Meteorological phenomena influences groundwater levels.

Rainfall is not an accurate indicator of groundwater recharge.

by

Dr. Nitish Priyadarshi

Water is essential to people and the largest available source of fresh water lies underground. Ground water is the part of precipitation that seeps down through the soil until it reaches rock material that is saturated with water. Water in the ground is stored in the spaces between rock particles. Increased demands for water have affected the level of under groundwater. The demand for water has increased over the years and this has led to water scarcity in many parts of the world. The situation is aggravated by the problem of water pollution or contamination. World is heading towards a freshwater crisis mainly due to improper management of water resources and environmental degradation, which has lead to a lack of access to safe water supply to millions of people. This freshwater crisis is already evident in many parts of world, varying in scale and intensity depending mainly on the time of the year.


Water-level changes can be divided into several categories. There are short-term changes that can only be seen when water-level measurements are made many times a day. There are long term changes that can only be seen after data are collected for many years. There are minor changes of only a few hundredths of a foot, and changes that are hundreds of feet.

Any phenomenon that produces a change in pressure on groundwater will cause the groundwater level to vary. Differences between supply and withdrawal of groundwater cause levels to fluctuate. Other diverse influences on groundwater levels include meteorological and tidal phenomena, urbanization, earthquakes, and external loads. And, finally, subsidence of the land surface can occur due to changes in underlying groundwater conditions.

Fluctuations due to meteorological phenomena:

Atmospheric Pressure:

Changes in atmospheric pressure can also cause groundwater levels to fluctuate. Atmospheric pressure is caused by the Earth’s gravitational attraction of air in the atmosphere. At sea level, the weight of the atmosphere exerts a pressure of about 14.7 pounds per square inch on the Earth’s surface.


Changes in atmospheric pressure produce sizable fluctuations in wells penetrating confined aquifers. The relationship is inverse; that is, increases in atmospheric produce decreases in water levels, and conversely.

For an unconfined aquifer, atmospheric pressure changes are transmitted directly to the water table, both in the aquifer and in a well; hence, no pressure difference occurs. Air entrapped in pores below the water table is affected by pressure changes, however, causing fluctuations similar to but smaller than that observed in confined aquifers. Temperature fluctuations in the capillary zone will also induce water table fluctuations where entrapped air is present.

Rainfall:

Rainfall is not an accurate indicator of groundwater recharge because of surface and subsurface losses as well as travel time for vertical percolation. The travel time may vary from a few minutes for shallow water tables in permeable formations to several months or years for deep water tables underlying sediments with low vertical permeabilities.

Precipitated water that reaches at the surface ground maybe partially discharge into streams as surface runoff or partially infiltrate into the ground. The latter further percolates into groundwater aquifers, eventually emerging in springs, seeping into streams to form surface runoff, or storing in subsurface. The soil stores infiltrated water to become soil moisture, and then it recharges to groundwater level if the soil is saturated. Nevertheless, it releases slowly as subsurface flow to enter the stream as baseflow during rainless period. This may also result from deeper percolation, evapotranspiration, or artificial discharge.

If no water supplies are continually provided from either rainfall or other sources of recharge, groundwater level would gradually decrease due to deeper percolation or evapotranspiration.

Furthermore, in arid and semiarid regions, recharge from rainfall may be essentially zero. Shallow water tables show definite response to rainfall where the unsaturated zone above a water table has a moisture content less than that of specific retention, the water table will not respond to recharge from rainfall until this deficiency has been satisfied.

Wind:
Minor fluctuations of water levels are caused by wind blowing over the tops of wells. The effect is identical to the action of a vacuum pump. As a gust of wind blows across the top of a casing, the air pressure within the well is suddenly lowered and, as a consequence, the water level quickly rises. After the gust passes the air pressure in the well rises and the water level falls.

Frost:

In the regions of heavy frost it has been observed that shallow water tables decline gradually during the winter and rise sharply in early spring before recharge from ground surface could occur. This fluctuation can be attributed to the presence of a frost layer above the water table. During winter water moves upward from the water table by capillary movement.

What is groundwater dating?

How we calculate age of Groundwater?

by

Dr. Nitish Priyadarshi

photo credit: http://water.usgs.gov/ogw/karst/img/features/arbuckle/vendomeWell.jpg

Groundwater age, based on measurement of the concentrations of chemical and isotopic substances in water, refers to the time elapsed since the introduction of the chemical or isotopic substance into the water, or to be more precise, the time elapsed since the chemical or isotopic substance was recharged and isolated from the modern reservoir. For example, some of the rain that falls on an area percolates (trickles) down through soil and rock until it reaches the water table. Once this water reaches the water table, it moves though the aquifer. The time it takes to travel to a given location, known as the groundwater age, can vary from days to thousands of years.
Although we often refer to dating of groundwater, we are actually dating a chemical substance that is dissolved in the groundwater, not the water itself. Rather than referring to groundwater dating, it would probably be more correct to say we are interpreting temporal aspects of chemical and isotopic substances in groundwater. Isotopes can be separated into environmental isotopes, which are found in the groundwater, and isotopes that are introduced into the ground as a part of a groundwater study.

Environmental isotopes can be either radioactive or stable. They can be used to determine the locations of groundwater recharge areas, circulation patterns in aquifers, sources of dissolved solids in groundwater, and the age of groundwater- the length of time it has been out of contact with the atmosphere.

Various environmental isotopes and tracers are used to determine the age of groundwater. Carbon-14 is used to date groundwater older than 1000 years. Chlorofluorocarbons (Freon) and tritium techniques are used to date groundwater that is less than 50 years old.

Theoretically, ages can be estimated by (1) the travel time of groundwater from the point of recharge to the subsurface point of interest as calculated by Darcy's law combined with an equation of continuity, (2) the decay of radionuclides which have entered the water from contact with the atmosphere, (3) the accumulation of products of radioactive reactions in the subsurface, (4) the degree of disequilibrium between radionuclides and their radioactive daughter products, (5) the time-dependent changes in the molecular structure of compounds dissolved in water, (6) the presence of man-made materials in groundwater, (7) the correlation of paleoclimatic indicators in the water with the known chronology of past climates, and (8) the presence or absence of ions which can be related to past geologic events that have been previously dated. Owing to uncertainties in each of the methods, as many methods as possible should be used in every field situation.

The residence time of water underground has always been a topic of considerable speculation. But with the advent of radioisotopes, determination of the age of groundwater has become possible.

Chlorofluorocarbons-

Hydrologists employ a variety of techniques to measure groundwater age. For relatively young groundwater, chlorofluorocarbons (CFCs) often are used. CFCs are human-made compounds that are stable in the environment. Atmospheric CFC concentrations increased from the time of their development in the 1930s until the 1990s, and hydrologists now know how atmospheric CFC concentrations have changed over time.
CFCs can be used to determine groundwater age because water that is in contact with the atmosphere picks up CFCs from the atmosphere. Thus, CFCs are incorporated in the water before it enters an aquifer. Once water enters an aquifer, it becomes isolated from the atmosphere, and it carries a CFC signature (a distinctive chemical composition) as it travels through the aquifer. This signature reflects the atmospheric concentration when the water was at the surface. By measuring the CFC concentration in groundwater, hydrologists know how long ago the water entered the aquifer.

In the United States and other developed countries, CFCs are being phased out of use because they contribute to atmospheric ozone depletion. As a consequence, atmospheric CFC concentrations have begun to decrease. Atmospheric concentrations of CFCs are not expected to decrease quickly, so CFC dating will continue to work for most young groundwater for many years to come. However, for very young groundwater (groundwater entering aquifers after the late 1990s), CFC dating soon will yield ambiguous results.

Sulfur hexafluoride (SF 6 )-

Hydrologists recently have developed another dating technique that may ultimately replace CFC dating. The new technique uses sulfur hexafluoride (SF 6 ) concentrations in groundwater to determine groundwater age. SF 6 is another stable, human-made compound that has exhibited increasing concentrations in the atmosphere. Unlike CFC concentrations, atmospheric SF 6 concentrations are expected to increase for the foreseeable future. The method, although relatively new, shows promise.

Tritium-

Tritium, 3H, is an unstable isotope of hydrogen with a half life of 12.4 years. It is produced in the upper atmosphere by cosmic radiation; carried to earth by rainfall and hence underground, this natural level of tritium begins to decay as a function of time, such that

A= A0 e-λt

Where A is the observed radioactivity, A0 is the activity at the time the water entered the aquifer, λ is the decay constant, and “t” is the age of the water.

Tritium in the atmosphere is typically in the form of the molecule H3 HO and enters the groundwater as recharging precipitation. Prior to 1953, rainwater had less than 10 tritium units (TU). Starting in 1953, the manufacturing and testing of nuclear weapons have increased the amount of tritium in the atmosphere, with a resulting increasing in tritium in the groundwater. As a result 3H can be used in a qualitative manner to date groundwater in the sense that groundwater with less than 2 to 4 TU is dated prior to 1953 and if the amount is significantly greater than 10 to 20 TU it has been in the contact with the atmosphere since 1953.

Tritium has been used to trace the seepage of contaminated groundwater from low-level nuclear waste disposal areas.

Radiocarbon dating of groundwater (Carbon-14)-

Radiocarbon dating methods can be applied to obtain the age of groundwater. Carbon exists in several naturally occurring isotopes, 12 C, 13 C and 14 C.

Like tritium, carbon 14 is produced in the atmosphere by interaction of cosmic rays with nitrogen, and was introduced in large amounts by nuclear weapons testing. Unlike tritium 14 C has a half life of 5730 years, making it a useful tool for dating water as old as 50,000 years. The 14 C generated in the atmosphere is carried down to the earth’s surface by precipitation, and becomes incorporated into the biomass or transported into water bodies such as lakes, the ocean and groundwater. 14 C undergoes radioactive decay (to 14 N), so that once isolated from the atmosphere, the amount of 14 C decreases with time according to the equation

(14 C)t = (14C)0e –Kt

Where (14 C)t is the amount present at time “t”, (14C)0 is the amount present at t = 0, and K is the decay constant, which is related to the half-life T1/2 by the equation

T1/2 = In2/k
To determine the time since a water was last in contact with atmosphere, it is necessary to know (14 C)0. This is determined by tree rings for the most recent 7000 years; there is no accurate way to determine it prior to 7000 years, so it is generally assumed arbitrarily to have been constant. This gives rise to a time scale in “14 C years”, which may be different from astronomical years.


There are some complications in the behavior of 14 C during recharge, so that the “absolute” age of a groundwater cannot be determined reliably. However, if the 14 C concentration is measured at several points along a flow line within an aquifer, the differences in age between the points and hence the flow velocity can be determined. One complication is that dissolution of carbonate minerals or oxidation of organic matter within an aquifer may add “old” or “dead” (no detectable 14 C) carbon to the water and give an erroneously old age. The contribution of carbon from these sources can sometimes be estimated from 13 C/ 12 C measurements and chemical arguments, so that corrections can be made. Another complication is mixing. A low 14 C concentration may mean that we are looking at relatively “old” water, or it may mean that we are looking at a mixture of relatively “young” water and “dead” water. 14 C measurements can be interpreted as ages only when mixing is insignificant.

Measurements of water samples taken from deep wells in deserts of the United Arab Republic and Saudi Arabia indicate ages of 20,000 to 30,000 years. This period is compatible with the Wisconsin Ice Age, when these desert areas last had high rainfall capable of recharging the underlying major aquifers.

Reference:
Drever, J.I.(1982). The Geochemistry of Natural Waters. Prentice-Hall, Englewood cliffs, NJ.

Thatcher, L. et.al. (1961). Dating desert groundwater. Science 134(3472): 105-106.

Todd, D.K. (1995). Groundwater Hydrology. Wiley, Toronto, pp. 24-25.

Wigley, T.M.L. (1975). Carbon-14 dating of groundwater from closed and open systems. Water Resour. Res. 11: 324-328.

Wigley, T.M.L. (1976). Effect of mineral precipitation on isotopic composition and 14C dating of groundwater. Nature 263: 219-221.
http://www.freedrinkingwater.com/water-education2/6-age-groundwater.htmhttp://www.waterencyclopedia.com/Ge-Hy/Groundwater-Age-of.html

Effect of Urbanization on Ground water in Ranchi city, India.

Ranchi people needs 33858160 liters of water per day.

by

Dr. Nitish Priyadarshi

Ranchi the capital city of Jharkhand state of India is located at 23.350 N and 85.330 E. The total area covered by Ranchi-Municipal area is about 141 square kilometers and the average elevation of the city is 645 m above Mean Sea Level (MSL). As of 2001 India census Ranchi had a population of 846,454.

Water supply, in adequate quantity and at desirable quality, is essential for any sustainable urbanization. Water supply in Ranchi dates back to more than 50 years ago.

There are three main dams ( Hatia, Rukka, and Kanke dam) from where the water is supplied to the city. Surface water is always vulnerable to pollution. People of Ranchi are dependent more on purer source like groundwater. Of the total consumption more than 60% comes from groundwater storage. Due to increasing population more pressure has developed on groundwater from the aquifer beneath the city.

The process of urbanization and industrialization from last 20 years has caused changes in the water table as a result of decreased recharge and increased withdrawal. Many of the small ponds which were main source of water in the surrounding areas are now filled for different construction purpose affecting the water table. Lots of DEEP- BORING in the Ranchi city has also forced the water table to move down as well as Ranchi plateau

Large scale abstractions always bring changes in the natural system of the aquifer and also in the environment. Over exploitation of groundwater beneath some large cities of the world has resulted in serious environmental hazards like groundwater quality deterioration.

The International Conference on Water and Environment, Dublin, 1992 enunciated two crucially important guideline principles, namely, that all human beings have a basic right to access to clean water and sanitation at an affordable price, and that water has an economic value in all its competing uses and should be recognized as economic good.

Groundwater in Ranchi city:

Groundwater in Ranchi city is mainly stored here in secondary porosity features or void spaces developed as result of weathering, fracturing, jointing, shearing or faulting phenomena. The gneisses and granitic rocks with associated schists and quartzites constitute the main consolidated rock terrain of Ranchi district.

Major portion of Ranchi city which is part of the Chotanagpur Plateau occupied by hard rock which are devoid of primary porosity and occurrence and movement of groundwater is controlled by the joints, fractures and fissures present in them.

During the long span of geological history these rocks have been deformed and tectonised in many ways including deep erosion.

In Ranchi city water table in the consolidated formations is now at its lowest from April to June. Water table is at its highest peak during August, gradually stabilizing in the month of November.

Sources of groundwater recharge in Ranchi city and the other parts of Jharkhand State is the vertical percolation of rain water. Although city experiences about 1000 to 1200 mm rainfall annually, the rate of vertical percolation is hindered by the presence of highly weathered and metamorphosed rocks. The Ranchi plateau gradually slopes down towards south east into the hilly and undulating region of Singhbhum. Due to this uneven topography the rain water are lost through surface runoff resulting in less water percolation below the surface. The thin soil layer of Ranchi plateau which is becoming more thin due to weathering is gradually loosing its water retaining capacity, Moreover, present land development practices in the recharge area and natural canals or rivulets in and around the city is also reducing the natural recharge significantly. More than 40% of the rain water is lost in the form of surface runoff. The rate of decline ranges between 1m/year to 5m/year at different observation locations within the city.
The daily physiological consumption of drinking water for human varies from 1- 4 L per capita per day, depending upon the climate ( high in the summer), the kind of work a person does (a manual worker working in open sun would need to drink more water, than a person working in an air-conditioned office), and social habits. If we calculate total consumption of water it increases up to 40 L per capita per day especially in the country like India . Seeing the population of Ranchi, i.e. 846454 we can easily calculate average consumption of water in Ranchi city for domestic purpose per day. It is 33858160 L per day and it is increasing many fold every year. If we add the water being used in construction of houses, malls, buildings the figure will be more. As Ranchi is becoming one of the important business center in Eastern India there is a rampant increase in construction and expansion of city. Due to inadequate water supply from the dams, dependency on ground water is increasing. Over pressed zones are Upper Bazar, Main Road, Ratu Road, Chutia, Hindhpirhi, Circular Road, Burdwan Compound, Lalpur and Harmu Road.

Long term large-scale abstraction of ground water have deleterious effects on water quality, resources and ecology over a wide area.

Water quality deterioration due to overexploitation can take place in a number of ways. Depression in ground water level may result in reversal in flow directions and restrict ground water circulation. Restricted ground water circulation favours mineralization and thus increases the total dissolved solids (TDS) in the ground water. The results of ground water analysis indicate that fluoride is distributed heterogeneously in ground water of the city. Fluoride in high concentration is found in ground water of southern, western and southwestern zones of the Ranchi city. The water is found to be slightly acidic in nature and high in iron concentration in most of the zones.

Potential sources of contamination:
From experiences of other major cities of the world and observation made in Ranchi city, the possible sources of contaminants can be categorized as follows.

Municipal wastes:
Ranchi city is growing faster and without any proper municipal waste dumping policy, municipal waste can be seen dumped here and there in the city. Most of the by lanes in the city are chocked with municipal solid wastes. This municipal waste poses a serious threat to ground water quality. Leachate from a landfill can pollute ground water if water moves through the fill material. Possible sources of water include precipitation, surface water infiltration, percolating water from adjacent land, and ground water in contact with the fill. The problem of pollution from landfills is greatest where high rainfall and shallow water tables occur. Municipal waste, also called urban solid waste, is a waste type that includes predominantly household waste (domestic waste) with sometimes the addition of commercial wastes collected by a municipality within a given area. They are in either solid or semisolid form and generally exclude industrial hazardous wastes.

Municipal waste of Ranchi city is composed mainly of:

1.Biodegradable waste: food and kitchen waste, green waste, paper (can also be recycled).
2.Recyclable material: paper, glass, bottles, cans, metals, certain plastics, etc.
3.Inert waste: construction and demolition waste, dirt, rocks, debris.
4.Composite wastes: waste clothing, waste plastics.
5.Domestic hazardous waste (also called "household hazardous waste") & toxic waste: medication, paints, chemicals, light bulbs, fluorescent tubes, spray cans, batteries.

Municipal wastes produce toxic and carcinogenic chlorinated hydrocarbon solvents (CHSs) which have been found to contaminate ground water in many urban areas of the world. The CHSs are the components of the leachate produced at the disposal sites. Alongside with CHSs, leachate also contains higher amount of other dissolved solids which can also be potential source of ground water pollution. The concentration of CHSs in potable water is very hazardous, even at very low concentrations. Important pollutants frequently found in leachate include BOD, COD, iron, manganese, chloride, nitrate, hardness, and trace elements. Hardness, alkalinity, and total dissolved solids are often increased, while generation of gases, such as methane, carbon dioxide, ammonia, and hydrogen sulfide, are further by-products of landfills.

Polluted surface water:

Ranchi city is bounded by several small rivulets like Harmu river, Jumar river, Potpoto river, etc. These rivers are becoming sites for indiscriminate disposal of municipal, household and industrial wastes which may contaminate the city groundwater. This is particularly true for the Harmu river as the flow of the river is chocked with different household and municipal wastes. This river may pose major threat to the ground water quality.

Liquid waste:
Due to lack of proper drainage system most of the house hold liquid waste are sent in the disposal wells underground. Such disposal wells or soak pit tanks have been criticized from a health standpoint because of the potential for pollutants to be released directly into an aquifer. The problem is most critical where disposal wells are near pumping wells. Leakage from these wells can introduce high concentration of BOD, COD, nitrate, organic chemicals, and possibly bacteria into ground water.

Protection of ground water:

It is evident from the foregoing discussion that the aquifer beneath the city is getting overexploited and as consequence ground water resources are being depleted. Quality deterioration, an associated phenomena, of overexploitation, may be encountered. This quality deterioration will be relatively high in the overexploited and thickly populated areas. Once pollution has occurred, the water has to be treated at the point of abstraction. The cleanup of an aquifer is a very difficult task. It follows that every effort should be made to prevent the contamination of the ground water in the first instance. Rain water harvesting, harvesting of surface runoff and ground water recharge should be done in community level.