ISLAND HYDROGEOLOGY
The islands of India are broadly classified into
Oceanic Island
These are originated within ocean by volcanic activity or tectonic or due to the activity of microorganism. E.g. Lakshadwep group of Islands.
Deltaic Island
These are formed at the confluence of river mouth. e.g Sunderban in Bay of Bengal.
Continental Island
These are part of Main Island but later separated from main land. e.g. Diu group of islands
Origin of Island
Volcanic eruption from oceanic ridge or sea beds results oceanic islands and submergence of these with gradual growth of corals around them results atolls and ultimately coral islands. They are generally formed in tropical climate. An atoll is characterized by ring like structure consisting of barrier reef system totally or partially by central lagoon.
The Coral Reef
The coral reefs are the largest animal mode structure in the world. The coral reef can be divided as:
Fringing reef : Grow along coastal region and connected to
shore.
Barrier reef : Grow along coastal region but are farther from
shore.
Atoll : Atolls are large rings of corals that exist under
sea volcanic cones.
The coral reef is composed of calcareous and material derived from atoll and has very high % of CaCo3 (87%). The beach rock consists of moderately well cemented calc arenites. The beach sediments of lagoon consist of various types of coral materials of fine sand to cobble size.
The volcanic island comes in contact with shore by both marine and fresh water environments. It is proved that digenesis process occurs more rapidly in fresh water phreatic environment and it results in dissolution and/or precipitation of carbonate minerals forming well cemented litho units than those of under vadose or marine environment. The end product is low magnesian calcite and caliches. The accumulation of CaCo3 decreases porosity and permeability of the profile.
Water needs is met in various way, as there is tremendous scarcity of water. People have adjusted their life style for bathing and washing and they generally use common ponds for this. Rainwater is the main source for fresh water. In these regions rainfall intensity is 1.5 to 3.5 and 80 to 200 days are rainy.
Although overall geology of an atoll is interesting, its significance is limited to few meters from mean sea level. In oceanic island fresh ground water occurs as a lens floating over saline water. The hydro dynamic balance of fresh and saline water determines the shape and movement of interface and may be controlled by some of the following factors:
1. Water table fluctuation due to diurnal tides
2. Seasonal fluctuation of water table due to recharge or draft
3. Dispersion
4. Molecular diffusion
Due to the above controlling factors there is an alternate up and down movement of the interface. If the fresh ground water flux is high the transition zone will be thin.
In small island due to high permeability almost all the rain falling onto the ground seeps below and a part about 7.15%of the rain fall is intercepted and evaporated before reaching the ground. After rainfall water table rises temporarily and within 2-3 days readjusted and water level stabilise and there is as much as only 5% outflow into the sea.
For water balance the following formula is used. The recharge to ground water lens = rain fall – interception – evapotranspiration.
Ground water utilisation = Evapotranspiration + mixing + pumping + outflow
For water balance study monthly water budgeting or weekly water budgeting gives appropriate value of recharge. The main consumer of ground water is coconut plant because one coconut tree consumes 40 lpd and density of coconut trees is 25 000 – 35000 sq km. But draft through plant is slow, steady and spread uniformly.
Control of saline Water up Coning
Well should be just few cm below mean water table & spread out all over the fresh water lens. Pumping should proceed in slow, steady rate and should be spread in wide area. Pumping site should located preferably in less permeable aquifer.
Ground Water Pollution
Disposal of pollutant in Small Island is a problem since water table is shallow. Special care should be taken for ground water contamination. The spacing between latrine and well should be at least 15m. But due to high density of population remedial measure is not easy implement. In such area precautions should be taken before using ground water.
Oceanic Island
These are originated within ocean by volcanic activity or tectonic or due to the activity of microorganism. E.g. Lakshadwep group of Islands.
Deltaic Island
These are formed at the confluence of river mouth. e.g Sunderban in Bay of Bengal.
Continental Island
These are part of Main Island but later separated from main land. e.g. Diu group of islands
Origin of Island
Volcanic eruption from oceanic ridge or sea beds results oceanic islands and submergence of these with gradual growth of corals around them results atolls and ultimately coral islands. They are generally formed in tropical climate. An atoll is characterized by ring like structure consisting of barrier reef system totally or partially by central lagoon.
The Coral Reef
The coral reefs are the largest animal mode structure in the world. The coral reef can be divided as:
Fringing reef : Grow along coastal region and connected to
shore.
Barrier reef : Grow along coastal region but are farther from
shore.
Atoll : Atolls are large rings of corals that exist under
sea volcanic cones.
The coral reef is composed of calcareous and material derived from atoll and has very high % of CaCo3 (87%). The beach rock consists of moderately well cemented calc arenites. The beach sediments of lagoon consist of various types of coral materials of fine sand to cobble size.
The volcanic island comes in contact with shore by both marine and fresh water environments. It is proved that digenesis process occurs more rapidly in fresh water phreatic environment and it results in dissolution and/or precipitation of carbonate minerals forming well cemented litho units than those of under vadose or marine environment. The end product is low magnesian calcite and caliches. The accumulation of CaCo3 decreases porosity and permeability of the profile.
Water needs is met in various way, as there is tremendous scarcity of water. People have adjusted their life style for bathing and washing and they generally use common ponds for this. Rainwater is the main source for fresh water. In these regions rainfall intensity is 1.5 to 3.5 and 80 to 200 days are rainy.
Although overall geology of an atoll is interesting, its significance is limited to few meters from mean sea level. In oceanic island fresh ground water occurs as a lens floating over saline water. The hydro dynamic balance of fresh and saline water determines the shape and movement of interface and may be controlled by some of the following factors:
1. Water table fluctuation due to diurnal tides
2. Seasonal fluctuation of water table due to recharge or draft
3. Dispersion
4. Molecular diffusion
Due to the above controlling factors there is an alternate up and down movement of the interface. If the fresh ground water flux is high the transition zone will be thin.
In small island due to high permeability almost all the rain falling onto the ground seeps below and a part about 7.15%of the rain fall is intercepted and evaporated before reaching the ground. After rainfall water table rises temporarily and within 2-3 days readjusted and water level stabilise and there is as much as only 5% outflow into the sea.
For water balance the following formula is used. The recharge to ground water lens = rain fall – interception – evapotranspiration.
Ground water utilisation = Evapotranspiration + mixing + pumping + outflow
For water balance study monthly water budgeting or weekly water budgeting gives appropriate value of recharge. The main consumer of ground water is coconut plant because one coconut tree consumes 40 lpd and density of coconut trees is 25 000 – 35000 sq km. But draft through plant is slow, steady and spread uniformly.
Control of saline Water up Coning
Well should be just few cm below mean water table & spread out all over the fresh water lens. Pumping should proceed in slow, steady rate and should be spread in wide area. Pumping site should located preferably in less permeable aquifer.
Ground Water Pollution
Disposal of pollutant in Small Island is a problem since water table is shallow. Special care should be taken for ground water contamination. The spacing between latrine and well should be at least 15m. But due to high density of population remedial measure is not easy implement. In such area precautions should be taken before using ground water.
posted by DR. JOJI at 2:39 AM
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