Objectives
By the end of the course, you will be able to:
- Explain the importance of groundwater
- Describe an aquifer and its types
- Describe the relationship between aquifers and ground water
- List the common misconceptions about aquifers and groundwater
- Explain the relationship between groundwater depletion and water contamination
- List the major contaminants of groundwater and their effects
- Explain how to approach groundwater recharge
What is groundwater ?
Groundwater lies below the earth’s surface and fills pores between soil, gravel, and sand. It exists below the level of water table in the ground. The hydrological cycle illustrates how groundwater is accumulated.
Groundwater plays an important role as a decentralized source of drinking water for millions of rural and urban families across the world. Large tracts of rural India depend on groundwater to meet both, agricultural and domestic needs. In fact, it is the only source of water in many parts of India. According to some estimates, it accounts for nearly 80 per cent of the rural domestic water needs, and 50 per cent of the urban water needs in India.
Groundwater may be one of the safest and most reliable source of available freshwater. Only 3 per cent of the earth's freshwater is located in streams, lakes, and reservoirs. The remaining 97 per cent of freshwater is underground.
Unfortunately, many of us still believe that groundwater is only a supplement to the more easily perceived surface water resource, and therefore only of secondary importance. Inadequate knowledge of groundwater resources has led to serious quantitative and qualitative consequences across user-bases in agriculture, industry, and households.
Groundwater and aquifers
Groundwater occurs in aquifers. Aquifers are geologic formations, that is layers of sand, gravel and rock, that can store, transport or supply significant amounts of water to a well or a spring. They are irregular in shape, and can be close to the surface, or very deep.
An aquifer stores groundwater and also allows it to flow in certain directions, depending upon the openings in the rocks or rock material. The flow is also governed by basic hydraulic principles. Aquifers are partially or fully saturated portions of rocks or rock matter. The saturation is usually a result of water infiltrating the ground after rainfall. The mechanism of natural infiltration allows slow passage of water through the ground, ensuring good quality and consistent chemical composition of the groundwater. Uncontaminated groundwater is usually suitable as drinking water in its in-situ state.
Ever wondered why two neighbouring wells can have different water quality?
At a specific location, there may be several aquifers layered one on top of another. Because of this, neighboring locations can have their wells in different aquifers and experience different water quality.
Aquifers
Based on how they are placed, aquifers can be classified as confined and unconfined aquifers.
Confined aquifers
Confined aquifers, have layers of impermeable material above and below them – so they are contained within these layers. The geologic barriers cause the water to be under pressure. Fractures, or cracks, in bedrock also are
capable of bearing water.
Unconfined aquifers
Unconfined aquifers, generally located near the land surface, have no layers of clay (or other impermeable geologic material) above their water table. However, they do lie above relatively impermeable clay beds. The upper limit of groundwater within an unconfined aquifer is the water table. In many places, the water table is actually above the surface of land. Wetlands are an example where groundwater becomes surface water.
Groundwater in an unconfined aquifer (sometimes called a “water table aquifer”) is more vulnerable to contamination from surface pollution than a confined aquifer because pollutants on the land surface can enter the unconfined aquifer as water infiltrates the soil.
Saturated and Unsaturated Zones
Groundwater flows vertically and horizontally through the aquifers at rates influenced by gravity and the geologic formations of the area. Groundwater can remain in an aquifer for a few days or for many centuries.
The earth's crust can be divided into two regions:
- Saturated zone (e.g. aquifers): Pressure head of the water is greater than atmospheric pressure (gauge pressure>0). The definition of the water table is surface where the pressure head is equal to atmospheric pressure (gauge pressure = 0).
- Unsaturated (or vadose) zone: Conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water that incompletely fills the pores of the aquifer material is under suction.
Misconceptions about groundwater
After learning about groundwater, aquifers, and their significance, it is important to be aware of some of the common misconceptions about them:
- Aquifers are large underground lakes.
- Aquifers once depleted stay depleted.
- Groundwater is an everlasting source of water.
- Groundwater is pure and hence a very safe source.
- Groundwater flows mainly in underground rivers.
- Groundwater is not connected to rivers and lakes.
- Contaminants from oil that is poured on the ground will be filtered by soil and gravel before reaching groundwater.
- If a well reaches groundwater, an unlimited amount of water can be pumped.
Groundwater and Aquifer Contamination
Groundwater and aquifer are prone to pollution. Although aquifers have some natural capacity to filter out wastes, there is a limit to their absorbing and neutralizing capacity. Intensification of agricultural activity, industrial growth and urbanization are some of the primary factors that have added to the potential risk of contamination of groundwater.
In some instances, over-exploitation of groundwater is causing aquifer contamination. Additionally, its unscientific development with insufficient knowledge of groundwater flow dynamics and geo-hydrochemical processes is leading to its mineralization.
Groundwater Contamination and Depletion
The increasing use of groundwater, not only for agriculture but also in the urban setup is a cause for great alarm. Already there are authenticated reports regarding the crises of groundwater from various regions of the country. These crises are compounded by the diversity in physiography, agro-climatic factors, and most significantly in the variable geological framework that hosts groundwater. The problems are manifold, and range from groundwater depletion to extreme deterioration of water quality.
Rampant over-exploitation, in the absence of any voluntary and/or legislative regulation and control, has caused a depletion of groundwater resources in many regions, especially within the domains of hard-rock, exposed over large tracts of the country. The obvious indication of over-exploitation has been a long-term decline of the water levels in wells from the affected areas. Groundwater quality deterioration has been reported from some parts due to a variety of causes, ranging from contamination by various wastes to the leaching of fertilizers and pesticides to the water table.
Principal sources of groundwater pollution
Before learning about the extent and impact of groundwater contamination, it is important to identify the primary sources of water pollution:
- Industrial discharge of chemical wastes and byproducts
- Discharge of poorly-treated or untreated sewage
- Surface runoff containing pesticides and herbicides
- Surface runoff containing spilled petroleum products
- Surface runoff from construction sites, farms, or paved and other impervious surfaces, for example, silt
- Discharge of contaminated and/or heated water used for industrial processes
- Acid rain caused by industrial discharge of sulphur dioxide (by burning high-sulphur fossil fuels)
- Excess nutrients added by runoff containing detergents or fertilizers
- Underground storage tank leakage, leading to soil contamination, and hence, aquifer contamination
Organic and inorganic groundwater pollutants
Water pollutants can be classified as organic and inorganic.
Some of the main organic groundwater pollutants are:
- Pesticides and herbicides, a huge range of organohalide and other chemicals
- Bacteria, often is from sewage or livestock operations
- Food processing waste, including pathogens
- Tree and brush debris from logging operations
Some of the main inorganic groundwater pollutants are:
- Heavy metals including acid mine drainage
- Acidity caused by industrial discharges (especially sulphur dioxide from power plants)
- Chemical waste as industrial byproducts
- Fertilizers, in runoff from agriculture including nitrates and phosphates
Extent and impact of groundwater contamination and pollution
According to various estimates, the incidence of the following main constituents has been found to be above permissible levels.
- Fluoride: The incidence of fluoride above permissible levels of 1.5ppm occur in 14 Indian states, namely, Andhra Pradesh, Bihar, Gujarat, Haryana, Karnataka, Kerala, Madhya Pradesh, Maharashtra, Orissa, Punjab, Rajasthan, Tamil Nadu, Uttar Pradesh and West Bengal, affecting a total of 69 districts. Some other estimates find that 65 per cent of India’s villages are exposed to fluoride risk.
- Salinity: High levels of salinity are reported from all these states, except West Bengal and the NCT of Delhi, and affects 73 districts and three blocks of Delhi.
- Iron content: Iron content above permissible level of 0.3 ppm is found in 23 districts from 4 states, namely, Bihar, Rajasthan, Tripura and West Bengal and coastal Orissa and parts of Agartala valley in Tripura.
- Arsenic: High levels of arsenic above the permissible levels of 50 parts per billion (ppb) are found in the alluvial plains of Ganges covering six districts of West Bengal.
- Heavy metals: Presence of heavy metals in groundwater is found in 40 districts from 13 states. Presence of heavy metals in groundwater is found in 40 districts from 13 states, namely, Andhra Pradesh, Assam, Bihar, Haryana, Himachal Pradesh, Karnataka, Madhya Pradesh, Orissa, Punjab, Rajasthan, Tamil Nadu, Uttar Pradesh, West Bengal, and five blocks of Delhi.
- Fertilizers and pesticides: Non-point pollution caused by fertilizers and pesticides used in agriculture, often dispersed over large areas, is a great threat to fresh groundwater ecosystems. Intensive use of chemical fertilizers in farms and indiscriminate disposal of human and animal waste on land result in leaching of the residual nitrate causing high nitrate concentrations in groundwater.
- Nitrate: Nitrate concentration is above the permissible level of 45 ppm in 11
Hydrogeology
Hydrogeology is the science of groundwater that helps develop a correct understanding of the aquifers. This understanding requires the interaction of various disciplines – geology, physics, chemistry, engineering, mathematics, and even biology.
However, the basis of developing an understanding of groundwater occurrence and movement is to be able to define the physical framework for groundwater occurrence. This understanding is achieved through the sciences of geology and hydrogeology, which describe the geometry of the rocks and rock material in which groundwater occurs and moves and the processes involved in the occurrence and movement of groundwater.
Hydrogeology describes the natural quality of groundwater within various media as well as the effects of human lifestyle and practices on groundwater. Hydrogeology also provides information on how groundwater supplies can be used efficiently.
Groundwater development
Groundwater development has progressed in a manner catering only to the growing demand of water for agriculture, industry and households. However, development opportunities, in present and future times, will be limited primarily by the scarcity of the groundwater resource itself and growing concerns about uncertainties in the equitable and sustainability of the resource, the former leading to conflicts at various levels.
The emphasis in planning will increasingly have to shift to the management of groundwater including its equitable distribution and sustainable use. Hence, even though the planning objectives are similar to earlier scenarios (food security, economic development, removal of socio-economic disparities, etc.), there ought to be greater emphasis on factors of environmental protection and sustainability of the groundwater resource. It is a complex task, more so because the level of understanding of this hidden resource is often quite rudimentary.
Groundwater management
Sustainable development of groundwater resources cannot be complete without efficient systems of groundwater management. Over-exploitation of groundwater resources is likely even after efficient measures like watershed development is put in place. A majority of watershed development programs assume the benefits to the groundwater regime within the respective watersheds after artificial recharge measures and water conservation mechanisms are put in place. Often, watershed development measures tend to create a tacit impression that recharge has increased groundwater resources and groundwater abstraction can therefore by sustained manifold. Many such programs suffer in terms of “optimization” of groundwater resources, although they are executed with a noble objective.
In a country like India, issues linked to groundwater are being brought to the forefront, with the media playing a major role in highlighting them. Using this as a platform, there is need to create sensitivity and awareness through the frontiers of knowledge available on groundwater in order to manage it over time. Moreover, groundwater conservation only can ensure its anthropogenic and environmental utilities. We must therefore change our perception, of perhaps the most misunderstood of all natural resources by treating it as a “resource” rather than as a mere source of water supply.
Summary
- Groundwater lies below the earth’s surface and fills pores between soil, gravel, and sand. It exists below the level of water table in the ground.
- Groundwater plays an important role as a decentralized source of drinking water for millions of rural and urban families across the world.
- Aquifers are geologic formations, that is layers of sand, gravel and rock, that can store, transport or supply significant amounts of water to a well or a spring.
- Aquifers are of two types, confined and unconfined.
- Aquifers and groundwater are prone to pollution and contamination.
- Hydrogeology is the science of groundwater that helps develop a correct understanding of the aquifers.
- The emphasis in planning will increasingly have to shift to the management of groundwater including its equitable distribution and sustainable use.