Water Crisis ( Wars Ahead )

Water Crisis ( Wars Ahead ) 

Water crisis

Water crisis is a situation where the available potable, unpolluted water within a region is less than that region's demand

Agricultural crisis

Although food security has been significantly increased in the past thirty years, water withdrawals for irrigation represent 66 % of the total withdrawals and up to 90 % in arid regions, the other 34 % being used by domestic households (10 %), industry (20 %), or evaporated from reservoirs (4 %).

As the per capita use increases due to changes in lifestyle and as population increases as well, the proportion of water for human use is increasing. This, coupled with spatial and temporal variations in water availability, means that the water to produce food for human consumption, industrial processes and all the other uses is becoming scarce.

Environmental crisis

It is all the more critical that increased water use by humans does not only reduce the amount of water available for industrial and agricultural development but has a profound effect on aquatic ecosystems and their dependent species. Environmental balances are disturbed and cannot play their regulating role anymore.

An increase in tensions

As the resource is becoming scarce, tensions among different users may intensify, both at the national and international level. Over 260 river basins are shared by two or more countries. In the absence of strong institutions and agreements, changes within a basin can lead to transboundary tensions. When major projects proceed without regional collaboration, they can become a point of conflicts, heightening regional instability. The Parana La Plata, the Aral Sea, the Jordan and the Danube may serve as examples. Due to the pressure on the Aral Sea, half of its superficy has disappeared, representing 2/3 of its volume. 36 000 km2 of marin grounds are now recovered by salt.

Water stress results from an imbalance between water use and water resources. The water stress indicator in this map measures the proportion of water withdrawal with respect to total renewable resources. It is a criticality ratio, which implies that water stress depends on the variability of resources. Water stress causes deterioration of fresh water resources in terms of quantity (aquifer over-exploitation, dry rivers, etc.) and quality (eutrophication, organic matter pollution, saline intrusion, etc.) The value of this criticality ratio that indicates high water stress is based on expert judgment and experience (Alcamo and others, 1999). It ranges between 20 % for basins with highly variable runoff and 60 % for temperate zone basins. In this map, we take an overall value of 40 % to indicate high water stress. We see that the situation is heterogeneous over the world.

The concept of Water Stress

Already there is more waste water generated and dispersed today than at any other time in the history of our planet: more than one out of six people lack access to safe drinking water, namely 1.1 billion people, and more than two out of six lack adequate sanitation, namely 2.6 billion people (Estimation for 2002, by the WHO/UNICEF JMP, 2004). 3900 children die every day from water borne diseases (WHO 2004). One must know that these figures represent only people with very poor conditions. In reality, these figures should be much higher.

While the world's population tripled in the 20th century, the use of renewable water resources has grown six-fold. Within the next fifty years, the world population will increase by another 40 to 50 %. This population growth - coupled with industrialization and urbanization - will result in an increasing demand for water and will have serious consequences on the environment

And in my point of view there will be a war for that matter.

Groundwater and global change (overview)

Groundwater and global change (Overview) 

Groundwater containing by far the largest volume of unfrozen fresh water on Earth is a hugely important natural resource. However, what the general public and most decision-makers know and understand about groundwater is usually very little. Today, knowledge of groundwater around the world, its functions and its use is increasing rapidly and views about the many ways in which groundwater systems are linked with other systems are changing accordingly.  

All around the world, groundwater is a resource in transition: its exploitation started booming only during the twentieth century (‘the silent revolution’). This boom has resulted in much greater benefits from groundwater than were ever enjoyed before, but it also triggered unprecedented changes in the state of groundwater systems. On a global level, the key issues that need to be addressed to ensure the sustainability of groundwater resources are the depletion of stored groundwater (dropping water levels) and groundwater pollution. Climate change will affect groundwater, but because of its characteristic buffer capacity, groundwater is more resilient to the effects of climate change than

surface water. Therefore, in areas where climate change is expected to cause water resources to become scarcer than they are at present, the role of groundwater in water supplies is likely to become more dominant. Their buffer capacity is one of the major strengths of groundwater systems. It allows long dry periods to be bridged (creating conditions for survival in semi-arid and arid regions) and generally reduces the risk of temporary water shortages. It also smooths out variations in water quality and causes a portion of the stored water (medium-deep to deep groundwater) to be relatively insusceptible to sudden disasters, thus making this portion suitable as an emergency water source.

In terms of making a contribution to securing water availability and groundwater-related environmental values, managing groundwater resources sustainably is of vital importance to society and the environment. Nevertheless, there are situations where sustainable exploitation of groundwater is unlikely to be achieved. Such situations include, for example, cases of tapped non-renewable groundwater resources, and many of the intensely exploited renewable groundwater systems in arid and semi-arid zones. Such cases should be identified and the population of the areas concerned should be prepared in good time to adapt effectively to a future when these resources will be exhausted.

Groundwater governance is complex and needs to be tailored to local conditions. In the case of transboundary aquifer systems, the international dimension adds complexity. International cooperation and a wide range of international initiatives produce significant added value. This cooperation is instrumental in enhancing and disseminating information about groundwater, in developing and promoting approaches and tools for its proper management, and in raising global commitment for action on priority issues, such as the millennium development goals (MDGs) and

sustainable development. Ensuring that groundwater is adequately incorporated into such global actions is a challenge for all groundwater professionals.

Aquifer Types

Aquifer Types

In more detail, there are three main classifications of aquifers, defined by their geometry and relationship to topography and the subsurface geology  The simple aquifer shown is termed an unconfined aquifer, because the aquifer formation extends essentially to the land surface. As a result, the aquifer is in pressure communication with the atmosphere. Unconfined aquifers are also known as water table aquifers, because the water table marks the top of the groundwater system.

A second common type of aquifer is a confined aquifer, which is isolated from pressure communication with overlying or underlying geologic formations – and with the land surface and atmosphere – by one or more confining layers or confining units. Confined aquifers differ from unconfined aquifers in two fundamental and important ways. First, confined aquifers are typically under considerable pressure, which may be derived from recharge at higher elevation or from the weight of the overlying rock and soil (known as the overburden). In some cases, the pressure is high enough that wells drilled into the aquifer are free-flowing. This condition requires that the water pressure in the aquifer is sufficient to drive water up the wellbore and above the land surface, and such wells are called artesian wells. Second, confined aquifers typically remain saturated over their entire thickness, even as water is removed by pumping wells. Water extracted from the aquifer comes only from the depressurization of the aquifer – a combination of depressurization and expansion of the water itself, and relaxation of the aquifer formation upon reduction in pressure .

The third main type of aquifer is a perched aquifer. Perched aquifers occur above discontinuous aquitards, which allow groundwater to “mound” above them. Thee aquifers are perched, in that they sit above the regional water table, and within the regional vadose zone (i.e. there is an unsaturated zone below the perched aquifer). The dimensions of perched aquifers are typically small (dictated by climate conditions and the size of aquitard layers), and the volume of water they contain is sensitive to climate conditions and therefore highly variable in time.

Well Development Techniques Part (4)

Well Development Techniques Part (4)

Well Jetting

Development by high velocity jetting may be done with either water or air. A jetting tool is attached to the lower end of the drill string and lowered to the bottom of the well screen. Rotation is controlled by the rotary rig. The jetting tool activated by either air or water forces high-pressure fluid out the nozzles of the tool very effectively, developing the formation. Because of the high pressures used damages to the well screens may result through improper use of jetting tools. However, jetting is seen as possibly the most highly effective development technique in terms of well yield after completion. The essential point to be made is that yield depends to a great extent on the development method used. Particles loosened by jetting tools may be later removed by pumping or bailing.

Well Development Techniques Part (3)

Well Development Techniques Part (3)

Air Development (air surging and pumping)

Several techniques for the air development of wells exist. However, all inject air into the borehole such that aerated slugs of water are lifted irregularly out the top of the well casing. Air pressure may be cycled on and off to create a surging action desirable in well development. Sufficient air pressure will result in a continuous flow of aerated water out the top of the well, removing sediment and fine particles from the borehole.

For small wells, air may be injected down the drill stem into the formation. For larger diameter wells a separate airline and eductor pipe are inserted into the borehole. The size of the eductor pipe and airline depend on air pressures and volume available as well as the casing diameter. Numerous sources caution drillers that under some conditions the use of air development approach can create aquifer air locks, in such cases a development with water is a wiser choice. Even so air as a development is probably the most popular and widely used method of well development today.

The type of discharge produced from a well during air development depends on the air volume available, total lift, submergence, and annular area. In practice, two different flow conditions can be recognized when air is used when air is used for water well development although other flow regimes may exist at much lower or higher velocities in smaller diameter pipes. The picture above provides an illustration of how multiphase flow (water and air) occurs in the casing during air development. The percent submergence, total lift, and capacity of the compressor will control the relative proportion of air and water for a particular well.

A. Introduction of a small volume or air under high head causes little change in the water level in the well. In this case, the air pressure available is just sufficient to overcome the head exerted by the water column.

B. As air volume increases, the column becomes partly aerated. Displacement of the water by the air causes the water column to rise in the casing. Drawdown does not change because no pumping is occurring.

C. Further increases in air volume cause aerated slugs of water to be lifted irregularly out the top of the casing. Between surges, the water level in the casing falls to the near the static level.

D. If enough air is available, the aerated water will continually flow out the top of the well. With average submergence and total lift, the volume of air versus water is about 10 to 1. Higher air volumes may increase the pumping rate somewhat, but still higher rates may actually reduce the flow rate because flow into the well is impeded by the excessive air volume.

Well Development Techniques Part (2)

Well Development Techniques Part (2)

Washing and Backwashing

Drillers working in different regions have, through experience, come to rely on those well development techniques producing the best results in their areas. However, new techniques should always be considered and tried with the goal of obtaining the cleanest well with the best possible yield.

Overpumping is the simplest method of removing fine particles from formations. The theory is that if a sand free yield can be achieved by overpumping then a sand free flow will be the result when pumping at the normally expected lower rate. However, overpumping by itself is not considered the best well development approach. Overpumping is considered a limited approach to well development because water flows in a single direction only.

Backwashing reverses water flow and helps in the dilution, agitation and removal of sediment, fine particles and drilling fluids. Backwashing requires the introduction of water back into the well. If water taken from the well is to be reintroduced for backwashing, care must be taken to allow the settling out of particles from the removed water before reintroduction. Even so backwashing should not be the final step in the well development process; rather it may be an effective beginning or intermediate step. Washing and backwashing reverses the flow in the borehole during development. This reversal causes the collapsing of bridges in the particles of the near well area. This is desirable because collapsing these bridges further removes fines from the near well creating a cleaner flowing well.

Mechanical Surging

The forcing of water into or out of a well screen by use of a plunger type action is called surging. Surging tools can be used by both cable drillers and rotary drillers and can be used in combination with other development methods. Surging promotes a repeated change of direction in the flow of water in the well screen area. This repeated change of direction can produce good porosity in the near-well zone.

Mechanical surging is the first of two methods of well development that removes particles and clogging materials by the force of water impinging on them. A development method such as mechanical surging is a vigorous development method not suited to all aquifer types. However, mechanical surging has less potential for aquifer damage if a continuous flow of water into the well from the aquifer is maintained. Mechanical plungers may be fitted with one-way valves allowing them to lift water and fine sand out of the hole. Solid plungers do exist but have more potential to damage the aquifer. The results of mechanical surging should be measured by checking the well yield periodically, every hour after the process begins. Surge plunger should be a good fit in the casing. The plunger may be attached directly to the drill stem or operated by hand depending on well depth

Mechanical surging does have potential to damage the aquifer and should be done with aquifer. The force exerted during mechanical surging depends on the length of the stroke and the vertical velocity of the surge block. Swabbing is another variation of surging. Swabbing does not depend on reversing flow into the well. Rather the swab is slowly lowered to the desired depth and then drawn upward. Swabbing creates a pressure differential below and above the swab during the up stroke. This differential creates a powerful action which draws fines from the near well area into the bore hole for removal.

Well Development Techniques Part (1)

Well Development Techniques Part (1)

Well development is the process of cleaning out the clay and silt introduced during the drilling process as well as the finer part of the aquifer directly around the well screen prior to putting the well into service:-

1. Increase the rate of water flow from the aquifer into the well.
2. Stabilize the aquifer to prevent sand pumping to produce better quality water
3. Increase service life of the water pump
4. Remove organic and inorganic materials.

Types of well development techniques:

1. Chemical
2. Washing and Backwashing
3. Mechanical Surging
4. Air Development
5. Jetting

Chemical

Chemical agents are introduced into the development zone as solvents. Their action is intended to dissolve or loosen any clogging or blocking materials to make them easier to remove. The action of chemicals may also enlarge aquifer pores and improve permeability. Chemical based well development techniques can be gentle or violent in their action.

All chemical agents introduced into potable wells should be approved for such use by local authorities. Chemical methods are often used in conjunction with other well development techniques. This is particularly true when additional action is needed to break up mud cakes or flush out gelled muds. The chemical solution is allowed to stand in contact with the aquifer for the recommended soak period. After the soak period the solution is pumped or bailed from the hole. While well drilling fluids will break down naturally, the breakdown process may be enhanced by the use of chemical agents. Once degraded, the drilling fluids are much more easily pumped from the aquifer. Other chemicals may be used to break down clay smears and gelled bentonite. Chlorine breaks down polymers.

A tremie pipe can be used in conjunction with packing devices to isolate the areas of the borehole to be subjected to chemical treatment. Chemical treatment can be used to break down drilling fluids, clays and polymers. Acids are often used for improving the yield in limestone, dolomite and other calcium carbonate formations

Controlling and Managing Saltwater Intrusion

Controlling and Managing Saltwater Intrusion

One key to controlling saltwater intrusion is to maintain the proper balance between water being pumped from an aquifer and the amount of water recharging it.  Constant monitoring of the salt-water interface is necessary in determining the proper management technique.  In the past, many communities who came across a saltwater intrusion problem simply set up new production wells further inland.  This only complicated the issue.

Since then, various methods have been employed to help alleviate the concerns of saltwater intrusion.  Efforts towards the promotion of water conservation, and restricting withdrawals from coastal aquifers have been the focus in many areas.  Using alternative freshwater sources has also been encouraged.  Ocean water desalination plants are showing up in coastal regions around the world.

Where there are no other options for fresh water, efforts to maintain groundwater levels by ponding surface water and stormwater runoff, or using river water to recharge the groundwater table have been successfully implemented.  Aquifer Storage and Recovery (ASR) systems can help restore aquifers that have experienced long-term declines in water levels due to over-pumping.

Deep recharge well creates groundwater ridge.

Other methods to control saltwater intrusion, such as using deep recharge wells, have also been successful.   These wells create a high potentiometric surface, which allows for the pumping of groundwater below sea level landward of a groundwater ridge created. In some instances, barrier wells have been set up near the shore to pump out salt water and recharge a fresh water gradient toward the sea.

In all of these cases, hydrologic studies and water quality monitoring are essential to help better understand the movement and interaction of fresh water and salt water in the subsurface, and determine the best method to manage saltwater intrusion.  Potentiometric surface mapping of an aquifer can provide important information determining the direction of groundwater flow within a confined aquifer.  Plotting water level elevations on a map and contouring the results determines this.  The contoured surface is known as the potentiometric surface, which is actually a map of the hydraulic head in the aquifer.

Monitoring well networks allow continuous observation of the saltwater interface, after management strategies have been put in place.  This provides early warnings of saltwater intrusion and tracks the effectiveness of the strategy.  Overall, proper groundwater monitoring techniques and groundwater management, combined with groundwater conservation are needed to keep saltwater intrusion under control, and ensure fresh water supplies are sustained for future generations.

Methods and Instrumentation used for Investigation of Saltwater Intrusion

Methods and Instrumentation used for Investigation of Saltwater Intrusion

Late in the 1960's, efforts rose toward drilling for chemical analysis of groundwater samples and the determination of flow patterns based on piezometric levels.  Geophysical methods of investigation were introduced later, and were found to provide more information faster than the drilling techniques.  Subsequently, geophysical methods became more important for saltwater intrusion monitoring.

Today, there are numerous methods available including: well logging, chemical analysis of groundwater samples, research into the interaction between aquifer matrix and groundwater, and most common, chloride concentration profiling, and vertical conductivity and temperature profiling.   

Conductivity and Temperature used to Estimate Salinity

An aqueous solution's ability to carry an electrical current by means of ionic motion is measured through conductivity.  Salinity is the measured mass of dissolved salts (ions) in a solution. As such, conductivity readings provide a good indication of salinity.  In general, as salinity increases, the total dissolved solids (TDS) of a solution increases, and so too does conductivity.

As defined by the USGS, salt water has a total dissolved concentration of 35,000 mg/L, of which, 19,000 mg/L is chloride (Barlow, 2003).  Being the major constitute of salt water, chloride concentration profiling is a very common method for saltwater intrusion investigations.  As the concentration of chloride increases in salt water, so does conductivity.   As such, conductivity is a very good indicator of chloride content and salinity.

Conductivity is interdependent with temperature; therefore profiling both of these variables becomes an important factor when determining the behavior of the transition zone and the salt-water interface. 

Through using devices such as the Solinst Model 107 TLC Meter(Temperature, Level, Conductivity), salinity can be estimated through conductivity and temperature readings, both taken at a discrete depth.  The TLC Meter features a 'smart' probe that provides accurate temperature and conductivity measurements, and is attached to high quality flat tape for depth readings.  The probe and tape are mounted on a sturdy reel making operation easy.  Instruments such as this make vertical temperature and conductivity profiling simple.

For example, using standard methods, a conductivity reading of 25,000 µS/cm and a temperature reading of 20˚C yield a salinity estimation of 17ppt (APHA et al, 2005).  Through this method of investigation, borehole profiles of salinity can be used to track the fluctuation of the salt-water interface.  This, in turn increases the potential to control saltwater intrusion problems.

For continuous monitoring of the salt-water interface, an instrument such as the LTC Levelogger Edge allows accurate datalogging of conductivity along with temperature and water levels as often as every 5 seconds.  The LTC Levelogger Edge is ideal for long-term saltwater intrusion monitoring applications due to its compact, low maintenance, waterproof design.

The LTC Levelogger Edge combines a datalogger, 5-year battery, memory for 16,000 sets of readings, pressure transducer, and temperature and conductivity sensors in a small 22 mm x 190 mm housing.  It is simple to deploy, calibrate, and program.

To easily create a network of monitoring wells, the LTC Levelogger Edge can be integrated into a Solinst STS Gold Telemetry System, which allows convenient access to remote, real-time data.  The STS system also sets alarms to trigger when a specific conductivity level is reached, notifying personnel of potential saltwater intrusion conditions.  The LTC Levelogger Edge is also SDI-12 compatible and can be integrated into an SDI-12 or SCADA network.

By using an instrument like the LTC Levelogger Edge, the salt-water interface can be tracked over time, and provide real-time warnings when intrusion conditions occur or worsen.

Saltwater Intrusion Basics

Saltwater Intrusion Basics

Groundwater Monitoring, Management and Conservation Keep

Saltwater Intrusion Under Control

Almost two thirds of the world's population lives within 400 km of the ocean shoreline; just over half live within 200 km, an area only taking up 10% of the earth's surface (Hinrichsen, 2007).  Most of these coastal regions rely on groundwater as their main source of fresh water for domestic, industrial and agricultural purposes.  As the world's population continues to grow at an alarming rate, fresh water supplies are constantly being depleted, bringing with it issues such as saltwater intrusion and increasing the importance of groundwater monitoring, management, and conservation.

Freshwater-Saltwater Interactions

Saltwater intrusion is a major concern commonly found in coastal aquifers around the world.  Saltwater intrusion is the induced flow of seawater into freshwater aquifers primarily caused by groundwater development near the coast.  Where groundwater is being pumped from aquifers that are in hydraulic connection with the sea, induced gradients may cause the migration of salt water from the sea toward a well, making the freshwater well unusable.

Because fresh water is less dense than salt water it floats on top.  The boundary between salt water and fresh water is not distinct; the zone of dispersion, transition zone, or salt-water interface is brackish with salt water and fresh water mixing.

Under normal conditions fresh water flows from inland aquifers and recharge areas to coastal discharge areas to the sea.  In general, groundwater flows from areas with higher groundwater levels (hydraulic head) to areas with lower groundwater levels.  This natural movement of fresh water towards the sea prevents salt water from entering freshwater coastal aquifers (Barlow, 2003).

Groundwater pumping/development can decrease the amount of fresh water flowing towards the coastal discharge areas, allowing salt water to be drawn into the fresh water zones of coastal aquifers.  Therefore, the amount of fresh water stored in the aquifers is decreased (Barlow, 2003).

The Ghyben-Herzberg Relation assumes, under hydrostatic conditions, the weight of a unit column of freshwater extending from the water table to the salt-water interface is balanced by a unit column of salt water extending from sea level to that same point on the interface.  Also, for every unit of groundwater above sea level there are 40 units of fresh water below sea level.

X  Groundwater Level        Y  Sea Water Level

Salt-water interface in an unconfined coastal aquifer according to the Ghyben-Herzberg relation.

This analysis assumes hydrostatic conditions in a homogeneous, unconfined coastal aquifer.  According to this relation, if the water table in an unconfined coastal aquifer is lowered by 1 m, the salt-water interface will rise 40 m.

Generally, saltwater intrusion into coastal aquifers is caused by two mechanisms:

· Lateral encroachment from the ocean due to excessive water withdrawals from coastal aquifers, or

· Upward movement from deeper saline zones due to upconing near coastal discharge/pumping wells.

Saltwater intrusion into freshwater aquifers is also influenced by factors such as tidal fluctuations, long-term climate and sea level changes, fractures in coastal rock formations and seasonal changes in evaporation and recharge rates.  Recharge rates can also be lowered in areas with increased urbanization and thus impervious surfaces.  Intrusion has also occurred in areas because of water levels being lowered by the construction of drainage canals .

Most incidents of saltwater intrusion occur in coastal regions, as has been the focus of discussion thus far, but inland areas can also be affected.  Salinity issues in some regions surrounding the Rio Grande in New Mexico and Texas have been attributed to upwelling of deep-circulating groundwater, which is more saline due to natural underlying geologic formations (Doremus, 2008).  The more saline groundwater is brought to the surface through pumping for irrigation and other uses.  Similar occurrences have been noted in the Mississippi River Valley Alluvial Aquifer in Arkansas, where in response to pumping, there is also upward movement of saline water from deeper formations

Intrusion Occurrence

Incidents of saltwater intrusion have been detected as early as 1845 on Long Island, New York.  Intrusion occurs in coastal aquifers worldwide, and is a growing issue in areas including North Africa, the Middle East, the Mediterranean, China, Mexico, and most notably, the Atlantic and Gulf Coasts of the United States, and Southern California. The increased use of groundwater has caused the salt-water interface to move inland and closer to the ground surface along much of the U.S. Atlantic Coast, as well as Southern California.

How Sinkholes Develop?... With Chemical Equation

How Sinkholes Develop?... With Chemical Equation

•Naturally occurring sinkholes are most commonly found in a type of terrain known as karst topography, which consists of bedrock (rock beneath the soil) filled with nooks and crannies. The underlying bedrock in karst landscapes is usually made of limestone. A great portion of the state of Florida is, in essence, sitting atop one continuous slab of limestone, making it vulnerable to sinkholes. Limestone is composed of calcium carbonate (CaCO3), which primarily comes from the remnants of corals and other types of marine organisms, whose shells are made of calcium carbonate.

•Sinkholes often form when acidic groundwater or acid rain dissolves limestone, a porous 9-rock present in the soil, creating voids and cavities. The soil resting on top of the limestone then sinks or collapses, causing a sinkhole.

•Limestone builds up slowly after these animals die and their shells are deposited and accumulate over time. Other substances composed of calcium carbonate include marble, chalk, Tums antacid tablets, and eggshells. To understand how limestone bedrock contributes to sinkholes, consider what happens when you place an egg in a glass of vinegar, which contains 5% acetic acid (CH3COOH). You will notice that little bubbles of carbon dioxide gas form almost immediately and, within a day or two, the eggshell will have completely disappeared, leaving you with the egg’s translucent membrane to protect the egg. The eggshell, which is composed of calcium carbonate, does not normally dissolve in water, but in the presence of acetic acid, calcium carbonate and acetic acid react with each other, causing the eggshell to dissolve according to the following chemical reaction:

•2 CH3COOH (aq) + CaCO3 (s) Acetic acid + Calcium carbonate
•➞ [Ca2+ (aq) + 2CH3COO– (aq)] + H2O (l)
•➞ Calcium acetate + Water + CO2 (g) + Carbon dioxide

•Any substance made of calcium carbonate will react with an acid. Limestone, being made of calcium carbonate, will react with an acid and will be slowly worn away. But are there acids underground?

•To answer this question, consider what happens to rainfall (which eventually 9-becomes groundwater) as it passes through the atmosphere. While falling through the air, the rain comes into contact with carbon dioxide. Although carbon dioxide comprises only about 0.04% of the atmosphere, that is enough to make rainfall acidic, lowering its Ph to about 5.6. So, by the time rainfall reaches the ground, it has turned into acid. The reaction is as follows:

•H2O (l) + CO2 (g) ➞ H2CO3 (aq)
•Water + Carbon dioxide ➞ Carbonic acid
•Carbonic acid then dissociates to give a hydrogen ion (H+) and a bicarbonate ion (HCO3 –):

•H2CO3 (aq) ➞ H+ (aq) + HCO3 – (aq)

•The ability of carbonic acid to dissociate by producing hydrogen ions is what makes this molecule an acid. Over time, acidic rainwater seeps into the ground and comes into contact with limestone bedrock. Water makes its way into cracks or pockets in the rock, reacting with the limestone and eventually making holes and fissures in the rock. Sinkholes occur when acidic rainwater has eaten away so much of the underlying limestone bedrock beneath the soil that the ground collapses.

•The more it rains, the greater the amount of carbonic acid leaching into the soil below.

•Humid areas have the most rainfall. High humidity in the air leads to cloud formation, which eventually produces rainfall. So it is no surprise that Florida leads the United States in the number of sinkholes because it has both limestone bedrock and high humidity.


•The acidity of rainwater is not the only reason water in the ground is acidic. Decaying organic materials and root respiration also produce carbon dioxide, which dissolves in soil water to form carbonic acid.

Land subsidence

Land subsidence

Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials.

Causes of  Subsidence 

Subsidence is caused by a diverse set of human activities and natural processes, including :-
1.mining of coal , metallic ores,
2.Limestone , salt, and sulfur; withdrawal of groundwater, petroleum, and geothermal fluids;
3.dewatering of organic soils;
4.pumping of groundwater from limestone;
5.wetting of dry, low-density deposits, which is known as hydro compaction; natural sediment compaction;
6.melting of permafrost; liquefaction; and crustal deformation

Catastrophic Subsidence as Result For Water Level Decline (Sinkholes).

•Water is stored in underlying carbonate rocks and moves through interconnected openings along bedding planes, joints, fractures, and faults, some of which are enlarged by solutioning.
•Recharge from precipitation, in response to gravity, moves downward into this system of openings or toward the stream channel, where it discharges and becomes streamflow

•Sinkholes can be separated into categories described as“induced” and “natural.” Induced sinkholes are those caused or accelerated by human activities, whereas natural ones occur in nature. Sinkholes resulting from water level declines

Induced sinkholes

•Induced sinkholes (catastrophic subsidence) are those caused, or accelerated, by human water development/management activities
•activities. These sinkholes commonly result from a water level decline due to pumpage.
•Are most predictable in a youthful karst area impacted by groundwater withdrawals.

Triggering mechanisms resulting from water level declines

(1)loss of buoyant support of the water
(2)increased gradient and water velocity
(3)water-level fluctuations