Groundwater occurs under phreatic condition in the coral sandy aquifer. The fresh groundwater is appeared as a lens over the brackish water underlain by saline water. The schematic diagram of groundwater system is given below.
The response of the groundwater system to changes in base level elevation is expected to be expressed in the field by the following: groundwater level changes vertical and lateral shift of the fresh-saline water interface in the case of a saline base level and changes in submarine or sublacustrine groundwater discharge. Water level measurements and monitoring are carried out employing manual meters, electric devices or pressure transducers at different resolutions and frequencies depending on the specific objectives. Twenty two observation wells were established and monitoring of depth to water level was carried out once in 10 days for a period of one year, to know the response of water level fluctuation to rainfall, pattern of seasonal fluctuation and height of annual fluctuation. The depth to water level varies from 1.0 m to 4.0 m depending on topography or ground elevation.
In general, groundwater is a very good source of drinking water because of its purity. It is also used for spraying and irrigation where surface water is scarce, for industrial purposes. The main source of this water is arid and semi arid zones. An aquifer constitutes a natural reservoir of usually high quality water. But although it is more protected than surface waters, groundwater appears to be subject to pollution. It can be defined as,
Pollution is a modification or change of the physical, biological and chemical properties of water, preventing or restricting its use in the various applications.
The main origin of groundwater pollution are: industrial, agricultural, domestic and environmental pollution.
The quality of groundwater continues to be threatened by natural and human activities, although contamination of underground aquifers can be a consequence of people's misuse of the environments. A groundwater contaminant is defined by most regulatory agencies as any physical, chemical, biological or radiological substance or matter in groundwater. The contaminants can be introduced in the groundwater by naturally occurring activities, such as natural leaching of the soil and mixing with other groundwater sources having different chemistry. They are also introduced by planned human activities, such as waste diposal, mining and agricultural operations. The contamination from naturally occurring activities are usually less. However, human activities are the leading cause of groundwater contamination. The most prevalent human activities that cause groundwater contamination are waste diposal, storage and transportation of commercial materials, mining operations etc.
Groundwater is a critical source of fresh water through out the world. Other than water stored in icecaps and glaciers, groundwater accounts for approximately 97% of fresh water on Earth. Quantification of natural rates of groundwater recharge (the rates at which aquifer waters are replenished) is imperative for efficient groundwater management. The rate, timing and location of recharge are important issues in areas of groundwater contamination as well as groundwater supply. In general, the likelihood for contaminant movements to the water table increases as the rate of recharge increase. Areas of high recharge are often equated with areas of high aquifer vulnerability to contamination.