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GeoScience Services 2263 Kingston Road <br />Grand Junction, CO 81503 <br />,.<- `~ (970) 374-3356 <br />The physical reason why water rises in a capillary tube can be mathematically described <br />in Equation (2). <br />~I',,, = P~ - Pj = pkT/m (P„~/P„~ (2) <br />Equation (2) states that the pressure under a curved surface of water or capillary meniscus <br />(P~) as denoted by point A in Figure 1 is less that the pressure under a flat surface of <br />water (Pf) as denoted by point B in Figure 1. To achieve equilibrium, water will rise in <br />the capillary until the pressure at point C is equal to the pressure at point B. The right <br />side of Equation (2) simply shows how the matrix potential can be measured in the soil <br />where the density of water (p), Boltzmann's constant (k), absolute temperature (T), the <br />mass of water (m), and the relative humidity in the soil (P~~/P~f) can be used to quantify <br />matrix potential. <br />The important implication to understand from this analysis is that the curvature of the <br />meniscus, controlled by adsorption and surface tension forces, is responsible for the <br />movement of water in the unsaturated zone. The tighter the curvature in smaller <br />capillaries or fine-grain soils, the greater the matrix potential exhibited by the soils. This <br />fact will result in higher water contents for fine-grain soils in the unsaturated zone. <br />Using this understanding of the role of the meniscus in unsaturated water transport <br />provides the basis for understanding capillary barriers and the implications of faults or <br />fractures in the unsaturated zone. Using the analogy of a small capillary tube for the soils <br />surrounding a fault and a large capillary tube for the fault, it is possible to illustrate the <br />effect of a capillary barrier. As the water meniscus in the fine-grain soil or rock <br />approaches the fault, the increase in the radius of the fault prevent the meniscus from <br />"jumping" across the soil/fault interface to transport water to the fault. Since the <br />meniscus controls water movement in the unsaturated zone, the fault acts as a barrier to <br />water movement or a capillary barrier. Capillary barriers are commonly used in <br />hazardous waste caps where coarse grain material is layered with fine grain material to <br />prevent infiltration from entering into the underlying waste. <br />For conditions where a fault is located near the surface, covered by a thin soil veneer, and <br />an extreme precipitation event results in a saturated wetting front reaching the subsurface <br />fault allowing water to enter the fault, the same capillary forces will limit the migration of <br />water. As water flows downward in the fault, capillary forces in the surrounding soil or <br />rock will "wick" water from the fault into the surrounding rock limiting the extent of <br />migration. <br />In summary, unlike saturated conditions, faults and fractures in the unsaturated zone act <br />as a barrier to water transport. The extremely low moisture contents of the soil and rock <br />underlying the waste dumps at the JD-6 and JD-8 mines result in strong capillary forces <br />that restrict water movement to porous soil and rock at the sites. The low permeabilities <br />