Abordaje teórico filosófico

Most of the techniques associated with the measurement and classification of water repellency are largely covered in reviews by Tschapek (1984) and Wallis and Horne (1992). The discussion below is a synopsis of their work and discusses the techniques most commonly employed to measure water repellency.

A thorough review regarding the manner in which a liquid and a planar solid surface interact to produce a contact angle (H) was given by Zisman (1964). The interaction between the cohesive forces within a water droplet and the adhesive forces between the water droplet and the solid surface are directly indicated by the contact angle (H) between the water droplet and the solid surface. A number of authors (Bond and Hammond, 1970; Fink and Myers, 1969; Mallik and Rahman, 1985) have directly measured the apparent contact angle of water droplets in contact with soil surfaces. Their studies served to emphasise that the technique was only applicable to very repellent soils (otherwise water droplets on less repellent soil surfaces will infiltrate over time and make measurement of the contact angle impossible). Furthermore, surface roughness and pore shape and size distribution appeared to affect H.

An indirect measurement of H was suggested by Letey et al. (1962) who used a capillary rise and infiltration technique to determine H. A number of assumptions were made regarding the approximation of pore behaviour to that of cylindrical tubes in the soil and of Poiseuille flow through these tubes. The authors found that ethanol produced the same H for all soils tested and so H was assumed to be zero for ethanol. On this basis they were able to use Poiseuille’s equation to calculate the pore radius which was then used in the calculation of H for other solutions. The method seemed to produce reasonable and consistent results. There were however, a number of drawbacks, notable of which was the time required to measure capillary rise and its unsuitability for use on intact soil cores.

In 1957, Philip (1957) developed the theory of intrinsic diffusivity and sorptivity which he later modified (Philip, 1969). Using this theory, Tillman et al. (1989) developed a method which determines the ratio of intrinsic sorptivity for ethanol to that of water for structurally stable soils, and argued that this ratio can be used as an index of water repellency. This repellency index was evaluated by Wallis et al. (1991) who measured the index on a range of

New Zealand soils. The index was compared with the ‘water drop penetration time (WDPT)’ and ‘molarity of an ethanol droplet (MED)’ techniques (described later in this section). The repellency index suggested that all soils were repellent to some extent at field moisture conditions and that the technique was more sensitive than either WDPT or MED, thus being of potential use for those soils exhibiting low levels of repellency. A number of other advantages were revealed whereby the index could be used to measure actual and potential short-term water infiltration which could be compared against rainfall and irrigation intensities. Additionally, the technique was able to be used in situ or on undisturbed field cores at field-moist or air-dry conditions.

Perhaps the simplest test for water repellency is the water drop penetration time (WDPT) method (Van't Woudt, 1959) which involves placing a drop of distilled water on the surface of a soil and measuring the time for the water to penetrate the sample. The soil surface is prepared beforehand to provide standard conditions since surface roughness and pore geometry affect WDPT. A number of techniques have been proposed to improve the repeatability of the method, but most researchers have opted for increased replication to improve their estimate of WDPT. Recent advances in the employment of this technique are provided by Doerr (1998). The relationship between WDPT and H has been considered by Letey (1969) and Letey et al. (2000) who suggested that WDPT classifies soils into two categories; those with an apparent H above 90o, and those below. If the water forms a droplet on the soil surface then H > 90o and the soil is considered repellent. Since the droplet penetrates the soil surface over time, then H changes also, suggesting that the WDPT test is a better indicator of the persistence of repellency, rather than a measure of the initial contact angle. A study of South Australian sandy soils by King (1981) suggested that WDPT is limited in its ability to measure repellency and is relevant only for a few degrees span around H = 90o. The study showed that WDPT is < 1 second for H < 75o, thus imposing limitations on using the technique for low repellency soils. Other authors have measured WDPT times greater than an hour for severely repellent soils, making the technique time-consuming for these types of soils. Despite the limitations just described, the WDPT method has distinct advantages in terms of its speed (except for severely repellent soils), simplicity, and its application to both in-situ and disturbed samples, and as a result has been used in many studies associated with water repellent soils.

Another simple technique used in the measurement of soil water repellency is the molarity of an ethanol droplet test (MED) developed by King (1981). The test measures the molarity of ethanol in an aqueous droplet required to infiltrate the soil surface within 10 seconds. In his study of a large number of Australian sandy soils, King (1981) found that measured MED values correlated very well with observed apparent contact angles. Variability however, increased when H > 92o and the test was not useful in those soils where low repellency was observed (H ≤ 81o) since at this angle MED = 0. For such soils, King (1981) argued that the WDPT test was a useful extension of the MED test. As with the WDPT test, the most important advantages of the MED test are speed and simplicity and its applicability to field and disturbed samples. Furthermore, it can be used on highly repellent soils where WDPT values are in excess of 1 hour. Its main disadvantage lies with its unsuitability for use with low repellency soils.

While direct measurement of apparent contact angles (H) between the water droplet and the soil surface gives a definitive value for the degree of water repellency of very repellent soils, the duration of the procedure and the measurement of very small areas of the soil surface makes this technique prohibitive in terms of replication and time. This has prompted the development of further techniques such as capillary rise and intrinsic sorptivity which, although also time-consuming, produce H and repellency index values more representative of the bulk soil. The most popular techniques used for the measurement of water repellency are the WDPT and MED tests due to their speed and simplicity. The main disadvantages with these techniques lie with their measurement of point surfaces making large replication sets necessary, and the limited contact angle range of their respective measurements. The latter drawback may be mitigated somewhat by performing both tests in combination.