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Ground Water in Porous Materials
The origin, number, and total volume of open spaces in the crustal materials in which ground water occurs vary considerably from one material to another, most noticeably between loose sediments and bedrock. Loose, unconsolidated materials have internal spaces among the constituent particles called interstices (Figure 7). A pail of sand, for example, will absorb a large volume of water before overflowing. The water fills the voids among the grains of sand. The percentage of the bulk volume of material occupied by interstices is its porosity. As indicated in Table 1, the porosity varies from one material to another. The greatest contrast in porosity is between unconsolidated sediment and dense rock types. Most rocks in Maine and New England have lost their original porosity due in part to recrystallization (metamorphism), which occurred under conditions of elevated temperature and pressure in the geologic past. This crystalline bedrock, as it is called, also includes large areas of intrusive bedrock, such as granite, that crystallized from a melt, and therefore contains few open spaces. There are essentially no primary openings in crystalline rocks, but there are secondary openings where the rock is broken by faults and joints. A fault is a fracture in rock along which there has been displacement of the two sides relative to one another parallel to the fracture, while a joint is a fracture along which no appreciable movement has taken place. These cracks, or fractures, are generally spaced several feet apart in crystalline rock. Thus, the bulk porosity of the underlying bedrock is very small.
If all the interconnected interstices of a material are filled with water, the material is considered saturated. The volume of water that drains freely from a saturated substance generally is less than the volume of water it initially absorbed when dry. Because of tensional forces, some water clings to the walls of the interstices and reduces the size of the interconnected openings that transmit ground water. These openings represent the effective porosity, which is the ratio of the void space through which flow can occur, compared to the total volume of the porous material. Similarly, specific yield is the ratio of the volume of water that drains from the material, as compared to the total volume of the porous material. Referring to Table 1, it is seen that the coarser texture and larger pore size of gravel allows more water to drain through it than does clay. Clay is very porous, but the individual openings are so tiny that tensional forces are relatively large. A cubic foot of gravel drains more water from its void spaces than does a cubic foot of clay, even though the total percent of void space in clay is nearly twice that of gravel.
Specific yield indicates the volume of water that drains from a porous substance, but does not tell how quickly the water drains out. Hydraulic conductivity (generally called permeability) refers to the relative ease with which a porous substance can transmit a liquid. For ground water work, it is typically defined as the rate of flow of water in gallons per day through an aquifer cross section of one square foot under a hydraulic gradient of one at the the prevailing temperature. The term relates physical properties of both a porous medium and the fluid passing through it. Table 1 compares the permeability of various geologic materials, and it can be seen that sand and gravel have high values, while clay and dense bedrock (typical of Maine) have very low values.
It is interesting to note that there is no direct relationship between porosity or specific yield and permeability. This is because the rate at which water passes through a substance is very sensitive to the arrangement of individual particles. The same material deposited under two sets of conditions has different permeabilities, even though the specific yields are similar. The permeability of a sand and gravel deposit in place in the field is likely to be different from the permeability of that same material dug up and measured in a laboratory.
Last updated on March 25, 2009
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