1 Environmental Sciences Division, Centre for Earth Science Studies, Trivandrum- 31
Introduction
Because of the proximity of the islands of Lakshadweep to Kerala, and partly on account of cultural lineage, they have always remained upper most in the research activities of the Centre for Earth Science Studies (CESS) right from its inception in 1978. The research programs pursued spans over a wide spectrum of topics, from geologic and geomorphological evolution of the islands to resource evaluation and environmental impact assessment of developmental projects. Of particular contextual interest are the programs on groundwater resource assessment and management, environmental impact assessment, shoreline dynamics, wave climate and power potential and pollution monitoring of the seas surrounding these islands. Following is a resume of the work that has been either completed or ongoing in CESS in these areas.
1. Studies on the groundwater resource potential and its management
The fresh groundwater resource of the tiny coral atolls of Lashadweep, by and large, occurs as lenses floating in hydraulic continuity with sea water. Cases of increased extraction from these fragile lenses and resultant salinisation were reported from many islands where pumping activity has been on the increase, especially, during the last two decades. This necessitated a detailed study in all the inhabited islands of Lakshadweep, to delineate the geometry of the fresh groundwater lenses, to evaluate their storage potential, and to assess the sustainable rate of production in order to suggest appropriate, futuristic groundwater development and management options for the island territory.
The hydrogeological environment of these islands is complex. The terrain is mostly flat and the surface and near-surface coral medium is porous and permeable. Rainfall, averaging about 1600 mm annually, is the sole source of groundwater recharge in these islands.
Hydrogeological and geophysical methods using innovative procedures were adopted for direct mapping of the fresh groundwater lens, for hydraulically characterizing the aquifer and for assessing the sustainable long-term production capability of the fresh groundwater lens system (Varma, et al., 1989; Varma and Ramachandran, 1992; Varma and Ramachandran, 1995 a&b; Varma, et al., 1995 a&b; Varma and Ramachandran, 1996; Varma, 1997). The geophysical method employed electrical resistively profiling and sounding, the result of which was used to describe the lateral extent and the thickness of fresh water and transition zone of the groundwater lens. The transmission and yield capabilities of the aquifer represented by its porosity, transmissivity, specific yield etc. were determined from pump tests, formation resistively factor analysis, tidal wave propagation method and permeability test on core samples. The sustainable production capability of the lenses was finally assessed based on the quantum of annual recharge determined from climatic water balance, chloride balance, water table fluctuation and groundwater storage change.
Based on the studies, maps depicting the lateral extent and isoplethes of the fresh groundwater lens of all the inhabited islands were prepared. A specimen map of Kavaratti on these aspects is shown in Figure 1.
Figure 1. Lateral extent and isopleths of fresh water portion of the groundwater lens
The study assessed that about 500 mm of rainfall reaches the groundwater body, annually, as recharge. Irrespective of this, it was found that the water table rise, upon incidence of rainfall, diminishes in a couple of days. It was suggested that the annual rainfall that infiltrates to the groundwater lens escapes to sea as rejected recharge. However, this was negated by the geophysically derived fresh groundwater lens model of premonsoon and postmonsoon period at Kavaratti (Figure 2).
(RL- Ground level, WT- Water Table, FZ- Base of the fresh groundwater lens, TZ- Base of the transition zone)
The static and dynamic storage potential of the inhabited islands of Lakshadweep, determined based on the geometry and specific yield of the fresh groundwater portion of the aquifer is given in Table 1. In general, it indicates that the dynamic storage of inhabited islands is adequate to serve the needs provided the development of the resource does not tell upon the sustainable yield, worked out to be about 10% of the average annual recharge (AAR). Contrary to this, the estimated draft of fresh groundwater at Kavaratti was about 0.17 MCM as against the AAR, It suggested that either an overdraft takes place or a part of the groundwater draft (about 25%) reenters the ground, which is logical in the island situation.
The salinity index of the island groundwater system indicates that no fresh groundwater without saline contamination is available in any island of Lakshadweep. However, the contamination fortunately is very marginal except at a few spots at Kavaratti and Minicoy. It is also observed that the quality of water deteriorated with longer duration of pumping. There is no doubt, therefore, that over extraction would deteriorate the quality of fresh groundwater in these islands.
The study also reveals that the thickness and shape of the lens derived using the classical Ghyben-Herzberg relationship, does not match with the one delineated by the electrical resistivity method. It is, however, found that the groundwater lenses of the islands comprise of a fresh water portion on the top and a transition zone at the bottom (Figure 2).
Table 1. Static storage, annual recharge and projected dynamic storage of fresh groundwater in various inhabited islands of Lakshadweep.
|
Name of Islands |
Area of fresh GW zone Km2 |
Sp. Yield % |
Volume of fresh GW portion of the aquifer MCM |
Static storage potential of fresh GW MCM |
Annual water surplus mm |
Recharge area Km2 |
Dynamic storage potential MCM |
|
Agatti |
1.070 |
21 |
2.344 |
0.4922 |
193 |
1.284 |
0.2478 |
|
Andrott |
4.077 |
11 |
63.633 |
6.9996 |
414 |
4.077 |
1.6879 |
|
Amini |
1.563 |
17 |
6.463 |
1.0987 |
193 |
1.876 |
0.3621 |
|
Chetlat |
0.514 |
22 |
2.381 |
0.5238 |
414 |
0.514 |
0.2128 |
|
Kadmat |
2.200 |
17 |
15.424 |
2.6221 |
414 |
2.200 |
0.9108 |
|
Kalpeni |
1.180 |
25 |
14.990 |
3.7475 |
414 |
1.180 |
0.4885 |
|
Kavaratti |
1.930 |
18 |
6.450 |
1.1614 |
474 |
1.930 |
0.9140 |
|
Kiltan |
1.172 |
26 |
10.667 |
2.7734 |
414 |
1.172 |
0.4852 |
|
Minicoy |
1.226 |
25 |
7.325 |
1.8313 |
459 |
1.226 |
0.5627 |
One of the prime considerations that evolved from the study for the development and management of the groundwater resources in the islands of Lakshadweep is the maintenance of a minimum thickness of lens as a critical factor for the upkeep of island groundwater system. It implies that the fresh groundwater extraction from island lenses has to be limited to the sustainable yield. Considering these factors, the study recommended stoppage of indiscriminate pumping and advocates the use of infiltration galleries and low capacity pumps (about 60 Ipm), if pumping is needed. It is also suggested to limit pumping locations to zones where the thickness of lens is greater. These measures alongwith popularization of water harvesting has done a world of good in protecting the fragile fresh groundwater lens from irreversible damage that would have happened, otherwise, if callous pumping had been continued.
2. Environmental Impact Assessment
Being one of the most thickly populated areas of the Indian Union (2000 per Km2 in 1991), they are overwhelmingly interfered by man, often to the detriment of himself. The increasing pace of developmental activities add further to the environmental deterioration of this highly sensitive coral ecosystem. One such activity has been the blasting and dredging of coral reefs for the development of a harbour in the island of Kavaratti (Ramachandran, 1981). While the primary impact was the destruction of living corals, the secondary impact that it generated was marine erosion. The study suggested minimization of establishment of institutions in a single island so that the magnitude and importance of impacts become minimum. It further emphasized the need for having mandatory environmental impact assessment (EIA) of all the developmental projects, irrespective of their size in these islands.
Two of the sectors identified by the Island Development Authority (IDA) for the economic development of the islands of Lakshadweep are fisheries and tourism. It is estimated that, annually more than 8000 tourists can be accommodated on these islands on a sustainable basis, which amounts to more than 5 times the number of tourists of both Indian and foreign origin, who visit these islands today. However, the experiences of many islands of the world, where the tourism industry has taken root, shows that unless proper care is taken right at the reinvestment stage of various development projects, things might turn suicidal in terms of environment, economy and culture, as pointed by Ramachandran (1982) in his study on the natural environment of Lakshadweep, landuse capability, problems and prospects.
The recently enacted Coastal Regulation Zone Act, 1991 and the overwhelming necessity to safe guard the sensitive coral environment coupled with public awareness have recently imposed various restrictions on any type of activity in these islands. This has obviously made it obligatory on the part of the Union Territory Administration to make any activity transparent by resorting to EIA and making it public. One such EIA was done by CESS (1997) with regard to construction of five air strips of 500 m x 50 m in the islands of Kavaratti, Andrott, Minicoy, Kadmat and Kiltan, primarily to examine its feasibility in terms of development of tourism. The feasibility study for air strips was based on Geographical Information System (CIS) based on thematic maps of 1:4,000 scale on Coastal Regulation Zone, Agricultural Landuse, Natural Vegetation Cover, Groundwater lens, Settlement and Built-up Area, Physical Factors etc. Linear features such as roads, communication lines etc. were also considered. This resulted in suitability assessment and overlay factor maps leading to demarcation of air strip corridors with very low environmental impacts. Supplementing this spatial approach, an interactive matrix analysis was also attempted to facilitate understanding the various dimensions of the likely environmental issues involved in the total tourism development and also the Environmental Management Plans (EMP) that might be needed to ameliorate the negative impacts, in future, and the probable costs involved in the process.
3. Shoreline Dynamics
Wave diffraction plays a significant role in the distribution of wave energy along the Coasts of the islands and thus affects the stability of the coast. To understand the relationship between the erosion-cycle and wave diffraction in selected islands, a beach monitoring survey involving beach profile measurements was carried out during 1991 - 1992 (Prakash et al., 1995). The beach profile data have been processed for the beach volume changes and critical areas of erosion and accretion demarcated. It is observed that the SW and SE portions experience critical erosion whereas the NE portion of the island shows a seasonal erosional/accretional behaviour. Wave data from a Directional Waverider Buoy deployed off Kavaratti show that wave heights vary from 0.56 to 8.95 m. The S-SW waves are dominant with periods ranging from 5 to 8 s during monsoon and 8 to 12 s during fair weather period. The diffraction coefficients were computed for the dominant wave directions and wave periods. The diffraction coefficient along the boundary of the islands vary from 0.71 to 1.45. Higher diffraction is observed at the SW and SE coasts where critical erosion is observed. At the NE portion of the islands where there is no net erosion the diffraction coefficient is less.
4. Wave Climate and Power Potential
The wave climate and power potential of the seas surrounding Lakshadweep islands were also studied by the CESS using wave data collected with a Directional Waverider Buoy deployed off Kavaratti (Baba et al., 1995). The study reveals that the wave climate of the Lakshadweep sea is influenced by the southwest monsoonal winds. The period from June to August is the roughest season for the Lakshadweep sea. The maximum wave height observed during the one year period is 8.95 m in August. Waves generally do not exceed the height of 5 m during November-March. During southwest monsoon the dominant values of maximum wave height is around 5 m and during the non-monsoon season it is around 1.4m.
The significant wave heights range from 0.4 to 4.7 m, the lowest being observed in February and the highest in August. Generally, Hs are higher during June, July and August when the range is 1.75-4.70 m and are lower during November-March. The zero-crossing period (Tz) ranges from 3.5 to 13.3 s. The maximum ranges are observed in February and April and the lowest ranges during June-August. The dominant Tz during southwest monsoon is in the range 7-8 s and during non-monsoon it is 5-7 s. The peak period ranges from 8.4 to 26 s during the year, the lowest being observed in November and the highest in February. Generally, Tp is lower during June-August with smaller variations, the wave direction ranged from 106° to 316° N. The waves are least persistent in direction during January and February. The easterly components are observed during the period November-January. The S-SW directions persist throughout the year and these directions dominate most of the period except when the southwest monsoon is intense with westerly waves.
The maximum wave power observed during the year is 110 kw/m in July. During November to March the dominant wave power is in the range 0-5 kw/m and during April, May, September and October it is the range 5-10 kW/m. The peaks of the distributions of the wave power are in the range 20-25 kw/m during June, 30-35 kw/m during July and 15-20 kw/m during August. The monthly mean wave power ranges from 28 to 40 kw/m during June-August. The average wave power for the complete year is about 14 kw/m and that for June, July and August is around 35 kw/m.
5. Monitoring of Marine Pollution Parameters
As part of the nation wide project on the Coastal Ocean Monitoring and Prediction System (COMAPS), the CESS has been continuing monitoring of various marine pollution parameters in the seas around Lakshadweep, particularly, off Kavaratti, Andrott and Minicoy (Ouseph, 1997). As part of the program, the marine water samples are being analyzed for total suspended solids, pH, salinity, Dissolved Oxygen, Biological Oxygen Demand, silicate, total Phosphorous, ammonia-Nitrogen, nitrite-Nitrogen, nitrate-Nitrogen, total-Nitrogen, petroleum hydrocarbon and trace metals. Chlorophyll- a pigment and primary productivity values were determined to assess the productivity level. Identification and assemblage of plankton communities and microbial parameters are also attempted.
Data so far collected provide the status of water quality and productivity in the region. Water temperature were almost uniform and the suspended solid concentration was recorded high at Kavaratti. High pH was noted at Minicoy. Low Dissolved Oxygen concentration was recorded at Kavaratti and Andrott. Nutrients concentration was almost constant in all the regions. High Biological Oxygen Demand (BOD) was noted at Kavaratti and Andrott. The low Dissolved Oxygen and high BOD recorded in this regions indicate the environmental stress. The petroleum hydrocarbon concentration was also noted to be high at Minicoy in comparison to Kavaratti and Andrott.
Minimum pigment concentration was noted at Minicoy. The highest biomass was recorded in the Kavaratti waters and the lowest in Andrott. Coscinodiscus and Chaetoceros were the dominant phytoplankton groups observed in the Lakshadweep waters. Copepods and Decapods were the dominant groups among he zooplankton.
Inter island comparison of Kavaratti, Minicoy and Andrott indicates that Kavaratti island is the most viable in terms of microbial activity. The highest Total Viable Count is noted in the shore sample of Kavaratti Airport point. Anthropogenic pollution of shore water, recorded as coliforms and faecal coliforms, was also maximum at Kavaratti. In Minicoy and Andrott faecal contamination is found spreading beyond 2.5 km distance. Faecal Streptococcus, another indicator of faecal contamination, is found maximum in Andrott harbour point. Faecal contamination of sea shore is obvious in all the three islands by the presence of Streptococcus sp. Other type of pollution indicators such as Salmonella sp, Shigella sp. and Vibrios sp. were also observed, the maximum being in Kavaratti island. The present study reveals that the coral environment is subjected to stress mainly by anthropogenic sources. The high microbial activity is likely to affect the coral ecosystem.
References
Baba, M., Shahul Hameed, T.S. and Kurian, N.P., 1995, Wave climate and power potential of the seas surrounding Lakshadweep islands, Abst. Vol., Workshop. Status of Sci. Database on Lakshadweep; Geological Survey of India, Trivandrum
CESS, 1997, Identification of airstrip corridors based on environmental impact assessment (EIA) in Kavaratti, Andrott, Kadmat, Kiltan and Minicoy Islands of Lakshadweep, Report (Volume I & II) submitted to the Administration, Union Territory of Lakshadweep, Kavaratti, Centre For Earth Science Studies (CESS), Trivandrum.
Ouseph, P.P., 1997, Coastal Ocean Monitoring and Prediction Systems (COMAPS) - Monitoring along Kavaratti, Andrott and Minicoy Islands, lakshadweep, Annual Report submitted to Department of Ocean Development, New Delhi. Centre For Earth Science Studies (CESS), Trivandrum
Prakash, T.N., Shahul Hameed, T.S, and Suchindan, G.K., 1995, Shoreline dynamics of selected islands of Lakshadweep Archipelago in relation to wave diffraction, Abst. Vol., Workshop. Status of Sci. Database on Lakshadweep; Geological Survey of India, Trivandrum
Ramachandran, K.K., 1981, Laksahdweep coral ecosystem and a case study on human impact on the Kavaratti Atoll, Proc. Sem. Status of EnvtI. Studies in India, Centre for Earth Science Studies, Trivandrum. pp. 255 -262.
Ramachandran, K.K., 1982, Lakshadweep coral islands (India), its natural environment, landuse capability, problems and prospects, First Intl. Sym. Soil, Geo. Landforms, Bankok, pp. B11.1 -B11.14.
Varma, Ajaykumar., 1997, Studies on groundwater resource potential and management of the coral atolls of Lakshadweep, Union Territory of India, P.hD Thesis, University of Kerala, Trivandrum.
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Varma, Ajaykumar and Ramachandran, K.K., 1995a, Assessment of groundwater recharge in Kavaratti Island, Union Territory of Lakshadweep, India, Project Report, Centre for Earth Science Studies, Trivandrum.
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Varma, Ajaykumar and Ramachandran, K.K., 1996, Resistivity survey for describing the fresh water lenses of Agatti Atoll, Lakshadweep, India, Jl. Assn. of Expl. Geophys., XVII: 11-16.
Varma, Ajaykumar, Unnikrishnan, K.R. and Ramachandran, K.K., 1989, Geophysical and hydrogeological studies for the assessment of groundwater resource potential in the Union Territory of Lakshadweep, India, Final Report, Centre for Earth Science Studies, Trivandrum.
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Varma, Ajaykumar, Unnikrishnan, K.R. and Ramachandran, K.K., 1995b, Location, geometry and storage potential of the fresh groundwater lenses of Lakshadweep islands, Abst. Vol., Workshop. Status of Sci. Database on Laksfiadweep', Geological Survey of India, Trivandrum.
