Introduction

The overall development such as industrial, agricultural as well as increasing population has lead to increase in demand for potable water. In the recent years, to meet the increasing demand there has been indiscriminate exploitation of groundwater resources particularly in the areas where the surface water potential is negligible. There are several tiny atolls off western coast of India (Mallik 2001), where the population density is very high. Groundwater is the only source of fresh water on these atolls. It is in the form of thin fragile floating lens, which is often vulnerable to overexploitation, tidal waves, tsunami and cyclones, causing seawater ingress (Chandramohan et al. 1993; Singh and Gupta 1999a). There is continuous pumping of groundwater to meet various needs on these islands, which has lead to deterioration in groundwater quality. The new technology such as solar pump has added further pumpage of groundwater beyond the actual need. As a consequence of indiscriminate exploitation, the quality in some of these islands has already started deteriorating. In order to develop sustainable scheme for groundwater management, it has become essential to assess the groundwater quality on the island. Keeping this in mind, such study was carried out in one of the Lakshadweep islands, i.e., Andrott Island.

About the study area

Lakshadweep is an archipelago of coral islands in the Arabian Sea, off the western coast of India. In the range of 220–400 km off the western coast of India, there are about 36 islands, 12 atolls, 3 reefs and 5 submerged coral banks, spread over an area of 32 km2 in the Arabian Sea (Chandramohan et al. 1993). Tens of these islands are habited. The eastern most habited island is Andrott Island, where more than 10,000 (10,720 as per 2001 census, population density as 2,233 km−2) people live on the island (Fig. 1). This island is nearest to the main land. The areal extent of the island is about 4.9 km2. Most of the islanders use groundwater for their various needs. There is no surface water storage on the island and there has been growing demand for potable water in the recent years, which has lead to increase in exploitation of groundwater. The overexploitation of groundwater on island has lead to decrease in fresh water potential as well as deterioration in groundwater quality due to seawater ingress.

Fig. 1
figure 1

Location map of the study area

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Hydrogeology

Earlier workers have described geology, geomorphology and hydrogeology of the island (Nazeeb 1995; Wagle and Kunte 1993; Singh and Gupta 1999b; Mallik 2001; Revichanndran et al. 2001). The group of atolls lies on the prominent N–S Lakshadweep ridge. It is speculated that islands are buried continuation of Aravalli Mountain chain (northwest of Indian continent) and the Deccan Traps have been faulted down along the western coast of India. The ridge rises from the deep sea and is composed of basalt, which is capped by recent coral reef in the form of atoll and coral banks. Andrott Island is occupied by coral sand. The topography of the island is undulating and the ground surface is about few meters to 8.0 m above mean sea level (amsl). The island is of elliptical shape with major axis in E–W direction, whereas all other islands have N–S. The island does not have any lagoon around it and is enveloped with sparkling white carbonate sand beach. There is no surface water storage on the island. The island is covered with coral sands and at some places shell limestone. Most of the rain occurs during the month of June to September (monsoon season). The average annual rainfall for the period of 2000–2005, on the island, is about 1,817 mm.

Due to high permeable coral sand on the surface, most of the rain percolate down and finally goes as subsurface runoff to sea. There are no signatures of drainage on the topography. Major vegetation on the island is coconut with about more than 100,000 trees.

The groundwater occurs in the coral sand underlined by shell limestone, in the form of floating lens. The groundwater is being exploited for various needs of islanders through hand-dug shallow wells. The recent development has brought many of these wells equipped with solar pumps, which in turn has increased the exploitation of groundwater. The diameter of the wells varies from less than a meter to about 2.0 m. The depth of the wells varies from less than a meter to about 7.0 m below ground surface. Groundwater is mainly used for domestic purposes, as hardly there is any industry or agriculture. Groundwater regime on the island floats on the seawater and tidal response to water table has been clearly noticed.

Hydrochemical studies

In order to assess groundwater quality, water samples have been collected from the existing open wells during pre-monsoon and post-monsoon seasons on the island. Total 35 samples were collected in each season and the locations of these wells are shown in Fig. 2. The analysis of water samples were carried out to assess major cation and anion such as calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), bicarbonates (HCO3), chloride (Cl), sulphate (SO4), nitrate (NO3) including total hardness (TH), pH and electrical conductivity (EC). The results of chemical analysis are given in Tables 1 and 2. All the concentrations are expressed in milligrams per liter, except pH. EC is measured in micro-siemens per centimeter. The chemical analysis data have been interpreted using various plots such as Piper’s trilinear, Gibbs and Wilcox diagrams, to assess the groundwater quality on the island. Further, principle component analysis (PCA) technique is also used to compare the similarities and dissimilarities of the ions simultaneously for pre- and post-monsoon samples.

Fig. 2
figure 2

Location of the observation wells at Andrott Island

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Table 1 Major cations and anions for pre-monsoon season, Andrott Island, Lakshadweep
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Table 2 Major cations and anions for post-monsoon season, Andrott Island, Lakshadweep
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Results and discussion

Comparison of some selected parameters of pre- and post-monsoon is shown in Fig. 3. It shows that EC and content of Cl are higher in pre-monsoon samples, whereas the content of Ca and HCO3 are higher in post-monsoon samples. The higher content of Cl is indication of mixing of seawater during non-monsoon period, whereas the higher content of Ca may be due to dissolution of Ca from soil (coral) during infiltration process that takes place during monsoon.

Fig. 3
figure 3

Comparison of selected parameters during pre- and post-monsoon

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To demarcate the zones of high EC and Cl content areally, these values are plotted and shown in Figs. 4 and 5 for pre- and post-monsoon periods, respectively. During the pre-monsoon period, higher content of Cl and EC is observed at the eastern side of the island and at the peripheral parts of the western side. But during the post-monsoon period, Cl content exceeds the permissible limit of drinking water quality standard [250 ppm, ISI (1983)] only at few locations in the eastern and western parts, whereas EC is within the permissible limit of drinking water standard [1,500 ppm, ISI (1983)]. As indicated in Fig. 4, the eastern part of the island is affected due to seawater ingress during non-monsoon period and it has reduced during post-monsoon period due to recharge, however, the effect remains visible in eastern as well as western part as shown by high Cl content in Fig. 5.

Fig. 4
figure 4

Areal distribution of a chloride and b electrical conductivity during pre-monsoon

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Fig. 5
figure 5

Areal distribution of a chloride and b electrical conductivity during post-monsoon

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In order to understand the role of various cation and anion in the groundwater chemistry during pre- and post-monsoon period, the data were plotted in the trilinear diagram (Piper 1944) as shown in Fig. 6. Majority of pre-monsoon samples have shown dominance of Na and HCO3 whereas during the post-monsoon Ca and HCO3 dominance are seen amongst cations and anions, respectively. This dominance of Na may be due to seawater mixing during pre-monsoon period. The dominance of bicarbonate as observed during both pre- and post-monsoon may be only due to interaction with coral formation. In the central diamond shaped field, majority of the pre-monsoon samples fall in “no cation or anions exceeds 50%” area, whereas all the post-monsoon samples fall in “carbonate hardness exceeds 50%” area, indicating that the chemical properties are dominated by alkaline earth (Ca, Mg) or corals and week acids (HCO3 + CO3).

Fig. 6
figure 6

Piper’s trilinear diagram

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In order to visualize the sodium hazard among the water samples of the study area, the data has been plotted in the form of Richard’s diagram (Richards 1954) as shown in Fig. 7. In this diagram sodium absorption ratio (SAR) is plotted against EC. The diagram classifies the water into two divisions: (a) conductivity classification, (b) sodium hazard classification. Both conductivity and sodium hazard are classified as low, medium, high and very high, they are represented by C1, C2, C3, C4 and S1, S2, S3, S4, respectively (Todd 1980). Majority of the pre-monsoon samples fall in very high to high sodium hazard and high salinity hazard zones indicating non-suitability of groundwater for agricultural purposes. The post-monsoon samples fall in the range of medium to low sodium hazard and high to medium salinity hazard zone indicating limited use for agricultural purposes. The number of samples of pre- and post-monsoon falling in different zones are given below:

Fig. 7
figure 7

Richard’s diagram

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Area

Character

No. of samples

S4C4

Very high SAR, very high conductivity

Pre-monsoon (1)

S4C3

Very high SAR, high conductivity

Pre-monsoon (12)

S3C3

High SAR, high conductivity

Pre-monsoon (7)

S2C3

Medium SAR, high conductivity

Pre-monsoon (10), post-monsoon (11)

S1C3

Low SAR, high conductivity

Pre-monsoon (3), post-monsoon (10)

S2C2

Medium SAR, medium conductivity

Post-monsoon (8)

S1C2

Low SAR, medium conductivity

Pre-monsoon (1), post-monsoon (6)

Various factors controlling groundwater chemistry are analyzed by the diagram (Gibbs 1970) shown in Fig. 8. The groundwater samples are scattered between the rock and evaporation dominance fields. Pre-monsoon samples are showing more of evaporation dominance whereas the post-monsoon samples are showing rock dominance. The host lithological units are mainly controlling the groundwater chemistry of post-monsoon and the evaporation, pre-monsoon samples.

Fig. 8
figure 8

Gibb’s diagram

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Principle component analysis

Principle component analysis (PCA) is a popular multivariate technique, which identifies the most important components contributing to the data structure and the interrelationships in large number of variables. In other words, PCA is a simple mathematical reduction of the data without any elaborate assumptions (Anderson 1958; Morrison 1964), which enables us to describe the information with considerably fewer variables than was originally present.

In the present investigation, the pre- and post-monsoon hydrochemical data are subjected for PCA using standard statistical packages on computer. The variables considered are TDS, TH, Ca, Mg, Na, K, HCO3, Cl, SO4, and NO3. Correlation matrices have been calculated to know the inter relationship between the variables and are presented in Tables 3 and 4. Some selected cross plots are shown in the Fig. 9. Cross plot of sodium with EC shows the correlation 0.88 during pre-monsoon, whereas during post-monsoon it is found 0.39. Similarly, the correlation between the chloride and EC is found 0.92 and 0.39 during pre- and post-monsoon, respectively. Na and Cl are showing strong correlation with EC during the pre-monsoon, whereas weak correlation during the post-monsoon season.

Table 3 Correlation matrix of pre-monsoon hydrochemical data
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Table 4 Correlation matrix of post-monsoon hydrochemical data
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Fig. 9
figure 9

Cross-plots of EC with (a) sodium (b) chloride during pre- and post-monsoon

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As explained above, the eigen values are helpful in deciding the number of components required to explain the data variation. In the present case, the cumulative percent of trace of first three eigen values account for 77.50 and 74.20% of the total variance for pre- and post-monsoon data, respectively. Hence, these first three component scores are used to explain the background hydrochemical processes without losing much of the significant characteristics. The component scores are cross-plotted, i.e., I versus II, I versus III and II versus III as shown in Fig. 10. Among the principle components of pre-monsoon, component I is negative for Ca, component II is negative for TH, Mg and HCO3, whereas component III is negative for TDS, Ca, Na, HCO3 and Cl. Cross plots of component I versus III, component II versus III and component I versus II separates out sample nos. 2, 3, 4, 10, 11, 13, 14, 19, 26, 27 and 33 which have TDS of range 790–2,272, Na of range 78–452 and Cl of range 92–865 mg/l. the rest of samples are having TDS of range 383–1,117, Na 16–202 and Cl 28–319 mg/l. Most of the samples separated out from the cluster are located nearer to the coast.

Fig. 10
figure 10

Principle component cross plots of pre- and post-monsoon

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Of the principle components of post-monsoon, component II is negative for TDS, TH, Mg, Cl, SO4 and NO3, whereas component III is negative for Mg, Na, K, SO4 and HCO3. Cross plots of component I versus III, component II versus III and component I versus II separates out samples no. 2, 3, 9, 11, 13, 14, 26, 32 and 33, which have TDS of range 274–909, Na of range 28–82 and Cl of range 40–349 mg/l. the rest of samples are having TDS of range 295–728, Na 22–70 and Cl 45–309 mg/l. Most of the samples separated out from the cluster are located nearer to the coast.

Conclusions

  • The various plots for chemical analysis data clearly suggest seawater ingress during non-monsoon period in the western as well as eastern part of the Andrott Island.

  • Groundwater is also affected due to dissolution of calcium from the coral soil and aquifer formation in the island.

  • The western and eastern parts of the island where groundwater quality begins to deteriorate during the non-monsoon period, the aquifer system becomes more vulnaerable to seawater ingress.

It is suggested to minimize the groundwater abstraction in these zones and implement rainwater harvasting measures to augment groundwater resources.