Where Does Groundwater Reside?

Where does groundwater reside?

Groundwater as we know the drinking water which is pulled out of the ground, where does it comes from?

The Underground Reservoir 

Water moves among various reservoirs during the hydrologic cycle. Of the water that falls on land, some evaporates directly back into the atmosphere, some gets trapped in glaciers, and some becomes runoff that enters a network of streams and lakes that drains to the sea. The remainder sinks or percolates downward, by a process called infiltration, into the ground. In effect, the upper part of the crust behaves like a giant sponge that can soak up water.

Of the water that does infiltrate, some descends only into the soil and wets the surfaces of grains and organic material making up the soil. This water, called soil moisture, later evaporates back into the atmosphere or gets sucked up by the roots of plants and transpires back into the atmosphere. But some water sinks deeper into sediment or rock, and along with water trapped in rock at the time the rock formed, makes up groundwater. Groundwater slowly flows underground for anywhere from a few months to tens of thousands of years before returning to the surface to pass once again into other reservoirs of the hydrologic cycle. 

Porosity: Open Space in Rock and Regolith 

Contrary to popular belief, only a small proportion of underground water occurs in caves. Most groundwater resides in  relatively small open spaces between grains of sediment or between grains of seemingly solid rock, or within cracks of various sizes. The term pore refers to any open space within a volume of sediment, or within a body of rock, and the term porosity refers to the total amount of open space within a material, specified as a percentage. For example, if we say that a block of rock has 30% porosity, then 30% of the block consists of pores. Geologists distinguish between two basic kinds of porosity primary and secondary.

Porosity is the open space in rock or sediment, whereas permeability is the degree to which the pores are connected.

Primary porosity develops during sediment deposition and during rock formation (figure above a,b). It includes the pores between clastic grains that exist because the grains don’t fit together tightly during deposition. Secondary porosity refers to new pore space produced in rocks some time after the rock first formed. For example, when rocks fracture, the opposing walls of the fracture do not fit together tightly, so narrow spaces remain in between. Thus, joints and faults may provide secondary porosity for water (figure above c). As groundwater passes through rock, it may dissolve and remove some minerals, creating solution cavities that also provide secondary porosity.

Permeability: The Ease of Flow 

If solid rock completely surrounds a pore, the water in the pore cannot flow to another location. For groundwater to flow, pores must be linked by conduits (openings). The ability of a material to allow fluids to pass through an interconnected network of pores is a characteristic known as permeability. Groundwater flows easily through a material, such as loose gravel, that has high permeability. In gravel, the water is able to pass quickly from pore to pore, so if you pour water into a gravel-filled jar, it will trickle down to the bottom of the jar, where it displaces air and fills the pores (figure above d). In tightly packed sediments or in rock, the water flows more slowly because it follows a tortuous path through tiny conduits. Water flows slowly or not at all through an impermeable material. Put another way, an impermeable material has low permeability or even no permeability. The permeability of a material depends on several factors:

• Number of available conduits: As the number of conduits increases, permeability increases. 
• Size of the conduits: More fluids can travel through wider conduits than through narrower ones. 
• Straightness of the conduits: Water flows more rapidly through straight conduits than it does through crooked ones. 

Note that the factors that control permeability in rock or sediment resemble those that control the ease with which traffic moves through a city. Traffic can flow quickly through cities with many straight, multilane boulevards, whereas it flows slowly through cities with only a few narrow, crooked streets. Porosity and permeability are not the same feature. A material whose pores are isolated from each other can have high porosity but low permeability. 

Aquifers and Aquitards 

Water in the ground-aquifers, aquitards and the water table.

With the concept of permeability in mind, hydrogeologists distinguish between an aquifer, sediment or rock with high permeability and porosity, and an aquitard, sediment or rock that does not transmit water easily and therefore retards the motion of water. An aquifer that is not overlain by an aquitard is an unconfined aquifer. Water can infiltrate down into an unconfined aquifer from the Earth’s surface, and groundwater can rise to reach the Earth’s surface from an unconfined aquifer. An aquifer that is overlain by an aquitard is a confined aquifer its water is isolated from the ground surface (figure above a).

The Water Table 

Infiltrating water can enter permeable sediment and bedrock by percolating along cracks and through conduits connecting pores. Nearer the ground surface, water only partially fills pores, leaving some space that remains filled with air (figure above b). The region of the subsurface in which water only partially fills pores is called the unsaturated zone. Deeper down, water completely fills, or saturates, the pores. This region is the saturated zone. In a strict sense, geologists use the term “groundwater” specifically for subsurface water in the saturated zone, where water completely fills pores. 

The term water table refers to the horizon that separates the unsaturated zone above from the saturated zone below. Typically, surface tension, the electrostatic attraction of water molecules to each other and to mineral surfaces, causes water to seep up from the water table (just as water rises in a thin straw), filling pores in the capillary fringe, a thin layer at the base of the unsaturated zone. Note that the water table forms the top boundary of groundwater in an unconfined aquifer. 

The depth of the water table in the subsurface varies greatly with location. In some places, the water table defines the surface of a permanent stream, lake, or marsh, and thus effectively lies above the ground level (figure above c). Elsewhere, the water table lies hidden below the ground surface. In humid regions, it typically lies within a few meters of the surface, whereas in arid regions, it may lie hundreds of meters below the surface. Rainfall rates affect the water table depth in a given locality  (figure above d) the water table drops during the dry season and rises during the wet season. Streams or ponds that hold water during the wet season may, therefore, dry up during the dry season because their water infiltrates into the ground below.

Topography of the Water Table 

Factors that influence the position of the groundwater.

In hilly regions, if the subsurface has relatively low permeability, the water table is not a planar surface. Rather, its shape mimics, in a subdued way, the shape of the overlying topography (figure above a). This means that the water table lies at a higher elevation beneath hills than it does beneath valleys. But the relief (the vertical distance between the highest and lowest elevations) of the water table is not as great as that of the overlying land, so the surface of the water table tends to be smoother than that of the landscape. 

At first thought, it may seem surprising that the elevation of the water table varies as a consequence of ground-surface topography. After all, when you pour a bucket of water into a pond, the surface of the pond immediately adjusts to remain horizontal. The elevation of the water table varies because groundwater moves so slowly through rock and sediment that it cannot quickly assume a horizontal surface. When rain falls on a hill and water infiltrates down to the water table, the water table rises a little. When it doesn't rain, the water table sinks slowly, but so slowly that when rain falls again, the water table rises before it has had time to sink very far. 

k (such as shale) may lie within a thick aquifer. A mound of groundwater accumulates above such aquitard lenses. The result is a perched water table, a groundwater top surface that lies above the regional water table because the underlying lens of impermeable rock or sediment prevents the groundwater from sinking down to the regional water table (figure above b).
Figures credited to Stephen Marshak.

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