The Udawalawe irrigation project which came into operation in 1967 has caused to release of excess freshwater to Kalametiya Lagoon through Kuchchigal ara (Dahdouh-Guebas et al. 2005a; Madarasinghe et al. 2020a) resulting in desalination. In addition, due to the fact that the natural sand bar is not seasonally opened and the artificial dyke was constructed during the irrigation project, the ebb-flow system of the lagoon has been upset. Therefore, the tidal effect becomes minimal [Sri Lanka, anyway has a microtidal system (Kodikara et al., 2017a)]. This scenario has eventually caused reduction of the overall mean salinity level of the lagoon. In contrast, during the dry season, seawater intrusion becomes prominent and leads to increase in the lagoon salinity. Due to this fact, a spatial disparity in salinity distribution could be observed and it was no surprise for us to record the highest salinity at the lagoon outlet and the lowest at the inlet. It is evident that the lagoon ecology and biology are affected with such salinity reduction (Madarasinghe et al. 2020b) and salinity further plays a crucial role since increased salinity results in increasing metal bioavailability (Hou et al. 2013). In term of water pH, Ramanathan et al. (2005) recommended optimum range of pH 6.8–8.7 for proper function of a lagoon and the obtained mean value for water pH was lower than that of the prescribed range. This directly indicates a deterioration of water quality of Kalametiya Lagoon. In addition to pH and salinity, it is recommended to check dissolved oxygen, total suspended materials, redox potential, and organic matter content which may be useful in figuring out the holistic picture of the lagoon (Lawson 2011).
Heavy metals, as one of the top contaminants, have recently gained worldwide attention due to their high toxicity, environmental persistence, and accumulation in the environment and organism (Zhang et al. 2014). Heavy metal contents are mainly attributed to lithogenic and anthropogenic inputs (Kabata-Pendias and Mukherjee 2007). Two major causes are discussed for Cr enrichment in the lagoon. It is apparent that sediments are brought to the lagoon with excess freshwater and that bulky sedimentation may have caused to add more Cr to the lagoon. There is evidence which reflects that lithogenic components; i.e. produced from the weathering of bedrocks and soils (Yunginger et al. 2018) in upper areas are transported by water which then settled at the bottom of downstream water bodies (Tamuntuan et al. 2015). The other probable source of Cr may be phosphate fertilizers, used in agricultural fields in the area (Dissanayaka and Chandrajith 2009). There is a high likelihood to get solubilized phosphate fertilizers when excess freshwater flows through the agricultural lands, situated at the upstream areas. In addition, Dissanayaka and Chandrajith (2009) reported that the phosphate fertilizers used in Sri Lanka contain not only Cr, but also high amounts of heavy metals including Ni, and Pb. Furthermore, Zn, Cu, Cd, Pb, and As have also been identified as widely used heavy metals in agricultural fertilizers, pesticides and fungicides (Gimeno-Garci’a et al. 1996; Kelepeitzis 2014). Therefore, As, Cd and Pb could be linked with agricultural fertilizers, being used in the area. Although Adikaram et al. (2016) reported that most marine algae produce organoarsenic compounds, for example in Batticaloa lagoon, it is unlikely to be applicable in Kalametiya Lagoon since material exchange is minimal at the lagoon outlet. When Hg is considered, that could be due to higher input from the domestic sewage and hospital effluents (Wang et al. 2017). Moreover, metal processing, stainless steel welding, chromate production, tannery facilities and ferrochrome and chrome pigment production could largely contribute to heavy metal release as well (ATSDR 2012). It is well-known that water pH directly influences the heavy metal concentrations by altering bioavailability and toxicity. Metals such as Cd, Pb and Hg are most likely to have significant correlation with water pH and are recorded to increase detrimental environmental effects with increasing acidity (DWAF 1996). It has been found that water pH governs the methylation of elements such as Pb and Hg (van Loon 1982) that was best reflected by significant correlation shown for Hg in this study.
It is commonly observed that, except Hg, the rest are largely accumulated at the Centre and Outlet of the lagoon. Higher accumulation of heavy metals at these regions is due to poor ebb-flow system of the lagoon which happened due to the artificial dyke construction and natural sand barrier. In general, mosaic distribution of heavy metals in Kalametiya Lagoon could be due to the limited circulation taking place inside the lagoon. Furthermore, the increasing trend of heavy metals in the lagoon from the Inlet (E) to the Outlet (A) might be due to the process of sediment accumulation pattern from upstream to downstream of the lagoon (Dahdouh-Guebas et al. 2005b). Though the sediment undergoes resuspension, redox reaction and biodegradation, considerably changing the affinity of the sediment, it is considered to be the depositing compartment of the marine environment. The distribution and accumulation of heavy metal depend on grain size. Therefore, more insight would be gained if grain size analysis would have been done (ElNemr et al. 2007).
When current pollution status is taken into consideration, Kalametiya Lagoon is not at stake as the heavy metal contents did not exceed TEL and PEL. In Sri Lanka, few studies have focused on heavy metal pollution in several water bodies and lagoons including Kumbichchankulama, Alankulama, Thuruwila, dry zone and Negombo Lagoon (Bandara et al. 2008; Chandrajith et al. 2012; Sivanantha et al. 2016). According to their results, Negombo Lagoon showed the highest content of heavy metals in the published data of Sri Lanka, for example, As: 9.89 mg/Kg; Cd: 2.63 mg/Kg; Cr: 26.1 mg/Kg ; Pb: 20.26 mg/Kg. In rest of the water bodies also, Cd content was greater than 2.18 mg/Kg. Therefore, the pollution level of Kalametiya Lagoon is far below the aforementioned figures in the other water bodies. Since this excess freshwater flow continues, higher levels of pollution can be expected soon. On the other hand, survey data clearly reflect that socio-economic interactions have become minimal and less than 5% of the dwellers now depend on lagoon fisheries. Further, the dwellers have indirectly experienced water quality changes including toxicity (sudden fish death, skin irritation). Due to this fact, majority of the dwellers are reluctant in using lagoon water for their daily use. However, such kind of livelihood transformations are not uncommon in Sri Lanka as well as the other parts of the world (Okello et al. 2019; Madarasinghe et al. 2020a, 2020b).
To minimize the impact of heavy metal pollution at this stage, it is recommended to implement eco-sustainable remedies like construction of a separate canal to dispose excess water coming from the Udawalawe irrigation project, introduction of a cascade system to freshwater canals before entering to the lagoon which would minimize sediment loading, periodic removal of accumulated sediments manually, and use of phytoremediation techniques.