Changes in groundwater chemistry have an influence on water quality, which may represent a threat to human health (Snousy et al., 2021). Understanding the processes involved in such changes can help in the development of countermeasures against such occurrences.
Based on the plotted Durof diagrams (Figure 13), changes in water chemistry observed in the study area are due to the processes of ion mixing and simple dissolution (Ravikumar et al., 2015). These chemical changes cause typical changes in shallow groundwater in the study area. The mixing of ions results in ion exchange, which can be described by the relationship between concentrations, i.e., (Ca2+ + Mg2+)−(SO42− + HCO3− ) and (Na+ + K+)−Cl− (Li et al., 2021), as well as chloro-alkaline indices (CAI) (Tiwari et al., 2019). The results of calculations using data from Tables 1 and 2 indicate that, regardless of when they were collected, all water samples from the dug wells had negative concentration relationship values with positive CAI. This suggests the withdrawal of Na+ + K+ ions and the release of Ca2+ and Mg2+ ions in the water samples from the dug wells; this process is referred to as reverse ion exchange (Li et al., 2021; Tiwari et al., 2019).
The CAI values were found to be consistent with the plotted Piper diagrams (Figure 14), which show that the well water facies at the study area are largely composed of mixed CaMgCl and primary or secondary saline water (Snousy et al., 2021). According to the classification of Gappuci et al. (2020), almost all wells in Pari Island cannot be categorized as freshwater; the sole exception is W7. Well W7 is located in the center of the island where the freshwater lens is thickest. In June 2021, the water quality in W7 was classified as secondary saline water; it was then reclassified to fresh water in November 2021. Dilution by rainwater probably improved the groundwater quality at well W7. The facies of other wells also shifted from primary saline water (NaCl) and secondary saline water (CaCl) in June to mixed water (CaMgCl) in November (Cappucci et al., 2020; Snousy et al., 2021).
In general, calcium is the dominant ion in freshwater, while magnesium is the dominant ion in seawater. Hence, the Mg2+/Ca2+ ratio can be used as an important delineator between freshwater and seawater (Tiwari et al., 2019). Figure 15 shows that the ratio of Mg2+/Ca2+ to Cl− is less than 1 for all wells, with exception of well W1 in June 2021. This suggests that freshwater characteristics in the study area were stronger than those of seawater during both sampling periods. This may explain the assumption by Pari Island residents that their well water was either quite safe or did not change during that period. The ratio of Mg2+/Ca2+ to Cl− decreased in November 2021 compared to June 2021. This is consistent with the interpretation of the occurrence of a reverse ion exchange process, where the abundance of Ca2+ or rainwater dilution causes the Cl− value to decrease.
Goddy and Hinsby (2008) found that the average TOC in natural groundwater was 2.7 mg C/L, while TOC levels in untreated domestic wastewater were reported between 80 and 260 mg/L (Metcalf and Eddy, 2003). The findings in this study as well as a consideration of the predominant local activities suggest that the groundwater in the study’s sampling locations was affected by domestic sources.
Tables 1 and 2 show a significant increase in EC values in the dug wells in the western to central regions of Pari Island (W1–W6) in November 2021 compared to June 2021. In contrast, the increase in EC values was not significant in the central and eastern regions of Pari Island (W7–W15). Figure 16 presents the distributions of water samples from the study area as plotted on a Gibbs diagram and suggests that the EC increase is related to rock weathering and evaporation–crystallization processes. Before June 2021, the groundwater chemistry of the dug wells on Pari Island was controlled by rock weathering processes; subsequently, between June and November, this chemistry was controlled by the processes of evaporation or crystallization (Li et al., 2021; Tiwari et al., 2019). There is a positive relationship between EC and TDS, in which an increase in TDS concentration will result in further increases in EC value (Rebello et al., 2020; Rusydi, 2018).
Evaporation is among the major factors that influence the chemical composition of shallow groundwater (Richter and Kreitler in Nguyen et al., 2015). Evaporation reduces the water content, consequently increasing the concentration of the residual chemical constituents. This process can also explain the similar behavior observed in the TOC values, especially for samples from the west and middle of the island (except for sample W2). There is the possibility that the organic content in the groundwater was diluted by rainwater during the period June to November 2021.
Figure 17 shows the TOC values of seawater recorded in June and November. In June, seawater from the west coast had very high TOC levels compared to that of the central and east coast. This value is also extremely high compared to TOC levels in the Arabian Gulf (0.8–3.9 mg/L; Emara, 1998). Emara (1998) also noted a significant correlation between TOC values and the presence of petroleum hydrocarbons.
Salinity also influences the organic content in groundwater. Based on Piper diagrams, when the entire groundwater area consists of mixed water and primary and secondary saline water, tidal seawater intrusions can influence the organic content of the groundwater. The salinity may change the chemical composition in the water, thus decreasing its dissolved oxygen abundance and increasing its TOC (Al-Said et al., 2018).
In samples from the east of the study area (see Figure 7), the organic matter content of the groundwater did not decrease between June and November despite increasing groundwater dilution by rainwater; some TOC values in this area are even higher in November than those recorded in June. The seawater origin of the organic matter can be potentially disregarded as the TOC value in the eastern part of the study area (W13) in November is relatively low (Figure 17); other activities should be investigated as a source of the organic matter.
The analysis of climate variability indicates that there has been a trend of more frequent extreme precipitation events, suggesting that the frequency of seawater floods could increase. In this regard, the low-level areas in the west of Pari Island are likely to become more vulnerable in the future, contributing to the increased vulnerability of water resources of the island.
The SPEI results at different timescales show varying trends (Table 4). The short- and medium-timescale SPEI results (SPEI-3 and SPEI-6) indicate an increasing frequency of very-to-extremely or severe dry and wet events, as well as a clear increasing trend in both dry and wet event intensity. An increasing intensity of wet events is beneficial for the sustainability of water resources, while an increasing trend in dry events is disadvantageous for water resources. The magnitudes of dry events on both short and medium timescales have slightly decreased over time (Figure 9). The long-timescale SPEI (SPEI-12) data show an increasing trend of wet events and a decreasing trend of dry events. However, the magnitude of these dry events has increased, suggesting that the increased dry event magnitudes from interannual variations could supersede or counteract any benefits to water resources from the occurrences of wet events from monthly and seasonal variations.
Seasonal variations are widely reported to affect the salinity of groundwater of small islands (Barbieri et al., 2021; Canul-Macario et al., 2020; Giridharan et al., 2008; Heiss & Michael, 2014; Isa et al., 2014). Salt concentrations are typically higher during the dry season and lower when rainwater dilutes the concentrated groundwater during the wet season. However, climatic seasonality has been altered as a result of climate change, with the start of seasons becoming increasingly less predictable. The correlation between the TDS content of groundwater and the SPEI on Pari island is strongest for SPEI-2 (-0.87), suggesting that two-monthly interval precipitation events have the most significant impact on groundwater quality.