG) is not directly measurable in the laboratory, soil water repellency must be assessed using thermodynamically related parameters such as the initial advancing contact angle (0) or the work of wetting (W-W). The MED test can be used to determine 0, and in turn W-W, but only if some simplifications are accepted and the test is performed under standard controlled conditions. Wetting theories exclude the dissolution or swelling of the solid by the liquid or chemical reactions between the liquid, solid, and gas phases that change system composition. Consequently, for MED tests to give valid 0 estimates, system composition must not change measurably during 10 s of solid/liquid contact. We discuss system conditions that should be controlled before and during MED testing to improve the validity of test results. Finally, we propose a detailed MED testing protocol for possible adoption by commercial analytical laboratories and the soils research community.