What makes an aquifer

Porosity is a description of how much space there could be to hold water under the ground, and permeability describes how those pores are shaped and interconnected. This determines how easy it is for water to flow from one pore to the next. Larger pores mean there is less friction between flowing water and the sides of the pores.

Smaller pores mean more friction along pore walls, but also more twists and turns for the water to have to flow-through. 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.

Sometimes caverns are formed that hold water and extend for thousands of feet. Often, the crevices and joints in limestone form a connecting network, further enhancing water flow. Gravel makes a good aquifer because it is extremely permeable and porous. The large pieces of sediment create significant pore spaces that water can travel through.

Often, gravel must be surrounded by a less permeable soil type, such as rich clay or impenetrable rock. When gravel cements, however, it becomes conglomerated and loses its permeability.

In some cases, fractured volcanic rocks, such as columnar basalts, make good aquifers. Rubble zones surround volcanoes and consist of large particles, which, like gravel, are very porous and permeable.

The variation among volcanic rock sediments largely result from the specific type of sediment, and the way it was ejected. Do not just look at the construction aspects or the beauty of the home and When you open the faucet you expect water to flow. And you expect it to flow night or day, summer or winter, whether you want to fill a glass or water the lawn.

It should be clean and pure, without any odor. You have seen or read about places where the water doesn't have these qualities. You may have lived in a city where you were allowed to water Below a certain depth, the ground, if it is permeable enough to hold water, is saturated with water. The upper surface of this zone of saturation is called the water table.

The saturated zone beneath the water table is called an aquifer, and aquifers are huge storehouses of water. What you are looking at in this photo is a "well" that exposes the water table, with an. The ground beneath our feet is not just rock, or at least, not just one kind of rock. Many different types of rock exist, and they have very different properties. Often, different types of rocks exist in horizontal layers beneath the land surface.

Some layers are more porous than others, and at a certain depth below ground the pores and fractures in these rocks can be. Pumping has removed water from storage in basalt aquifers and caused declines in many areas of the Columbia Plateau. Skip to main content. Search Search. Water Science School. Aquifers and Groundwater. Groundwater Photo Gallery Learn about groundwater through pictures Visit the gallery. Get GW data. Groundwater Information by Topic Learn more. Science Center Objects Overview Related Science Publications Multimedia A huge amount of water exists in the ground below your feet, and people all over the world make great use of it.

Learn the basics about water using our Water Primers! Credit: Environment and Climate Change Canada. Below are other science topics related to aquifers and groundwater. Date published: October 9, Filter Total Items: 7. Year Select Year Apply Filter.

Date published: November 8, Date published: June 18, Date published: June 11, Date published: June 6, Attribution: Water Resources. Year Published: A primer on ground water Most of us don't have to look for water. Baldwin, Helene L. Asides the use of sounding curves, empirical formulae have also been adapted in relating measurement of apparent resistivity with hydrological parameters of interest as this relates to aquifers. The empirical formula developed in the laboratory by Archie [ 4 ] relates these parameters:.

The equation presented by Eq. This concept was utilized by Pfeifer and Anderson [ 7 ] to observe and monitor the migration of tracer-spiked water through the subsurface using resistivity array. In conclusion, it could be said that the complexities that exist in the interpretation of sounding curves and the non-unique solution it gives, suggests the suitability of surface resistivity in determined subsurface geology.

Electromagnetic EM techniques as tool for geophysical exploration has dramatically increased in recent years served as a useful tool for groundwater and environmental site assessment. It involves the propagation of continuous-wave or transient electromagnetic fields in and over the earth through resulting in the generation of time-varying magnetic field.

For any of such surveys to be carried out three components are essential; a transmitters, receivers, buried conductors or conductive subsurface. These three form a trio of electric-circuit coupled by an EM induction with currents been introduced into the ground directly or through inductive means by the transmitters.

The Primary field travels from the transmitter coil to the receiver coil via paths above and below the surface. Where a homogenous subsurface is detected no difference is observed between the fields propagated above, below and within the surface other than a slight reduction in amplitude. However, the interaction of the time-varying field with a conductive subsurface induces eddy currents, which gives rise to a secondary magnetic field Figure 9.

The attributes of the fields generated, such as amplitude, orientation and phase shift can be measured by the receiver coil and compared with those of the primary field as such information about the presence of subsurface conductors, or subsurface electrical conductivity distribution can be inferred.

The acquisition of EM data requires less time, achieving greater depth of investigation than resistivity techniques.

However, the equipment used are expensive and the methods used to qualitatively interpret data from EM surveys is complicated than those used in resistivity methods. This is because a conductive subsurface environment is essential to set up a secondary field measured with inductive EM methods Figure 9. Electromagnetic methods as a tool for geophysical investigation and exploration is most suited for the detection of water—bearing formation aquifers and high — conductive subsurface target such as salt water saturated sediments.

Electromagnetic induction technique from Ref. Instrumentation could take in varying forms; but mainly consist of a source and receiver or receiver units. The source transmitter transmits time-varying magnetic fields with the receiver measuring components of the total primary and secondary field, magnetic field, sometimes the electric field and the necessary electronic circuitry to process, store and display signals [ 9 , 10 ].

Data obtained from electromagnetic surveys, like their resistivity counterpart can be collected in profile and sounding mode with their information been presented as maps or pseudo-section to give a better picture of the subsurface. Acquisition, resolution and depth of investigation from this survey are been governed by mostly by conditions of the subsurface and domain of measurement.

EM surveys are divided into two domain system of measurement namely; frequency and time domain system. For frequency domain EM systems, we have the transmitter classed as either high or low frequency transmitters; high transmitter frequencies permits high- resolution investigation of subsurface conductors at near-surface or shallow depths while lower transmitter frequencies allows for deeper depth of investigation at the expense of resolution.

This implies that high frequency EM surveys yield better result for near-surface due to high resolution, however if interested in deeper subsurface investigation low frequency EM surveys then we have need a way around the low resolution. In the case of time domain system, secondary magnetic field is measured as a function of time, with early — time measurement being suited best for near-surface information while late- time measurement yields results of the deeper subsurface.

It is paramount to note that depth of penetration or investigation and resolution is also been governed by coil configuration; while measurements from coil separations are influenced by electrical properties thus the larger coil separation investigates greater depths while smaller coil separation investigates near-surface. Because Electrical Conductivity is related inversely to Electrical Resistivity, as such discussions relating electrical resistivity to lithology or hydrological properties can be applied in an inverse manner to measurements involving electrical conductivity.

Electrical conductivity for example is higher for saturated sediments , clayey materials than for unsaturated sediments and sandy materials respectively. Some examples of investigations involving EM surveys include Sheets and Hendricks [ 11 ], who used EM induction methods to estimate soil water content and McNeill [ 12 ] that discussed the relation between electrical conductivity and hydrogeological parameters of porosity and saturation.

Ground Penetrating Radar GPR as a geophysical technique is relative new and becoming increasingly popular critically understanding the events of the near-surface or shallow subsurface.

Davis and Annan [ 13 ] viewed the Ground Penetrating Radar GPR as a technique of imaging the subsurface at high resolution using electromagnetic waves transmitted at frequencies between 10 to MHz. GPR could also be viewed as a non-destructive geophysical technique due to its successful geological applications in urban and sensitive environments.

Some of these applications include the subsurface mapping of water table soils and rocks structures e. It is similar in principle to seismic reflection profiling in however, propagation of radar waves through the subsurface is controlled by electrical properties at high frequencies. The GPR survey system is made up of three vital components; a transmitter , a receiver directly connected to the antenna and the control unit Figure The transmitter radiates EM waves into the subsurface that could be refracted, diffracted or primarily reflected depending on the dielectric permittivity and electrical conductivity nature of the subsurface interfaces encountered.

Recorded radar data received after the survey is first been observed, analyzed and interpreted by the aid of inbuilt radar processing software like RADPro, Ekko depending on the system type and make. These data are presented in form of radargrams which could either be presented as 2D or 3D subsurface images depending on the combination of the different axes x, y and z involved. Interpreting of radargrams is performed by interface mapping which is quite similar to the technique used in interpreting of seismograms.

Here each band within on a radargram is presumably classed and identified as a distinct geological horizon; this would have been correct except for the effects of multiples, interference with previous reflections, noise etc. All this effects on the radargram need to be removed to correctly identify the different geological horizons and geological structures as present within the radargram as such radargram are subjected to varying radar processing operations depending on the aims, objective of the survey been undertaking through the help of inbuilt system radar processing software like RADpro, Pulse Ekko system software etc.

Flow chart for a typical GPR system after [ 13 ]. Processing of the radargram could be simplified by processing operations such as dewowing removal of low frequency components , Gain Control strengthen weaker events , deconvolution restores shape of downgoing wave train such that primary events could be recognized more easily , Migration useful in removing diffraction hyperbolae and restoring dips.

The resultant radargram when correlated with the subsurface geology shows varying interfaces, geological structures that might be present Figure 11a and b. Though GPR has successfully been utilized in unsaturated non-electrically conductive or highly resistive and saturated electrically conductive environment [ 14 ], however performance is higher in unsaturated non-conductive than in saturated conductive such as non-expanding clay environment such as at Savannah River Site in South Carolina [ 15 ].

A Interpretation of a GPR profile image B interpretation of the prominent stratigraphic units, structures and faults. The depth of penetration or investigation of GPR survey is function of the frequency of the EM waves or radar waves and nature of the subsurface material been investigated as shown in Figure 12 for varying subsurface materials at frequencies ranging between 1 and MHz.

If the nature of subsurface material is highly resistive and has low conductivity then we expect a higher depth of penetration however for subsurface materials that are less resistivity and very conductive we expect low depth of penetration. Depth of penetration asides from been dependent on nature of the subsurface material i. Thus at low frequencies, we expect a greater depth of penetration at the expense of resolution while at high frequencies, we achieve a lower depth of penetration at higher resolution.