Keywords

Highlights

  • The seasonal changes affect the groundwater quantity and quality.

  • Variation of major ions concentration in the hydrochemical processes.

  • PCA reveals the main controlling factors of groundwater composition.

  • PHREEQC calculation indicates mineralization processes.

Introduction

Small tropical islands became substantial issues regarding freshwater resources as its insularity character may expose to too many sources of defilement. The quality of fresh groundwater in small islands usually depends on the surrounding activities while, the presence of freshwater are based on its quantity, its surface storage and subsurface recharge (Aris et al. 2010). Groundwater pollution has been documented worldwide as results from anthropogenic disturbances into natural systems (Rosenthal et al. 1992). One of the most acute water resources problems in small tropical islands is the continuous salinization of groundwater. Seasonal changes have become vital contribution to groundwater hydrochemistry either in saline or freshening status. Therefore, a temporal distribution of seasons which are pre-monsoon (South–West Monsoon; dry season) and post-monsoon (North–East Monsoon; particularly wet season) were taken into consideration in present study (Desa and Niemczynowicz 1996; Wong et al. 2009). The objective of this paper is to determine the controlling factors affecting the groundwater hydrochemistry especially in different season.

Materials and Methods

Kapas Island is located at 5° 13.140′ N, 103° 15.894′ E with an area about 2 km² (Abdullah 1981; Shuib 2003). The climate is typically tropical climate with annual rainfall between 451 and 1102 mm. Kapas Island experiences constant mean temperature at 29.88 °C and has average daily relative humidity around 70–80 %.

The sampling design for this study was based on spatial and temporal scales. A total of 216 groundwater samples with replicates were collected bimonthly from six constructed boreholes during pre-monsoon (August–October 2010) and post monsoon (February–April 2011) at Kapas Island.

Results and Discussion

The cation concentrations order were Ca > Na > Mg > K while for anions were HCO3 > Cl > SO4. As different monsoons were concern, two types of groundwater were found during pre-monsoon which are Ca–HCO3 and Na–HCO3 while post-monsoon is having only Ca–HCO3 type.

Principal component analysis (PCA) was used in this study to reduce large of dataset by explaining the correlation among variables (Stetzenbach et al. 1999; Yongming et al. 2006). Pre and post-monsoon extract four and three components with 81.6 and 78.9 % of total variances respectively. Pre-monsoon shows salinization processes while having components TDS, EC, Salinity, Eh, pH, Cl, and Na. On the contrary, variables components viz. Mg, TDS, EC, Salinity, Ca, Na, pH, Eh and HCO3 during post-monsoon described the mineralization process. Figure 32.1 shows the distribution of components in different monsoons.

Fig. 32.1
figure 1

Insert (i) and (ii) explained the variables of principal component in different monsoons

Full size image

Saturation indices (SI) indicate the behavior of carbonate minerals in groundwater. Most of groundwater samples during pre-monsoon are in dissolution state with 76 %. Meanwhile, the limitation of CO2 during post-monsoon explained the super-saturation of carbonate minerals and only 30 % of dolomite mineral were found in dissolution state.

Conclusion

Present study indicated that the geostatistical tool of PCA and geochemical technique of SI rendered an important data reduction to identify the factor controlling groundwater hydrochemistry especially in seasonal variation. Comprise of two different groundwater types; Ca–HCO3 (pre-monsoon) and Na–HCO3 (post-monsoon), these analyses revealed the most significant factors responsible to groundwater evolution which are salinization and mineralization processes.