Hydrogeology definition
Hydrogeology (hydro-importance water, and - topography significance the Earth's investigation) is the range of geography that arrangements with the appropriation and development of groundwater in the dirt and rocks of the Earth's outside layer (regularly in aquifers). The term geohydrology is frequently utilized reciprocally. Some make the minor refinement between a hydrologist or specialist putting forth a concentrated effort to topography (geohydrology), and a geologist putting forth a concentrated effort to hydrology (hydrogeology).
Hydrogeology Introduction
Hydrogeology is an interdisciplinary subject; it can be hard to account completely for the synthetic, physical, organic and even lawful collaborations between soil, water, nature and society. The cooperation's investigation between groundwater development and geography can be very mind boggling. Groundwater not generally stream in the subsurface down-slope taking after the surface geography; groundwater takes after weight inclinations (stream from high weight to low) frequently taking after breaks and conductors in roaming ways. Considering the interchange of the distinctive features of a multi-segment framework regularly obliges information in a few different fields at both the exploratory and hypothetical levels. The accompanying is a more customary prologue to the routines and classification of soaked subsurface hydrology, or basically the investigation of ground water content.
Hydrogeology in relation to other fields
Hydrogeology, as expressed above, is a world's branch sciences managing the stream of water through aquifers and other shallow permeable media (normally under 450 m or 1,500 ft beneath the area surface.) The exceptionally shallow stream of water in the subsurface (the upper 3 m or 10 ft) is applicable to the fields of soil science, farming and structural building, and additionally to hydrogeology. The general stream of liquids (water, hydrocarbons, geothermal liquids, and so on.) in more profound arrangements is additionally a worry of geologists, geophysicists and petroleum geologists. Groundwater is a moderate moving, thick liquid (with a Reynolds number not as much as solidarity); large portions of the experimentally inferred laws of groundwater stream can be on the other hand got in liquid mechanics from the extraordinary instance of Stokes stream (thickness and weight terms, yet no inertial term).
The numerical connections used to portray the stream of water through permeable media are the dispersion and Laplace comparisons, which have applications in numerous differing fields. Enduring groundwater stream (Laplace comparison) has been recreated utilizing electrical, flexible and heat conduction analogies. Transient groundwater stream is closely resembling the dissemination of warmth in a strong, accordingly a few answers for hydrological issues have been adjusted from warmth exchange writing.
Generally, the development of groundwater has been concentrated independently from surface water, climatology, and even the concoction and microbiological parts of hydrogeology (the procedures are uncoupled). As the field of hydrogeology develops, the solid associations between groundwater, surface water, water science, soil dampness and even atmosphere are turning out to be all the more clear.
For instance: Aquifer drawdown or overdrafting and the pumping of fossil water may be a contributing variable to ocean level ascent.
What does a Hydrogeologist do?
One of the principle errands a hydrogeologist normally performs is the forecast of future conduct of an aquifer framework, in view of examination of over a wide span of time perceptions. Some theoretical, yet trademark inquiries asked would be:
- Will the aquifer support another subdivision?
- Will the waterway go away if the rancher pairs his watering system?
- Did the chemicals from the laundry office set out through the aquifer to my well and make me wiped out?
- Will the tuft of profluent leaving my neighbor's septic framework stream to my drinking water well?
The vast majority of these inquiries can be tended to through reenactment of the hydrologic framework (utilizing numerical models or systematic comparisons). Exact recreation of the aquifer framework obliges information of the aquifer properties and limit conditions. Along these lines, a typical assignment of the hydrogeologist is deciding aquifer properties utilizing aquifer tests.
Keeping in mind the end goal to further describe aquifers and aquitards some essential and inferred physical properties are presented underneath. Aquifers are comprehensively named being either limited or unconfined (water table aquifers), and either immersed or unsaturated; the sort of aquifer influences what properties control the stream of water in that medium (e.g., the arrival of water from capacity for kept aquifers is identified with the storativity, while it is identified with the particular yield for unconfined aquifers).
What does a Hydrogeologist do?
One of the principle errands a hydrogeologist normally performs is the forecast of future conduct of an aquifer framework, in view of examination of over a wide span of time perceptions. Some theoretical, yet trademark inquiries asked would be:
- Will the aquifer support another subdivision?
- Will the waterway go away if the rancher pairs his watering system?
- Did the chemicals from the laundry office set out through the aquifer to my well and make me wiped out?
- Will the tuft of profluent leaving my neighbor's septic framework stream to my drinking water well?
The vast majority of these inquiries can be tended to through reenactment of the hydrologic framework (utilizing numerical models or systematic comparisons). Exact recreation of the aquifer framework obliges information of the aquifer properties and limit conditions. Along these lines, a typical assignment of the hydrogeologist is deciding aquifer properties utilizing aquifer tests.
Keeping in mind the end goal to further describe aquifers and aquitards some essential and inferred physical properties are presented underneath. Aquifers are comprehensively named being either limited or unconfined (water table aquifers), and either immersed or unsaturated; the sort of aquifer influences what properties control the stream of water in that medium (e.g., the arrival of water from capacity for kept aquifers is identified with the storativity, while it is identified with the particular yield for unconfined aquifers).
Hydrogeologist predicts the following things
Hydraulic head
Contrasts in pressure driven head (h) reason water to move starting with one place then onto the next; water streams from areas of high h to areas of low h. Water driven head is made out of weight head (ψ) and rise head (z). The head angle is the change in pressure driven head per length of flowpath, and shows up in Darcy's law as being corresponding to the release.
Water powered head is a specifically quantifiable property that can tackle any worth (due to the self-assertive datum included in the z term); ψ can be measured with a weight transducer (this quality can be negative, e.g., suction, yet is sure in soaked aquifers), and z can be measured in respect to a studied datum (commonly the highest point of the well packaging). Normally, in wells tapping unconfined aquifers the water level in a well is utilized as an intermediary for water driven head, accepting there is no vertical inclination of weight. Regularly just changes in pressure driven head through time are required so the consistent height head term can be forgotten (Δh = Δψ).
A record of water driven head through time at a well is a hydrograph or, the progressions in pressure driven head recorded amid the pumping of a well in a test are called draw.
Porosity
Porosity (n) is a straightforwardly quantifiable aquifer property; it is a division somewhere around 0 and 1 demonstrating the measure of pore space between unconsolidated soil particles or inside of a broke rock. Ordinarily, the larger part of groundwater (and anything broke down in it) travels through the porosity accessible to stream (some of the time called successful porosity). Porousness is an outflow of the pores' connectedness. Case in point, an unfractured rock unit may have a high porosity (it has heaps of openings between its constituent grains), yet a low porousness (none of the pores are associated). A case of this marvel is pumice, which, when in its unfractured state, can make a poor aquifer.
Porosity not straightforwardly influence the appropriation of pressure driven head in an aquifer, however it has an extremely solid impact on the movement of disintegrated contaminants, since it influences groundwater stream speeds through a conversely corresponding relationship.
Water content
Water content (θ) is additionally a straightforwardly quantifiable property; it is the absolute's portion rock which is loaded with fluid water. This is additionally a division somewhere around 0 and 1, yet it should likewise be not exactly or equivalent to the aggregate porosity.
The water substance is critical in vadose zone hydrology, where the pressure driven conductivity is an emphatically nonlinear capacity of water substance; this entangles the arrangement of the unsaturated groundwater stream comparison.
Hydraulic conductivity
Water powered conductivity (K) and transmissivity (T) are roundabout aquifer properties (they can't be measured specifically). T is the K coordinated over the vertical thickness (b) of the aquifer (T=Kb when K is steady over the whole thickness). These properties are measures of an aquifer's capacity to transmit water. Characteristic porousness (κ) is an auxiliary medium property which not rely on upon the thickness and thickness of the liquid (K and T are particular to water); it is utilized more as a part of the petroleum business.
Specific storage and specific yield
Particular stockpiling (Ss) and its profundity coordinated comparable, storativity (S=Ssb), are roundabout aquifer properties (they can't be measured straightforwardly); they show the measure of groundwater discharged from capacity because of a unit depressurization of a kept aquifer. They are divisions somewhere around 0 and 1.
Particular yield (Sy) is additionally a proportion somewhere around 0 and 1 (Sy ≤ porosity) and shows the measure of water discharged because of seepage from bringing down the water table in an unconfined aquifer. The worth for particular yield is not exactly the quality for porosity on the grounds that some water will stay in the medium even after waste because of intermolecular powers. Regularly the porosity or powerful porosity is utilized as an upper bound to the particular yield. Normally Sy is requests of extent bigger than Ss.
Contaminant transport properties
Frequently we are keen on how the moving groundwater will transport disintegrated contaminants around (the sub-field of contaminant hydrogeology). The contaminants can be man-made (e.g., petroleum items, nitrate, Chromium or radionuclides) or normally happening (e.g., arsenic, saltiness). Other than expecting to comprehend where the groundwater is streaming, taking into account the other hydrologic properties talked about above, there are extra aquifer properties which influence how broken down contaminants move with groundwater.
Hydrodynamic dispersion
Hydrodynamic dispersivity (αL, αT) is an exact element which evaluates the amount of contaminants stray far from the way of the groundwater which is conveying it. A contaminants' percentage will be "behind" or "ahead" the mean groundwater, offering ascent to a longitudinal dispersivity (αL), and some will be "to the sides of" the immaculate advective groundwater stream, prompting a transverse dispersivity (αT). Scattering in groundwater emerges in light of the fact that every water "molecule", passing past a dirt molecule, must pick where to go, whether left or right or up or down so that the water "particles" (and their solute) are bit by bit spread in all headings around the mean way. This is the "minute" system, on the size of soil particles. More vital, on long separations, can be the naturally visible inhomogeneities of the aquifer, which can have districts of bigger or littler penetrability, with the goal that some water can locate a particular way in one course, some other in an alternate bearing so that the contaminant can be spread in a totally unpredictable manner, similar to in a (three-dimensional) delta of a stream.
Dispersivity is really a component which speaks to our absence of data about the framework we are mimicking. There are numerous little insights about the aquifer which are being found the middle value of when utilizing a naturally visible methodology (e.g., minor beds of rock and dirt in sand aquifers), they show themselves as an evident dispersivity. In view of this, α is regularly asserted to be reliant on the length size of the issue — the dispersivity found for transport through 1 m3 of aquifer is unique in relation to that for transport through 1 cm3 of the same aquifer material.
Molecular Diffusion
Dissemination is a central physical wonder, which Einstein portrayed as Brownian movement, that depicts the irregular warm development of atoms and little particles in gasses and fluids. It is a critical wonder for little separations (it is vital for the accomplishment of thermodynamic equilibria), in any case, as the time important to cover a separation by dispersion is relative to the separation's square itself, it is ineffectual for spreading a solute over plainly visible separations. The dispersion coefficient, D, is regularly very little, and its impact can frequently be viewed as insignificant (unless groundwater stream speeds are to a great degree low, as they are in dirt aquitards).
It is essential not to mistake dissemination for scattering, as the previous is a physical wonder and the last is an experimental element which is thrown into a comparable structure as dispersion, in light of the fact that we definitely know how to take care of that issue.
Retardation by adsorption
The impediment element is another imperative element that make the contaminant's movement to digress from the normal groundwater movement. It is comparable to the impediment variable of chromatography. Dissimilar to dissemination and scattering, which essentially spread the contaminant, the hindrance element changes its worldwide normal speed, with the goal that it can be much slower than that of water. This is because of a chemico-physical impact: the adsorption to the dirt, which keeps the contaminant down and not permit it to advance until the amount relating to the synthetic adsorption harmony has been adsorbed. This impact is especially imperative for less dissolvable contaminants, which in this way can move even hundreds or thousands times slower than water. The impact of this wonder is that just more dissolvable species can cover long separations. The hindrance element relies on upon the compound way of both the contaminant and the aquifer.
Hydrologic Cycle
The water cycle, otherwise called the hydrologic cycle or the H2O cycle, depicts the constant development of water on, above and underneath the Earth's surface. The mass of water on Earth remains genuinely steady after some time yet the water's dividing into the significant repositories of ice, crisp water, saline water and barometrical water is variable relying upon an extensive variety of climatic variables. The water moves starting with one repository then onto the next, for example, from stream to sea, or from the sea to the environment, by the physical procedures of vanishing, build up, precipitation, invasion, overflow, and subsurface stream. In doing as such, the water experiences distinctive stages: fluid, strong (ice), and gas (vapour).
The hydrologic cycle includes the trading of vitality, which prompts temperature changes. Case in point, when water dissipates, it takes up vitality from its surroundings and cools the earth. When it consolidates, it discharges vitality and warms nature. These warmth trades impact atmosphere.
The evaporative period of the cycle sanitizes water which then recharges the area with freshwater. The stream of fluid water and ice transports minerals over the globe. It is likewise included in reshaping the topographical elements of the Earth, through procedures including disintegration and sedimentation. The hydrologic cycle is likewise fundamental for the upkeep of most life and biological systems on the planet.
Description of hydrologic cycle
The sun, which drives the hydrologic cycle, warms water in seas and oceans. Water vanishes as water vapour into the air. Ice, rain and snow can sublimate straightforwardly into water vapour. Evapotranspiration is water unfolded from plants and dissipated from the dirt. Water vapour atom H2O, has less thickness contrasted with the significant parts of the air, nitrogen and oxygen, N2 and O2. Because of the huge contrast in atomic mass, water vapour in gas structure pick up stature in outside as a consequence of lightness. Then again, as elevation expands, pneumatic force diminishing and temperature drops (see Gas laws). The brought temperature reasons water vapour down to consolidate into minor fluid water beads which is heavier than the air, such that it falls unless bolstered by an up draft. A tremendous centralization of these beads over a substantial space up in the climate get to be unmistakable as cloud. Haze is framed if the water vapour gather close ground level, as a consequence of soggy air and cool air crash or a sudden decrease in pneumatic force. Air streams move water vapour around the world, cloud particles impact, develop, and drop out of the upper air layers as precipitation. Some precipitation falls as snow or hail, slush, and can gather as ice tops and ice sheets, which can store solidified water for a large number of years. Most water falls once more into the seas or onto land as downpour, where the water streams over the ground as surface spillover. A bit of overflow enters waterways in valleys in the scene, with stream flow moving water towards the seas. Spillover and water rising up out of the ground (groundwater) may be put away as freshwater in lakes. Not all spillover streams into waterways, quite a bit of it splashes into the ground as invasion. Some water invades profound into the ground and recharges aquifers, which can store freshwater for drawn out stretches of time. Some penetration remains nearby to the area surface and can leak once more into surface-water bodies (and the sea) as groundwater release. Some groundwater discovers openings in the area surface and turns out as freshwater springs. In waterway valleys and surge fields there is regularly nonstop water trade between surface water and ground water in the hyporheic zone. After some time, the water comes back to the sea, to proceed with the hydrologic cycle.
Processes
Precipitation
Dense water vapor that tumbles to the Earth's surface . Most precipitation happens as downpour, additionally incorporates snow, hail, haze trickle, graupel, and slush. Around 505,000 km3 (121,000 cu mi) of water falls as precipitation every year, 398,000 km3 (95,000 cu mi) of it over the seas. The downpour ashore contains 107,000 km3 (26,000 cu mi) of water every year and a snowing just 1,000 km3 (240 cu mi). 78% of worldwide precipitation happens over the sea.
Canopy interception
The precipitation that is blocked by plant foliage, in the end dissipates back to the climate as opposed to tumbling to the ground.
Snowmelt
The runoff produced by melting snow.Runoff
The assortment of courses by which water moves over the area. This incorporates both surface spillover and channel overflow. As it streams, the water may saturate the ground, vanish into the air, get to be put away in lakes or repositories, or be separated for horticultural or other human employments.
Infiltration
The stream of water from the beginning into the ground. Once invaded, the water gets to be soil dampness or groundwater. A late worldwide study utilizing water stable isotopes, on the other hand, demonstrates that not all dirt dampness is similarly accessible for groundwater energize or for plant transpiration.
Subsurface flow
The stream of water underground, in the vadose zone and aquifers. Subsurface water may come back to the surface (e.g. as a being so as to spring or pumped) or in the long run saturate the seas. Water comes back to the area surface at lower height than where it penetrated, under the power of gravity or gravity prompted weights. Groundwater tends to move gradually, and is renewed gradually so it can stay in aquifers for a great many years.
Evaporation
The change of water from fluid to gas stages as it moves starting from the earliest stage waterways into the overlying climate. The wellspring of vitality for dissipation is fundamentally sun oriented radiation. Vanishing frequently verifiable incorporates transpiration from plants, however together they are particularly alluded to as evapotranspiration. Downright yearly evapotranspiration adds up to roughly 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which dissipates from the seas. 86% of worldwide dissipation happens over the sea.
Sublimation
The state change directly from solid water (snow or ice) to water vapor.Deposition
This refers to changing of water vapour directly to ice.Advection
The development of water in strong, fluid, or vapour states through the environment. Without shift in weather conditions, water that vanished over the seas couldn't encourage over land
Condensation
The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.Transpiration
The release of water vapor from plants and soil into the air. Water vapor is a gas that cannot be seen.Percolation
Water flows vertically through the soil and rocks under the influence of gravity.Plate tectonics
Water enters the mantle via subduction of oceanic crust. Water returns to the surface via volcanism.Groundwater
Hydrogeology is the study of ground water. When a hole is drilled in the Earth's crust after a few meters to kilometres water appears. This water exists even if there is no stream or river nearby which recharge the ground but water zone is present as subsurface. Rain water percolates in the ground and reaches the vast reservoir of water. This water is clean and is used for drinking purposes.
Characteristics
Porosity and permeability
In the upper region of crust in few kilometers, bedrock and soil contains small void spaces which is filled with air or ground water. The proportional volume of these spaces are called porosity. As water fills in these pores therefore porosity is the indicator a rock or soil can hold water and permeability is the interconnection of pores so it indicates the transmission of water through the rocks. Sand and gravel have larger pores so they are the most suitable comprising of effective porosity. Clay is the vice versa because of the smaller pores which can store but do not transmits with no effective permeability.
Water table
When it rains so the water soaks in the soil same like sponge soaking water. These rain droplets then descends in the crust but a few kilometers in the Earth's crust due to above lying over burden pressure, the pores are closed. The closing of pores makes it nonporous and impermeable which is called bedrock. Bedrock acts as barrier so soil and rocks above it are wet and is called zone of saturation. On its top is the water table and above water table is zone of aeration where it is moist but not saturated. Water is pulled downward by gravity action but due to electrical charges between clay and water particles it moves against gravity in small channels called capillary action. Soil moisture belt is the top soil capability where humus presence keep it moist which is greater than unsaturated zone. Aquifer is the body of rock and soil which can provide economical significance of water, it is both porous and permeable which allows water to be extracted from a single well.
Movement of ground water
Water flows through ground and flow rates depends upon permeability of the rocks. Aquifer are sponge like body through which water seeps. Ground water movement can be rapid if the rock is fractured. Osmosis is a process in which water flows from area of highest water table to lowest water table areas. The above figure shows the movement of water. In (a) water recharges the stream as water table in the ground is higher than that of the stream and in (b) stream recharges the ground water. Streams flows through many valleys because of the high ground water table which continuously recharge stream and this is the factor which allows stream flowing even if it not rains for weeks or months. The stream which is recharged by the ground water is called effluent stream. In desert areas the water table is low so streams recharges the ground water and theses streams are called infuent streams.
Springs
A spring occurs when ground water intersects the surface and water seeps through and flows on land. Above the water table in hill mostly the water is trapped by an impermeable layer and water flows from them, this is called perched aquifer.
Artesian wells
An inclined aquifer where its top and bottom is bounded by impermeable layer is an artesian aquifer. The underlying water is pressurized by the above water so when well is drilled the water comes out without being pumped this is called artesian well.
Ground water depletion
When water is pumped at high rate than of water inflow to the well, a cone of depletion appears. This cone can disappear if the pumped is turned of and water in a few weeks recharges the water table below well. If the water is pumped more rapidly or if there are many wells pumping from the same aquifer than the water table will drop down. By the new technologies, high efficiency pumps production where it can pump rapidly than that of the aquifer recharge, it will deplete.
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.
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| 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
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| 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
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| 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).
Tapping Groundwater Supplies
We can obtain groundwater at wells or springs. Wells are holes that people dig or drill to obtain water. Springs are natural outlets from which groundwater flows. Wells and springs provide welcome sources of water but must be treated with care if they are to last.
Wells
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| Pumping groundwater at a normal well affects the water table. |
In an ordinary well, the base of the well penetrates an aquifer below the water table (figure above a). Water from the pore space in the aquifer seeps into the well and fills it to the level of the water table. Drilling into an aquitard, or into rock that lies above the water table, will not supply water, and thus yields a dry well. Some ordinary wells are seasonal and function only during the rainy season, when the water table rises. During the dry season, the water table lies below the base of the well, so the well is dry.
To obtain water from an ordinary well, you either pull water up in a bucket or pump the water out. As long as the rate at which groundwater fills the well exceeds the rate at which water is removed, the level of the water table near the well remains about the same. However, if users pump water out of the well too fast, then the water table sinks down around the well, in a process called drawdown, so that the water table becomes a downward-pointing, cone-shaped surface called a cone of depression (figure above b, c). Drawdown by a deep well may cause shallower wells that have been drilled nearby to run dry.
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| Artesian wells, where water rises from the aquifer without pumping. |
An artesian well, named for the province of Artois in France, penetrates confined aquifers in which water is under enough pressure to rise on its own to a level above the surface of the aquifer. If this level lies below the ground surface, the well is a nonflowing artesian well. But if the level lies above the ground surface, the well is a flowing artesian well, and water actively fountains out of the ground (figure above a). Artesian wells occur in special situations where a confined aquifer lies beneath a sloping aquitard.
We can understand why artesian wells exist if we look first at the configuration of a city water supply (figure above b). Water companies pump water into a high tank that has a significant hydraulic head relative to the surrounding areas. If the water were connected by a water main to a series of vertical pipes, pressure caused by the elevation of the water in the high tank would make the water rise in the pipes until it reached an imaginary surface, called a potentiometric surface, that lies above the ground. This pressure drives water through water mains to household water systems without requiring pumps. In an artesian system, water enters a tilted, confined aquifer that intersects the ground in the hills of a high-elevation recharge area (figure above c). The confined groundwater flows down to the adjacent plains, which lie at a lower elevation. The potentiometric surface to which the water would rise, were it not confined, lies above this aquifer. Pressure in the confined aquifer pushes water up a well.
Springs
Many towns were founded next to springs, places where groundwater naturally flows or seeps onto the Earth’s surface, for springs can provide fresh, clear water for drinking or irrigation, without the expense of drilling or digging. Some springs spill water onto dry land. Others bubble up through the bed of a stream or lake. Springs form under a variety of conditions:
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| Geological settings in which springs form. |
- Where the ground surface intersects the water table in a discharge area (figure above a); such springs typically occur in valley floors, where they may add water to lakes or streams.
- Where flowing groundwater collides with a steep, impermeable barrier, and pressure pushes it up to the ground along the barrier (figure above b).
- Where a perched water table intersects the surface of a hill (figure above c).
- Where downward-percolating water runs into a relatively impermeable layer and migrates along the top surface of the layer to a hillslope (figure above d).
- Where a network of interconnected fractures channels groundwater to the surface of a hill (figure above e).
- Where the ground surface intersects a natural fracture (joint) that taps a confined aquifer in which the pressure is sufficient to drive the water to the surface; such an occurrence is an artesian spring.
Springs can provide water in regions that would otherwise be uninhabitable. For example, oases in deserts may develop around a spring. An oasis is a wet area, where plants can grow, in an otherwise bone-dry region.
The Messinian Salinity Crisis
You might have heard of The Messinian Salinity Crisis no doubt. From learned articles, geology textbooks, probably lectures at your college or University. Or possibly not. This was not always the hot topic it is now. In fact, the very idea of this happening, was for a while, challenged, even ridiculed. It seemed too incredible that this could happen as it did and Dessication/Flood theories took time to gain traction. But, if you had heard about it, you would remember that The Messinian Salinity Crisis, was a time when the Mediterranean Sea, very much as we know it today, evaporated – dried out, almost completely.
You will have heard of the rates of desiccation, influx and yet more desiccation, repeated in endless cycles over tens, even hundreds of thousands of years. On a human temporal scale, this would have been a long drawn out affair, covering a time hundreds of generations deep, more than the span of Homo sapiens existence. In Geologic terms however, it was a string of sudden events. Of incredibly hot and arid periods followed by rapid ingress of waters, either via spillways through what is now modern day Morocco and the southern Iberian peninsular, or headlong through a breach in the sill between the Pillars of Heracles, the modern day Straights of Gibraltar.
There were prolonged periods of
dessication, of desolate landscapes beyond anything seen today in Death Valley
or The Afar Triangle. These landscapes were repeatedly transgressed by brackish
waters from storm seasons far into the African and Eurasian interiors, or the
Atlantic, and these in turn dried out. Again and again this happened. It had to
be so because the vast deposits of rock salt, gypsum and anhydrites could not
have been emplaced in a single evaporite event. The salt deposits in and around
the Mediteranean today represent fifty times the current capacity of this great
inland sea. You may have heard too of the variety of salts production, as
agglomerating crystals fell from the descending surface to the sea floor, or as
vast interconnected hypersaline lakes left crystalline residues at their
diminishing margins, as forsaken remnant sabkhas, cut off from the larger
basins, left behind acrid dry muds of potassium carbonates – the final arid
mineral residue of the vanished waters.
Just under six million years
ago, Geologic processes isolated what was left of the ancient Tethys ocean, the
sea we know as the Mediterranean, home to historic human conflicts and marine
crusades of Carthage, Rome, Athens and Alexandria, a Sea fringed by modern day
Benidorm, Cyprus, Malta and Monaco. At a time 5.96 million years ago –
evaporation outpaced replenishment. Indeed, just as it does today, but without
the connecting seaway to replenish losses. Inexorable tectonic activity first
diverted channels, then – sealed them. Cut off from the Atlantic in the West,
water levels fell, rose briefly and fell again, and again. The mighty Nile - a
very different geophysical feature of a greater capacity than today, and the
rivers of Europe cut down great canyons hundreds and thousands of metres below
present Eustatic sea and land surface levels, as seismic cross sections show in
staggering detail. The cores taken at depth in the Mediterranean, show Aeolian
sands above layers of salt, fossiliferous strata beneath those same salts, all
indicating changing environments. The periods of blackened unshifting desert
varnished floors and bleached playas, decades and centuries long, were
punctuated often by catastrophic episodes, with eroded non conformable surfaces
of winnowed desert pavement, toppled ventifracts, scours and rip up clasts.
Species of fossilised terrestrial plant life, scraping an arid existence have been
found, thousands of meters down, in the strata of the Mediterranean sea floor.
There is much evidence too, in the uplifted margins of Spain, France, and Sicily, of those hostile millennia when the sea disappeared. Incontrovertible evidence, painstakingly gathered, analysed and peer reviewed, demonstrates via the resources of statistical analysis, calculus and geophysical data that the Messinian Salinity Crisis was a period during the Miocene wherein the geology records a uniquely arid period of repeated partial and very nearly complete desiccation of the Mediterranean Sea over a period of approximately 630,000 years. But for the Geologist, the story doesn’t end there. The Geologists panoptic, all seeing third eye, sees incredible vistas and vast panoramas. Of a descent from the Alpine Foreland to the modern day enclave of Monaco, gazing out southwards from a barren, uninhabited and abandoned raised coast to deep dry abyssal plains, punctuated by exposed chasms, seamounts and ridges, swirling and shifting so slowly in a distant heat haze. A heat haze produced by temperatures far above any recorded by modern man and his preoccupation with Global Warming. An unimaginable heat sink would produce temperatures of 70 to 80 degrees Celsius at 4000M depth within the basins.
Looking down upon this Venusian landscape, the sun might glint on remaining lakes and salt flats so very far away and so very much farther below. Hills and valleys, once submerged, would be observed high and dry – from above, as would the interconnecting rivers of bitter waters hot enough to slowly broil any organism larger than extremophile foraminifer. All this, constantly shimmering in the relentless heat. Only the imagination of the geologist could see the vast, hellish, yet breathtaking landscape conjured up by the data and the rock record. And finally, the Geologist would visualise a phenomenon far greater in scope and magnitude than any Biblical flood – The Zanclean Event.
Also known as The Zanclean
Deluge, when the drought lasting over half a million years was finally ended as
the Atlantic Ocean breached the sill/land bridge between Gibraltar and North
West Africa. Slowly perhaps at first until a flow a thousand times greater than
the volumetric output of the Amazon cascaded down the slopes to the parched
basins. Proximal to the breach, there would be a deafening thunderous roar and
the ground would tremor constantly, initially triggering great avalanches above
and below the Eustatic sea level as the far reaching and continuous concussion
roared and rumbled on, and on, and on. For centuries great cataracts and
torrents of marine waters fell thousands of metres below and flowed thousands
of kilometers across to the East. Across to the abyssal plains off the
Balearics, to the deeps of the Tyrrhenian and Ionian seas, into the trenches
south of the Greek Islands and finally up to the rising shores of The Lebanon.
The newly proximal waters to the final coastal reaches and mountains that
became islands, must have had a climatological effect around the margins of the
rejuvenated Mediterranean. Flora and Fauna both marine and terrestrial will
have recolonised quickly. Species may have developed differently, post
Zanclean, on the Islands. And in such a short period, there must surely have
been earthquakes and complex regional depression and emergence. Isostacy
compensated for the trillions of cubic meters of transgression waters that now
occupied the great basins between the African and Eurasian plates, moving the
land, reactivating ancient faults and within and marginal to the great inland
sea, a region long active with convergent movements of a very different
mechanism.
Hollywood and Pinewood have yet
to match the imagination of the Earth Scientist, of the many chapters of Earths
dynamic history held as fully tangible concepts to the men and women who study
the rocks and the stories they tell. The movies played out in the mind of the
geologist are epic indeed and – as we rightly consider the spectre of Global
Warming, consider too the fate of future populations (of whatever evolved
species) at the margins of the Mediterranean and the domino regions beyond,
when inexorable geologic processes again isolate that benign, sunny holiday
sea. Fortunately, not in our lifetime, but that of our far off descendants who
will look and hopefully behave very differently from Homo Sapiens.
Guest Blog: How Speleothems Are Used To Determine Past Climates?
Speleothems, more commonly known
as stalactites or stalagmites, consist of calcium carbonate (calcite or
aragonite) crystals of various dimensions, ranging from just a few micrometers
to several centimetres in length, which generally have their growth axis perpendicular
to the growth surface. Speleothems are formed through the deposition of calcium
carbonate minerals in karst systems, providing archives of information on past
climates, vegetation types and hydrology, particularly groundwater and
precipitation. However, they can also provide information on anthropogenic
impacts, landscape evolution, volcanism and tectonic evolution in mineral deposits
formed in cave systems.
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| Stalagmite Formation |
Rainfall containing carbonic acid
weathers the rock unit (generally either limestone or dolomite) and seeps into
the cracks, forming caverns and karst systems. The groundwater, percolating
through such cracks and caverns, also contains dissolved calcium bicarbonate.
The dripping action of these groundwater droplets is the driving force behind
the deposition of speleothems in caves.
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| Core drilling of an active stalagmite in Hang Chuot cave. |
Speleothems are mainly studied as
paleoclimate indicators, providing clues to past precipitation, temperature and
vegetation changes over the past »500,000 years.
Radioisotopic dating of speleothems is the primary method used by researchers
to find annual variations in temperature. Carbon isotopes (d^13C) reflect C3/C4 plant compositions and plant productivity, where
increased plant productivity may indicate greater amounts of rainfall and
carbon dioxide absorption. Thus, a larger carbon absorption can be reflective
of a greater atmospheric concentration of greenhouse gases. On the other hand,
oxygen isotopes (d^8O) provide researchers with past rainfall temperatures and quantified
levels of precipitation, both of which are used to determine the nature of past
climates.
Groundwater Flow
What happens to groundwater over time? Does it just sit, unmoving, like the water in a stagnant puddle, or does it flow and eventually find its way back to the surface? Countless measurements confirm that groundwater enjoys the latter fate groundwater indeed flows, and in some cases it moves great distances underground. Let’s examine factors that drive groundwater flow.
In the unsaturated zone the region between the ground surface and the water table water percolates straight down, like the water passing through a drip coffee maker, for this water moves only in response to the downward pull of gravity. But in the zone of saturation the region below the water table water flow is more complex, for in addition to the downward pull of gravity, water responds to differences in pressure. Pressure can cause groundwater to flow sideways, or even upward. (If you've ever watched water spray from a fountain, you've seen pressure pushing water upward.) Thus, to understand the nature of groundwater flow, we must first understand the origin of pressure in groundwater. For simplicity, we’ll consider only the case of groundwater in an unconfined aquifer.
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| The shape of water table beneath hilly topography. |
Pressure in groundwater at a specific point underground is caused by the weight of all the overlying water from that point up to the water table. (The weight of overlying rock does not contribute to the pressure exerted on groundwater, for the contact points between mineral grains bear the rock’s weight.) Thus, a point at a greater depth below the water table feels more pressure than does a point at lesser depth. If the water table is horizontal, the pressure acting on an imaginary horizontal reference plane at a specified depth below the water table is the same everywhere. But if the water table is not horizontal, as shown in above, the pressure at points on a horizontal reference plane at depth changes with location. For example, the pressure acting at point p1, which lies below the hill in figure above, is greater than the pressure acting at point p2, which lies below the valley, even though both p1 and p2 are at the same elevation.
Both the elevation of a volume of groundwater and the pressure within the water provide energy that, if given the chance, will cause the water to flow. Physicists refer to such stored energy as potential energy. The potential energy available to drive the flow of a given volume of groundwater at a location is called the hydraulic head. To measure the hydraulic head at a point in an aquifer, hydrogeologists drill a vertical hole down to the point and then insert a pipe in the hole. The height above a reference elevation (for example, sea level) to which water rises in the pipe represents the hydraulic head water rises higher in the pipe where the head is higher. As a rule, groundwater flows from regions where it has higher hydraulic head to regions where it has lower hydraulic head. This statement generally implies that groundwater regionally flows from locations where the water table is higher to locations where the water table is lower.
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| The flow of groundwater. |
Hydrogeologists have calculated how hydraulic head changes with location underground, by taking into account both the effect of gravity and the effect of pressure. These calculations reveal that groundwater flows along concave-up curved paths, as illustrated in cross section (figure above a, b). These curved paths eventually take groundwater from regions where the water table is high (under a hill) to regions where the water table is low (below a valley), but because of flow-path shape,
some groundwater may flow deep down into the crust along the first part of its path and then may flow back up, toward the ground surface, along the final part of its path. The location where water enters the ground (where the flow direction has a downward trajectory) is called the recharge area, and the location where groundwater flows back up to the surface is called the discharge area (see figure above a).
Flowing water in an ocean current moves at up to 3 km per hour, and water in a steep river channel can reach speeds of up to 30 km per hour. In contrast, groundwater moves at less than a snail’s pace, between 0.01 and 1.4 m per day (about 4 to 500 m per year). Groundwater moves much more slowly than surface water, for two reasons. First, groundwater moves by percolating through a complex, crooked network of tiny conduits, so it must travel a much greater distance than it would if it could follow a straight path. Second, friction between groundwater and conduit walls slows down the water flow.
Simplistically, the velocity of groundwater flow depends on the slope of the water table and the permeability of the material through which the groundwater is flowing. Thus, groundwater flows faster through high-permeability rocks than it does through low-permeability rocks, and it flows faster in regions where the water table has a steep slope than it does in regions where the water table has a gentle slope. For example, groundwater flows relatively slowly (2 m per year) through a low-permeability aquifer under the Great Plains, but flows relatively quickly (30 m per year) through a high-permeability aquifer under a steep hillslope. In detail, hydrogeologists use Darcy’s Law to determine flow rates at a location.
Darcy’s Law for Groundwater Flow
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| The level to which water rises in a drill hole is the hydraulic head (h). The hydraulic gradient (HG) is the difference in head divided by the length of the flow path. |
The rate at which groundwater flows at a given location depends on the permeability of the material containing the groundwater; groundwater flows faster in a more permeable material than it does in a less permeable material. The rate also depends on the hydraulic gradient, the change in hydraulic head per unit of distance between two locations, as measured along the flow path.
To calculate the hydraulic gradient, we divide the difference in hydraulic head between two points by the distance between the two points as measured along the flow path. This can be written as a formula:
hydraulic gradient = h1 - h2/j
where h1 - h2 is the difference in head (given in meters or feet, because head can be represented as an elevation) between two points along the water table, and j is the distance between the two points as measured along the flow path. A hydraulic gradient exists anywhere that the water table has a slope. Typically, the slope of the water table is so small that the path length is almost the same as the horizontal distance between two points. So, in general, the hydraulic gradient is roughly equivalent to the slope of the water table.
In 1856, a French engineer named Henry Darcy carried out a series of experiments designed to characterize factors that control the velocity at which groundwater flows between two locations (1 and 2), each of which has a different hydraulic head (h1 and h2). Darcy represented the velocity of flow by a quantity called the discharge (Q), meaning the volume of water passing through an imaginary vertical plane perpendicular to the groundwater’s flow path in a given time. He found that the discharge depends on the the hydraulic head (h1- h2); the area (A) of the imaginary plane through which the groundwater is passing; and a number called the hydraulic conductivity (K). The hydraulic conductivity represents the ease with which a fluid can flow through a material. This, in turn, depends on many factors (such as the viscosity and density of the fluid), but mostly it reflects the permeability of the material. The relationship that Darcy discovered, now known as Darcy’s law, can be written in the form of an equation as:
Q = KA(h1 - h2)/j
The equation states that if the hydraulic gradient increases, discharge increases, and that as conductivity increases, discharge increases. Put in simpler terms, the flow rate of groundwater increases as the permeability increases and as the slope of the water table gets steeper.
Groundwater Problems
Since prehistoric times, groundwater has been an important resource that people have relied on for drinking, irrigation, and industry. Groundwater feeds the lushness of desert oases in the Sahara, the amber grain in the North American high plains, and the growing cities of sunny arid regions.
Though groundwater accounts for about 95% of the liquid freshwater on the planet, accessible groundwater cannot be replenished quickly, and this leads to shortages. Groundwater contamination is also a growing tragedy. Such pollution, caused when toxic wastes and other impurities infiltrate down to the water table, may be invisible to us but may ruin a water supply for generations to come. In this section, we’ll take a look at problems associated with the use of groundwater supplies.
Depletion of Groundwater Supplies
Is groundwater a renewable resource? In a time frame of 10,000 years, the answer is yes, for the hydrologic cycle will eventually resupply depleted reserves. But in a time frame of 100 to 1,000 years the span of a human lifetime or a civilization groundwater in many regions may be a non-renewable resource. By pumping water out of the ground at a rate faster than nature replaces it, people are effectively “mining” the groundwater supply. In fact, in portions of the desert Sunbelt region of the United States, supplies of young groundwater have already been exhausted, and deep wells now extract 10,000-year old groundwater. Some of this ancient water has been in rock so long that it has become too mineralized to be usable. A number of other problems accompany the depletion of groundwater.
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| Effects of human modification of the water table. |
- Lowering the water table: When we extract groundwater from wells at a rate faster than it can be resupplied by nature, the water table drops. First, a cone of depression forms locally around the well; then the water table gradually becomes lower in a broad region. As a consequence, existing wells, springs, and rivers, and swamps dry up (figure above a, b). To continue tapping into the water supply, we must drill progressively deeper. Notably, the water table can also drop when people divert surface water from the recharge area. Such a problem has developed in the Everglades of southern Florida, a huge swamp where, before the expansion of Miami and the development of agriculture, the water table lay at the ground surface (figure above c, d). Diversion of water from the Everglades’ recharge area into canals has significantly lowered the water table, causing parts of the Everglades to dry up.
- Reversing the flow direction of groundwater: The cone of depression that develops around a well creates a local slope to the water table. The resulting hydraulic gradient may be large enough to reverse the flow direction of nearby groundwater (figure below a, b). Such reversals can allow contaminants, seeping out of a septic tank, to contaminate the well.
- Saline intrusion: In coastal areas, fresh groundwater lies in a layer above saline (salty) water that entered the aquifer from the adjacent ocean (figure below c, d). Because fresh water is less dense than saline water, it floats above the saline water. If people pump water out of a well too quickly, the boundary between the saline water and the fresh groundwater rises. And if this boundary rises above the base of the well, then the well will start to yield useless saline water. Geologists refer to this phenomenon as saline intrusion.
- Pore collapse and land subsidence: When groundwater fills the pore space of a rock or sediment, it holds the grains apart, for water cannot be compressed. The extraction of water from a pore eliminates the support holding the grains apart, because the air that replaces the water can be compressed. As a result, the grains pack more closely together. Such pore collapse permanently decreases the porosity and permeability of a rock, and thus lessens its value as an aquifer (figure below e, f).
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| Some causes of groundwater problems. |
Pore collapse also decreases the volume of the aquifer, with the result that the ground above the aquifer sinks. Such land subsidence may cause fissures at the surface to develop and the ground to tilt. Buildings constructed over regions undergoing land subsidence may themselves tilt, or their foundations may crack. In the San Joaquin Valley of California, the land surface subsided by 9 m between 1925 and 1975, because water was removed to irrigate farm fields.
Natural Groundwater Quality
Much of the world’s groundwater is crystal clear, and pure enough to drink right out of the ground. Rocks and sediment are natural filters capable of removing suspended solids these solids get trapped in tiny pores or stick to the surfaces of clay flakes. In fact, the commercial distribution of bottled groundwater (“spring water”) has become a major business worldwide. But dissolved chemicals, and in some cases methane, may make some natural groundwater unusable. For example, groundwater that has passed through salt-containing strata may become salty and unsuitable for irrigation or drinking. Groundwater that has passed through limestone or dolomite contains dissolved calcium (Ca2 ) and magnesium (Mg2 ) ions; this water, called hard water, can be a problem because carbonate minerals precipitate from it to form “scale” that clogs pipes. Also, washing with hard water can be difficult because soap won’t develop a lather. Groundwater that has passed through iron-bearing rocks may contain dissolved iron oxide that precipitates to form rusty stains. Some groundwater contains dissolved hydrogen sulphide, which comes out of solution when the groundwater rises to the surface; hydrogen sulphide is a poisonous gas that has a rotten-egg smell. In recent years, concern has grown about arsenic, a highly toxic chemical that enters groundwater when arsenic-bearing minerals dissolve in groundwater.
Human-Caused Groundwater Contamination
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| Contamination plumes in groundwater. |
As we’ve noted, some contaminants in groundwater occur naturally. But in recent decades, contaminants have increasingly been introduced into aquifers because of human activity (figure above a). These contaminants include agricultural waste (pesticides, fertilizers, and animal sewage), industrial waste (dangerous organic and inorganic chemicals), effluent from “sanitary” landfills and septic tanks (including bacteria and viruses), petroleum products and other chemicals that do not dissolve in water, radioactive waste (from weapons manufacture, power plants, and hospitals), and acids leached from sulfide minerals in coal and metal mines. The cloud of contaminated groundwater that moves away from the source of contamination is called a contaminant plume (figure above b).
The best way to avoid such groundwater contamination is to prevent contaminants from entering groundwater in the first place. This can be done by placing contaminants in sealed containers or on impermeable bedrock so that they are isolated from aquifers. If such a site is not available, the storage area should be lined with plastic or with a thick layer of clay, for the clay not only acts as an aquitard, but it can bond to contaminants. Fortunately, in some cases, natural processes can clean up groundwater contamination. Chemicals may be absorbed by clay, oxygen in the water may oxidize the chemicals, and bacteria in the water may metabolize the chemicals, thereby turning them into harmless substances.
Where contaminants do make it into an aquifer, environmental engineers drill test wells to determine which way and how fast the contaminant plume is flowing; once they know the flow path, they can close wells in the path to prevent consumption of contaminated water. Engineers may attempt to clean the groundwater by drilling a series of extraction wells to pump it out of the ground. If the contaminated water does not rise fast enough, engineers drill injection wells to force clean water or steam into the ground beneath the contaminant plume (figure above c). The injected fluids then push the contaminated water up into the extraction wells.
More recently, environmental engineers have begun exploring techniques of bioremediation: injecting oxygen and nutrients into a contaminated aquifer to foster growth of bacteria that can react with and break down molecules of contaminants. Needless to say, cleaning techniques are expensive and generally only partially effective.
Unwanted Effects of Rising Water Tables
We’ve seen the negative consequences of sinking water tables, but what happens when the water table rises? Is that necessarily good? Sometimes, but not always. If the water table rises above the level of a house’s basement, water seeps through the foundation and floods the basement floor. Catastrophic damage occurs when a rising water table weakens the base of a hillslope or a failure surface underground triggers landslides and slumps.
Courtesy: Stephen Marshak (used with permission)
Courtesy: Stephen Marshak (used with permission)
