Spatial distribution of heavy metals in surface sediments of Kalametiya Lagoon in southern Sri Lanka; insight towards pollution status and socio-economic interactions

Heavy metal (HM) pollution has become a serious threat to coastal aquatic ecosystems. This study, therefore, aimed at assessing the spatial distribution of selected heavy metals/metalloids including Arsenic (As), Cadmium (Cd), Chromium (Cr), Lead (Pb), and Mercury (Hg) in surface sediment (0–15 cm) samples collected across Kalametiya Lagoon in southern Sri Lanka. Forty-one (41) grid points of the lagoon were sampled and the sediment samples were analyzed for HM content by using ICP-MS. A questionnaire survey was carried out to investigate the possible sources for HM pollution in Kalametiya Lagoon. Water pH and salinity showed signicant variation across the lagoon. Overall mean value of pH and salinity were 6.68 ± 0.17 and 2.9 ± 2.2 PSU respectively. The spatial distribution of the heavy metals was not monotonic and showed a highly spatial variation. The kernel density maps of the measured heavy metals demarcated several different areas of the lagoon. The mean contents of As, Cd, Cr, Hg, and Pb were lower than that of threshold effect level (TEL) however, higher for Hg at the North Inlet. Nevertheless, it was still lower than potential effect level (PEL). Socio-economic interactions have dramatically reduced during the past two decades. Industrial sewage, river suspended sediments and agrochemicals such as fertilizers, pesticides were reportedly identied as the possible sources for heavy metal loads. Accumulation of toxic heavy metals can be minimized by detouring the water inow to the lagoon. Cr, Hg and Pb. heavy metal contents were in the descending order of Cr > Pb > As > Hg > Cd. The spatial distribution of the heavy metals was not monotonic and showed a highly spatial variation. According to the kernel maps, the following descending order could be observed: A (Outlet): Cr > Pb > As > Cd > Hg; B (Centre):


Introduction
Wetlands, as natural habitats, are dispersed worldwide, providing niches for oral and faunal communities, ecological services and economic bene ts for human welfare (UNESCO 1971;McCartney 2010). In a Sri Lankan context, Bambaradeniya (2004)  In this respect, heavy metal (HM) pollution in aquatic environments has evidently become a menace due to their high toxicity, low solubility, wide range of sources, and bioaccumulation behavior of heavy metals (Yu et al. 2008). HM pollution has further been much concerned since it poses a high risk to aquatic organisms at different tropic levels due to biomagni cation of xenobiotics through the food chain (Liu et al. 2018). In recent decades, a large amount of agricultural, industrial and domestic pollutants especially heavy metals, have been ended up in coastal aquatic ecosystems like lagoons, estuaries etc. (Wuana and Okieimen 2011;Kodikara 2021). Sediments at the bottom of the water body act as an indicator that is sensitive for monitoring of contaminants. They can also act as a sink and a transport agent for aquatic pollutants (Ulbrich et al. 1997). Therefore, determination of heavy metal contents in aquatic sediments is imperative to identify sources of heavy metal pollution (Diop et al. 2014). Further, spatial distribution of heavy metals in aquatic sediments can also provide valuable information regarding the impact of discharged waste on ecosystems and associated risk (Yan et al. 2010). This approach assists effective management of polluted aquatic ecosystems and also serves to monitor such systems (Li et al. 2016).
Whereas several studies in Sri Lanka have been carried out to assess the heavy metal content in food crops, land sediments, and atmospheric deposition (Herath et  Taking these facts into account, this study set out to assess selected heavy metals/metalloids in surface sediments in Kalametiya Lagoon and their spatial distribution. The following research questions were addressed a) what is the average content of major heavy metals in Kalametiya Lagoon in 2020; b) what is the spatial distribution of HMs in the lagoon; c) Do the inhabitants around the lagoon still have interactions with Kalametiya lagoon.

Methodology Study site
This study was conducted on the southern coast of Sri Lanka during the period of February-2019 and December-2020. The study site; Kalametiya Lagoon was selected for the study based on the fact that this coastal lagoon is highly affected by the aforementioned Udawalawe irrigation project. These intensive activities have caused deposition of loads of heavy metals in the lagoon resulting in changes in the lagoon biology and ecology. Therefore, this coastal ecosystem on the southern coast is considered as a model site to study HM pollution. Kalametiya Lagoon (6 0 04' 26" -6 0 07'19" N and 80 0 54' 43" -80 0 57' 25" E), located in the dry zone (annual rainfall is < 1750 mm) about 65 km east from Matara ( Fig. 1), is the largest lagoon on the southern coast of Sri Lanka, having an area of about 4.8 km 2 at present. Several canals, streams and reservoirs feed Kalametiya Lagoon and Kuchchigal Ara, a seasonal river, is the main freshwater in ow to the lagoon. Most of the lagoon is muddy and covered with marsh vegetation. A narrow canal, reinforced by a rocky dyke, has been constructed under the Udawalawa irrigation scheme as a pathway for continuous out ow from the lagoon to the sea.

Questionnaire survey
A questionnaire survey was conducted to obtain an overall view of the current socio-economic status of the lagoon and the possible effects of heavy metal pollution on socio-economy of the coastal dwellers. A standardized questionnaire (open-ended) was designed to evaluate the socio-economic status of Kalametiya Lagoon using a preliminary eld survey and interviews. The key stakeholders were identi ed through a combination of methods: expert knowledge elicitation and preliminary eld survey. The snowball sampling technique (Atkinson and Flint 2001) was used to identify further resource persons from whom more information could be collected. A face-to-face interview was carried out as a two-way interaction between respondent and interviewer (Kelley et al. 2003). In total, 45 people, including ve community leaders, were interviewed individually in the survey. All men and women who have been living in the proximity of the lagoon, not less than 18 years were selected. Also, data from the questionnaire were cross-checked with secondary literature and eld surveys to the best of our abilities.

Data Analyses and kernel density mapping
Heavy metal contents and physicochemical parameters were treated as continuous variables and descriptive statistics were studied along with data distribution patterns. Parametric assumptions such as normality of the data by the Shapiro-test, homogeneity of variances by the Levene's test were veri ed. As all the conditions were met for the dependent variables at 95% con dence level, One-way ANOVA was performed to check the level of signi cance of heavy metal distribution among the sampling regions. Similarly, the level of signi cance was tested for physicochemical parameters among the sampling regions using One-way ANOVA. Pearson correlation test was performed to study correlation between physicochemical parameters of lagoon water and the heavy metal contents. All statistical analyses were performed using R-3.2.2 statistical software. Kernel density maps were prepared using the heavy metal contents obtained from 60 sediment samples representing the total spatiality of the lagoon. The maps were processed on ArcMap v. 10.6 .

Physicochemical parameters
The results of the physicochemical parameters of water samples from ve regions (A-E) in Kalametiya Lagoon are presented in Table1. The pH ranged between 6.58 -6.92 with overall mean value of 6.68±0.17. Water acidity varied with different regions of the lagoon and water collected near the North Inlet showed the highest acidity. The water samples collected from B (Centre), D (Wide East Stream), E (North Inlet) showed signi cantly higher (p<0.05) water acidity as compared to A (Outlet) and C (Narrow West Stream). Similarly, level of salinity also showed some variations across the lagoon. The highest salinity (i.e. 5.1±0.06 PSU) was recorded near region A (Outlet) while the lowest (i.e. 1.1±0.25 PSU) was given for water near region E (North Inlet). The overall mean salinity of the lagoon for the studied period was 2.9±2.2 PSU. Signi cantly higher salinity (p<0.05) was recorded in region A (Outlet) as compared to region E (North Inlet). The level of salinity in regions B (Centre), D (Wide East Stream) and C (Narrow West Stream) was signi cantly higher (p<0.05) than region E (North Inlet) while signi cantly lower than (p<0.05) in region A (Outlet).

Spatial distribution of heavy metals
The analytical results of the studied heavy metals are presented in Table 1. When overall mean content was considered, heavy metal contents were in the descending order of Cr > Pb > As > Hg > Cd and the content ranges (mg/Kg) of the samples were 1. The heavy metal content results further showed that the mean contents of As, Cd, Cr, Hg, and Pb were lower than that of threshold effect level (TEL) while it was higher for Hg at the E (North Inlet) region. However, it was still lower than potential effect level (PEL). Similarly, none of the studied heavy metal exceeded effect range low (ER-L) and effect range medium(ER-M). According to correlation results, Cr (r= 0.56) and Pb (r=0.45) showed positive signi cant correlation with water pH while Hg (r= -0.53) had a negative signi cant correlation with salinity. The rest did not show any signi cant correlation with water pH and salinity.
Socio-economic status of the lagoon There were 65% male and 35% female respondents. According to the questionnaire survey, 32% of the respondents engaged in lagoon shery for their daily subsistence, 16% were farmers and the rest (52%) involved in various occupations. Moreover, 53% of the farmers claimed that they had been using fertilizers such as Urea, Potash, Rock dust fertilizer, and general fertilizers for paddy lands (i.e., nitrogen [N], phosphorus [P], and potassium [K]). Moreover, 21% of interviewees stated that they had not used fertilizers and the rest (26%) did not clearly answer the question. Dwellers in the area mentioned about the bene ts that they have obtained from the lagoon during 2000-2019; 79% said that the local people largely engaged in shing in the past, but now many of them are working in the bird sanctuary as tourist leaders in the lagoon. In addition, 90% of the respondents mentioned about the changes that had taken place during the past two decades; a) decrease of bird diversity and density; b) increase of mangroves and Typha (water sedge) plants; c) decrease of water salinity and lagoon depth; d) increase of freshwater sh species with shallow water column; e) bulky sedimentation has started with the onset of Udawalawe irrigation project; f) 75% of the respondents mentioned that lagoon water has become more toxic and hence was not no longer using used for drinking and bathing purposes. Furthermore, the respondents mentioned about the managerial actions that should have been taken to conserve Kalametiya Lagoon. The suggestions included a) removal of excess sedimentation; b) removal of aquatic invasive plants; c) widening of lagoon outlet; d) plantation of brous root plants along the main freshwater channel; e) strengthening institutional coordination; both government and non-governmental institutions.

Discussion
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 arti cial dyke was constructed during the irrigation project, the ebb-ow 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 guring 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 re ects 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 elds in the area (Dissanayaka and Chandrajith 2009). There is a high likelihood to get solubilized phosphate fertilizers when excess freshwater ows 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 identi ed 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 e uents (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 in uences the heavy metal concentrations by altering bioavailability and toxicity. Metals such as Cd, Pb and Hg are most likely to have signi cant 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 re ected by signi cant 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-ow system of the lagoon which happened due to the arti cial 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 a nity 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 gures in the other water bodies. Since this excess freshwater ow continues, higher levels of pollution can be expected soon. On the other hand, survey data clearly re ect that socio-economic interactions have become minimal and less than 5% of the dwellers now depend on lagoon sheries. Further, the dwellers have indirectly experienced water quality changes including toxicity (sudden sh 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. 2020aMadarasinghe et al. , 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.

Concluding Remarks
Kalametiya Lagoon in southern Sri Lanka is polluted with heavy metals like As, Cd, Cr, Hg and Pb. heavy metal contents were in the descending order of Cr > Pb > As > Hg > Cd. The spatial distribution of the heavy metals was not monotonic and showed a highly spatial variation. According to the kernel maps, the following descending order could be observed: A (Outlet): Cr > Pb > As > Cd > Hg; B (Centre): Cr > Pb > AS > Cd > Hg; C (Narrow West Stream): Cr > Pb > As > Cd > Hg; D (Wide East Stream): Cr > Pb > As > Cd > Hg and E (North Inlet): Cr > Hg > Pb > As > Cd. The mean contents of As, Cd, Cr, Hg, and Pb were lower than that of Threshold Effect Level (TEL) and Potential Effect Level (PEL). Similarly, none of the studied heavy metals/metalloids exceeded Effect Range Low (ER-L) and Effect Range Medium(ER-M). Therefore, Kalametiya Lagoon is not at stake at this point. However, if this trend continues, higher levels of pollution can be expected soon. On the other hand, survey data clearly re ect that socio-economic interactions have become minimal and less than 5% of the dwellers now depend on lagoon sheries.

Declarations Con icts of interest/Competing interests
The authors have no con icts of interest to declare.  Tables   Table 1 Descriptive details of heavy metals contents (mg/kg) in the surface sediments of the study area. Mean ± SD of heavy metals are reported for the results obtained from the present study.