Reference

1. Understanding Earth. Second Edition. Harvard University. W.H. Freeman and Company. New York. 1998. - 682 p.

2. Groundwater in Geologic Processes. Cambridge University Press. S.E. Ingebristen, w.E. Sanford. 1998. - 341 p.

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Glossary ” Conceptualizing Groundwater Systems”.

brittle	ломкий, хрупкий
ductile	вязкий (пластичный) , ковкий, пластичный, тягучий
subdued	подавленный, подчиненный, угнетенный
replica	1) дубликат 2) копия 3) отпечаток 4) реплика 5) точная копия
fraction (of the water)	доля, порция, часть
attraction	притяжение
to hold (held)	удерживать, задерживать
surface tension	поверхностное натяжение
roughly	приблизительно, на глаз, ориентировочно
likelihood	вероятность
dominate	доминировать, преобладать
connate water	реликтовая вода (в пустых породах) ; ископаемая вода
complacent	услужливый, почтительный, любезный, обходительный
debunk	разоблачать, развенчивать
conservative force	консервативная сила, недиссипативная сила
height	высота
arbitrary	произвольный, случайный
datum	уровень приведения; уровенная поверхность
flux	поток (энергии или вещества)

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solute	растворённое вещество
off-diagonal	недиагональный
coupled	двухсторонний; двойной, парный, сдвоенный, спаренный
osmotic	осмотический
conservative force	консервативная сила, недиссипативная сила
height	высота
arbitrary	произвольный, случайный
datum	уровень приведения; уровенная поверхность
flux	поток (энергии или вещества)

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Theme 4. “Conceptualizing Groundwater Systems”

Практичне заняття

Conceptualizing Groundwater Systems

План:

• How water flows through soil and rocks;

• The speed of groundwater flows;

• The limits of Darcy's law;

• Artesian flows

• Balancing recharge and discharge

How water flows through soil and rocks.

If water moves into and through the ground, what determines where and how fast it flows? With the exception of caves, there are no large open spaces for pools or rivers of water underground. The only space available for water is the pore space between grains of sand and other particles that make up the soil and bedrock and the space in fractures. Some pores, however small and few, are found in every kind of rock and soil, but large amounts of pore space most often found in sandstones and limestones.

The amount of pore space in rock, soil, or sediment is its porosity – the percentage of its total volume that is taken up by pores. Porosity depends on the size and shape of the grains and how they are packed together. The more loosely packed the particles, the greater the pore space between the grains will be. In many sandstones, porosity is as high as 30 percent (Figure 11.1a). Minerals that cement grains reduce porosity (Figure 11.1b). The smaller the particles (Figure 11.1c) and the more they vary in shape (Figure 11.1d), the more tightly they will fit together. Porosity is higher in sediments and sedimentary rocks (10 to 40 percent) than in igneous or metamorphic rocks (as low as 1 to 2 percent).

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Pore space in limestones varies, depending on how many pores were created through dissolution by groundwater or during weathering. In most unfractured shales, porosity is well under 10 percent (Figure 11.1e). Fractured rocks may contain appreciable pore space – up to 10 percent of the volume – in their cracks (Figure 11.1f). Porosity is highest of all – over 40 percent of volume – in soils and underlying loose sand and gravel layers.

Although porosity tells us how much water the rock can hold if all pores are filled, it gives us no information how rapidly water can flow through the pores. Water travels through a porous material by winding between grains and through cracks. The smaller the pore spaces and the more tortuous the path, the more slowly the water travels. The ability of a solid to allow fluids to pass through is its permeability. Generally, permeability increases as porosity increases, but not always. Permeability also depends on the sizes of the pores, how well they are connected, and how tortuous a path the water must travel to pass through the material.

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Figure 11.1 Pores in rocks are normally filled partly or entirely by water. (Pores in oil- or gas-baring sandstones and limestones are filled with oil or gas). (a) A highly porous, well-sorted sandstone has large amounts of pore space between grains. (b) A cemented sandstone has lower porosity because a cementing mineral has been precipitated in some of the pore space by fluids moving through the pores. (c) Porosity is reduced in fine-grained and poorly sorted sandstone. (d) Porosity is reduced in sandstone with irregularly shaped grains. (e) Unfractured shale has very low porosity (less than 10 percent). (f) Fractured shale has higher porosity, because some water can be stored in the fissures and cracks.

Both porosity and permeability are important factors when one is searching for a groundwater supply. In general, a good groundwater reservoir will be a body of rock, sediment, or soil with both high porosity (so it can hold large amounts of water) and high permeability (so the water can be pumped from it easily). A rock with high porosity but low permeability may contain a great deal of water, but because the water flows so slowly, it is hard to pump it out of the rock. Table 1.1 summarizes the degree of porosity and permeability in various rock types.

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Table 1.1

Porosity and Permeability of Aquifer Rock Types
Rock Type	Porosity (Pore space that may hold fluid)	Permeability (Ability to allow fluids to pass through)
Gravel	Very high	Very high
Coarse- to medium-grained sand	High	High
Fine-grained sand and silt	Moderate	Moderate to low
Sandstone, moderately cemented	Moderate to low	Low
Fractured shale or metamorphic rocks	Low	Very Low
Unfractured shale	Very Low	Very Low