What makes an aquifer able to store water

A permeable material has a greater number of larger, well-connected pores spaces, whereas an impermeable material has fewer, smaller pores that are poorly connected.

Permeability is the most important variable in groundwater. Permeability describes how easily water can flow through the rock or unconsolidated sediment and how easy it will be to extract the water for our purposes. The characteristic of permeability of a geological material is quantified by geoscientists and engineers using a number of different units, but the most common is the hydraulic conductivity.

The symbol used for hydraulic conductivity is K. The materials in Figure Unconsolidated materials are generally more permeable than the corresponding rocks compare sand with sandstone, for example , and the coarser materials are much more permeable than the finer ones.

The least permeable rocks are unfractured intrusive igneous and metamorphic rocks, followed by unfractured mudstone, sandstone, and limestone. The permeability of sandstone can vary widely depending on the degree of sorting and the amount of cement that is present. Fractured igneous and metamorphic rocks, and especially fractured volcanic rocks, can be highly permeable, as can limestone that has been dissolved along fractures and bedding planes to create solutional openings.

The surface of most silicate mineral grains has a slight negative charge due to imperfections in the mineral structure. Water H2O is a polar molecule. This means that while it has no overall electrical charge, one side of the molecule has a slight positive charge the side with the two hydrogens , compared to a slight negative charge on the other side. Water is strongly attracted to all mineral grains and water within that bound water layer a few microns around each grain is not able to move and flow along with the rest of the groundwater.

In the lower diagrams shown here, the bound water is represented by dark blue lines around each grain and the water that can move is light blue. We have now seen that there is a wide range of porosity in geological materials and an even wider range of permeability.

Groundwater exists everywhere there is porosity. However, whether that groundwater is able to flow in significant quantities depends on the permeability.

An aquifer is defined as a body of rock or unconsolidated sediment that has sufficient permeability to allow water to flow through it. Unconsolidated materials like gravel, sand, and even silt make relatively good aquifers, as do rocks like sandstone. How Does an Aquifer Work? An aquifer is filled with moving water and the amount of water in storage in the aquifer can vary from season to season and year to year.

Ground water may flow through an aquifer at a rate of 50 feet per year or 50 inches per century, depending on the permeability. But no matter how fast or slow, water will eventually discharge or leave an aquifer and must be replaced by new water to replenish or recharge the aquifer.

Thus, every aquifer has a recharge zone or zones and a discharge zone or zones. Figure 2 is a simple cartoon showing three different types of aquifers: confined, unconfined, and perched. Recharge zones are typically at higher altitudes but can occur wherever water enters an aquifer, such as from rain, snowmelt, river and reservoir leakage, or from irrigation. Discharge zones can occur anywhere; in the diagram, discharge occurs not only in springs near the stream and in wetlands at low altitude, and also from wells and high-altitude springs.

The amount of water in storage in an aquifer is reflected in the elevation of its water table. If the rate of recharge is less than the natural discharge rate plus well production, the water table will decline and the aquifer's storage will decrease. A perched aquifer's water table is usually highly sensitive to the amount of seasonal recharge so a perched aquifer typically can go dry in summers or during drought years.

Why is Groundwater So Clean? Aquifers are natural filters that trap sediment and other particles like bacteria and provide natural purification of the ground water flowing through them.

Like a coffee filter, the pore spaces in an aquifer's rock or sediment purify ground water of particulate matter the 'coffee grounds' but not of dissolved substances the 'coffee'. Also, like any filter, if the pore sizes are too large, particles like bacteria can get through. This can be a problem in aquifers in fractured rock like the Snake River Plain, or areas outside the sediment-filled valleys of southeast Idaho. Clay particles and other mineral surfaces in an aquifer also can trap dissolved substances or at least slow them down so they don't move as fast as water percolating through the aquifer.

Groundwater can become depleted if we use it at a faster rate than it can replenish itself. The replenishment of aquifers by precipitation is called recharging. Depletion of aquifers has increased primarily due to expanding agricultural irrigation. Groundwater can become contaminated when an excessive amount of pesticides and herbicides are sprayed on agricultural fields, septic tanks leak, or landfills are improperly lined or managed and toxic materials seep through the soil into the aquifer.

Aquifers naturally filter groundwater by forcing it to pass through small pores and between sediments, which helps to remove substances from the water. This natural filtration process, however, may not be enough to remove all of the contaminants. Pesticides can be fungicides which kill harmful fungi , insecticides which kill harmful insects , herbicides which kill harmful plants , or rodenticides which kill harmful rodents. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.

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