The circulation of water:
There is a continual movement of the earth’s water. The main reservoir is the ocean. From its surface water vapor is formed by the heat from the sun and carried up into the atmosphere by the air movement we called winds and breezes. The water vapor in the air is condensed to drops as the air rises and becomes visible as clouds. The contained moisture may then be precipitated as rain or snow or hail or mixture of these. The rain or snow which falls on the land may be partly re-evaporated, a part may flow into the streams and rivers and be returned to the ocean, and a part may sink into the ground where it supplies the moisture to the soil and also infiltrates downward into the rocks to form groundwater.
Water occupying openings, cavities, and spaces in rocks is commonly known as groundwater. There are two main sources of such water: Juvenile water, which rises from a deep, magmatic source, and meteoric water, which is due to rainfall having soaked into the underlying rocks. Water may be held in space between the grains of a rock (porosity) or in joints, cleavage, bedding planes etc.
Groundwater is an important source of water supply throughout the world. Its use in irrigation, industries, municipalities, and rural homes continue to increase. Cooling and air-conditioning have made heavy demands on groundwater because of characteristic uniformity in temperatures.
Groundwater pollution may be defined as the artificially or geologically induced degradation of natural groundwater quality. A large number of sources and causes can modify groundwater quality, ranging from septic tanks to irrigated agriculture. In contrast with surface water pollution, subsurface pollution is difficult to detect, is even more difficult to control, and may persist for decades. With the growing recognition of the importance of under groundwater, efforts are increasing to prevent, reduce, and eliminate groundwater pollution.
General mechanisms of groundwater contamination-
Contaminant releases to groundwater can occur by design, by accident, or by neglect. Most groundwater contamination incidents involve substances released at or only slightly below the land surface. Consequently, it is shallow groundwater which is affected initially by contaminant releases. In general shallow groundwater resources are considered more susceptible to surface sources of contamination than deeper groundwater sources. There are at least four ways by which groundwater contamination occurs: infiltration, direct migration, interaquifer exchange, and recharge from surface water.
Infiltration: contamination by infiltration is probably the most common groundwater contamination mechanism. A portion of the water which has fallen to the earth slowly infiltrates the soil through pore spaces in the soil matrix. As the water moves downward under the influence of gravity, it dissolves materials with which it comes into contact. Water percolating downward through a contaminated zone can dissolve contaminants, forming the leachate. Depending on the composition of the contaminated zone, the leachate formed can contain a number of inorganic and organic constituents. The leachate will continue to migrate downward under gravity’s influence until the saturated zone is contacted, horizontal and vertical spreading of the contaminants in the leachate will occur in the direction of groundwater flow.
Direct Migration: contaminants can migrate directly into groundwater from below ground sources (e.g. storage tanks, pipelines) which lie within the saturated zone. Storage sites and landfills excavated to a depth near the water table also may permit direct contact of contaminants with groundwater.
Interaquifer Exchange: contaminated groundwater can mix with uncontaminated groundwater through a process known as interaquifer exchange in which one water –bearing unit “communicates” hydraulically with another. This is most common in bedrock aquifers where a well penetrates more than one water-bearing formation to provide increased yield. Each water-bearing unit will have its own head potential, some greater than others. When the well is not being pumped, water will move from the formation with the greatest potential to formations of lesser potential. If the formation with the greater potential contains contaminated or poorer quality water, the quality of water in another formation can be degraded. Similar to the process of direct migration, old and improperly abandoned wells with deteriorated casings or seals are a potential contributor to interaquifer exchange.
Different sources of groundwater contamination-
1. Liquid wastes: Groundwater pollution can occur where industrial wastewaters are discharged into pits, ponds, or lagoons, thereby enabling the wastes to migrate down to the water table.
2. Tank and pipeline leakage: underground storage and transmission of a wide variety of fuels and chemicals are common practices for industrial and commercial installations. These tanks and pipelines are subject to structural failures so that subsequent leakage becomes a source of groundwater pollution. Petroleum and petroleum products are responsible for much of the pollution.
3. Mining activities: Mines can produce a variety of groundwater pollution problems. Pollution depends on the material being extracted and the milling process: coal, phosphate, and uranium mines are major contributors; metallic ores for production of iron, copper, zinc, and lead are also important. Heavy metal pollution is caused when such metals as arsenic, cobalt, copper, cadmium, lead, silver and zinc contained in excavated rock or exposed in an underground mine come in contact with water. Metals are leached out and carried downwards as water washes over the rock surface. Although metals can become mobile in neutral pH conditions, leaching is particularly accelerated in the low pH conditions such as are created by Acid Mine Drainage.
Irrigation return flows: Approximately one-half to two thirds of the water applied for irrigation of crops is consumed by evapotranspiration; the remainder, termed irrigation return flow, drains to surface channels or joins the underlying groundwater. Irrigation increases the salinity of irrigation return flow from three to ten times that of applied water. The degradation results from the addition of salts by dissolution during the irrigation process, from salts added as fertilizers or soil amendments.
1 Residential: Residential wastewater systems can be a source of many categories of contaminants, including bacteria, viruses, nitrates from human waste, and organic compounds. Injection wells used for domestic wastewater disposal (septic systems, cesspools, drainage wells for storm water runoff, groundwater recharge wells) are of particular concern to groundwater quality if located close to drinking water wells. Improperly storing or disposing of household chemicals such as paints, synthetic detergents, solvents, oils, medicines, disinfectants, pool chemicals, pesticides, batteries, gasoline and diesel fuel can lead to groundwater contamination. When stored in garages or basements with floor drains, spills and flooding may introduce such contaminants into the groundwater. When thrown in the household trash, the products will eventually be carried into the groundwater because community landfills are not equipped to handle hazardous materials. Similarly, wastes dumped or buried in the ground can contaminate the soil and leach into the groundwater.
2. Natural: groundwater contains some impurities, even if it is unaffected by human activities. The types and concentrations of natural impurities depend on the nature of the geological material through which the groundwater moves and the quality of the recharge water. Groundwater moving through sedimentary rocks and soils may pick up a wide range of compounds such as magnesium, calcium, and chlorides. Some aquifers have high natural concentration of dissolved constituents such as arsenic, boron, and selenium. The effect of these natural sources of contamination on groundwater quality depends on the type of contaminant and its concentrations.
Saline water intrusion.