4.1 Radioactivity level variations of 210Pb in the sediment between Sunda Shelf seas (sSCS and Malacca Strait)
210Pb radioactivity levels in the sediment of the sSCS recorded from this study is comparably close, with previous studies conducted by (Bakar et al. 2010). A few other shallow water regions around the world have also recorded values that are comparably close to Malaysian waters for 210Pb in their surface sediments such as in the Ghazaouet bay in Algeria, Izmir Bay and Aegean Sea in Turkey (Noureddine and Baggoura 1996; Saçan et al. 2010) as listed in Table 5.
Table 5
Comparison of 210Pb radioactivity in surface marine sediments with data from previous studies and other regions of the world (Bq/kg).
Location | 210Pb (range) | 210Pb (average) | References |
Present study, Malaysia |
offshore Malaysian Peninsular | 14.71–109.16 | 46.69 ± 2.31 | This study |
sSCS, Malaysian Peninsular | 14.71–65.52 | 40.91 ± 2.18 | This study |
Malacca Strait, Malaysia | 26.86-109.16 | 58.25 ± 4.45 | This study |
Previous study, Malaysia |
sSCS, Malaysian Peninsular | 18.3-123.1 | 54.38 | Ahmad et al. (2010)/ [16] |
Global comparison |
Ghazaouet bay, Algeria | 60–131 | - | Noureddine & Baggoura (1997)/ [18] |
Izmir Bay, Turki | - | 44 | Sacan et al. (2010)/ [19] |
wSCS, China (eastern Hainan island) | 37.4–199 | 110 | Huang et al. (2013)/ [21] |
The nearest study was conducted by (Huang et al. 2013) of the sediments off eastern Hainan Island, China located in the western SCS (wSCS). Result from the study recorded a range that is comparable to this study, which was conducted in the sSCS where 210Pb (excess) in surface sediment of wSCS falls within a range of 37.4–199 Bq/kg. However, the average value of 210Pb for this region’s water is much higher; recording 110 Bq/kg compared to the average value obtained in this study in the sSCS, which only recorded a value of 47 Bq/kg. (Huang et al. 2013) explained that the low sedimentation rate in the study area were among the causes that lead to the result of a higher reading of 210Pb compared to other shallow water areas as reported by other studies (Allison et al. 2000; Dai et al. 2011; Du et al. 2010).
In addition, geographical factors and latitude differences might also play a significant role and cause lower radioactivity of 210Pb in the sSCS if compared to wSCS. This is supported by a study conducted by (Preiss et al. 1996) showing that 210Pb radioactivity in the atmosphere is generally higher in northern latitudes than southern due to high asymmetry between continent/ocean ratios which consequently affects the sources of 210Pb between the two hemispheres. The study found increasing values of 210Pb from latitude of 0°N to 30°N while the recorded result in the southern hemisphere is lower and drops dramatically when it reaches latitudes of 40°S which is known to have zero continental sources [4]. The lack of continental sources in the southern 45°S, wSCS with strong circumpolar circulation, contributes to very low 210Pb content in Antarctica (Preiss et al. 1996). This phenomenon can reasonably explain the differences in 210Pb concentration recorded between the wSCS and the sSCS found in this study. The sampling site studied by (Huang et al. 2013) in wSCS is located in higher latitudes (18°N-20°N) of the northern hemisphere while the sSCS area in this study is located at 1°N-7°N. Thus, it can be concluded that the higher 210Pb radioactivity recorded in the wSCS is because this region’s water is more directly exposed to the larger continental source from the Asian mainland if compared to sSCS which only has direct exposure to continental sources in Southeast Asia.
Table 5. Comparison of 210Pb radioactivity in surface marine sediments with data from previous studies and other regions of the world (Bq/kg).
The significantly higher activity in the Kelantan Delta compared to the Pahang Delta as can be seen from the result indicates that the former received higher anthropogenic inputs than the latter. The Kelantan River is known to have become more turbid since the early 1990s because of high TSS due to upstream logging and sand mining activities (Yen and Rohasliney 2013). Additionally, pollution from agriculture, which has become the main activity on the East Coast of Peninsular Malaysia and Thailand is also a major contributing factor to higher 210Pb activity as recorded in this Delta.
Both Kelantan and Pahang Delta show same pattern of 210Pb distribution at which the stations that located closer to the river mouth have higher 210Pb readings compared to those further from the river towards the open sea. This phenomenon shows significant fluvial roles in the contribution of anthropogenic inputs of 210Pb to the marginal sea that originally come from the nearest mainland and major rivers affected by intense human activities.
Apart from frequently being identified as being closely associated with pollution activities, the content of organic matter in sediment is also known to be a factor that can affect radionuclide behaviour in sediment (Koch-Steindl and Pröhl 2001). Results from a study on the distribution of organic matter in the Kelantan Delta by (Naam et al. 2018) showed the same distribution pattern as 210Pb in this study. The percentage of organic matter and organic carbon in Kelantan Delta sediments have the highest reading of all the stations nearest to downstream and coastal areas when compared to outermost stations that record much lower readings. This indicates that offshore stations in close proximity to deltas have a high potential to receive 210Pb influenced by anthropogenic sources such as mining, agriculture, municipalities and industrial areas that are continuously developed on the East Coast of Peninsular Malaysia
There are a number of possible sources that could cause the high 210Pb radioactivity level found in surface sediments of the studied area as can be concluded from Fig. 4. The first is due to the high input of sources from continental areas such as from Sumatra Island, from Peninsular Malaysia itself, and the countries in Indo-China through atmospheric transport contributing to the high input of airborne 210Pb or their parent, 222Rn gas in the study area. Huge resource transportation in the atmosphere will eventually cause 210Pb excess (unsupported) to be highly accumulated in surface sediments on the seafloor.
The second possibility is closely related to the highly dynamic sediment deposition environment of the Straits of Malacca attributed to its complex physiography, oceanographic and climate system (Keller and Richards 1967) with the addition of myriad sediment sources contributed by the bordering mainland of Sumatra and Peninsular Malaysia. In fact as was mention in (Keller and Richards 1967), the depositional environment for the Strait of Malacca can be divided into four areas based on sedimentary texture parameters: North (north of Penang Island), Central (Penang Island to Klang), "Narrow" (Klang to Malacca) and South (Malacca to Singapore). Each of these areas show differences in oceanography and the influence of the river onto its ecosystem that eventually leads to the conclusion that sediment distribution in the north of the Strait of Malacca is almost dominantly controlled by the current; while in the south, both currents and streams are taken into account. This simply means that the Strait of Malacca always has huge and varied sources of material input into its marine ecosystem which also involves radionuclide sources. A lot of factors influence their fate and distribution after entering the strait’s ecosystem due to its highly dynamic environment. These factors involve all physical, chemical and biological processes such as sediment resuspension, ion-exchange, sorption-desorption, bioturbation and many more.
Because of the monsoonal effects on its neighboring seas, the Strait of Malacca generally has currents that flow in the northwestern direction throughout the year (Keller and Richards 1967). However, during the southwest monsoon, this current will become weak and often results in southeastern flow intrusion that comes from the Indian Ocean current pattern, consequently causing a pile up of water masses in the Andaman Sea (Keller and Richards 1967). A lot of recent studies such as work by (Rizal et al. 2010) are in agreement with an earlier study that derived a simulation model of the northwest current pattern dominating the strait. Other studies also revealed the intrusion of water masses from the Indian Ocean and the Andaman Sea in the northern part of the Strait of Malacca resulting from southwesterly winds (Mayer et al. 2015; Muhaimin et al. 2011). This theory is more convincing; there is a record of high percentage of organic matter from fine sediments deposited in the Andaman Sea and the influx of these sediments into the Malacca Strait are reported to consist mainly of planktonic foraminifera transported by the bottom current from the southeast current circulation (Keller and Richards 1967).
The Andaman Sea is thus seen as a capable source that supplies and contributes to significantly high 210Pb radioactivity in SM1 waters, especially in September. The statistical results of this study show that the northern part of the Strait of Malacca (SM1) is significantly different from 210Pb in both sSCS zones (sSCS1 & sSCS2). This strongly proves that the Andaman Sea is the most possible source of high radioactivity levels in SM1 as the sources cannot otherwise significantly reach the sSCS region. As mentioned earlier, in the southern part of the Strait of Malacca, sediment distribution is controlled by both currents and river runoff. Thus, the comparatively high 210Pb in SM2 most probably comes from both current and river sources that flow from the bordering land masses as there are 14 rivers in Sumatra and 12 rivers on the west coast of Peninsular Malaysia (Muhaimin et al. 2011). These rivers are known to produce comparatively significant amounts of annual discharge into the Strait of Malacca (Muhaimin et al. 2011).
Looking at this phenomenon in the eastern maritime region of sSCS (SCS1), the farthest areas from shore towards the open ocean have recorded a reading of 210Pb radioactivity, which is comparatively higher than stations located near the east coast of Peninsular Malaysia. The results of this study are in agreement with an earlier study by (Bakar et al. 2010) which report that terrestrial inputs from natural processes cannot explain such high radioactivity; the study speculate that the phenomena is likely derived from lithogenic sources in the area.
Higher 210Pb activity recorded in areas closer to shore in the southern maritime zone of sSCS (sSCS2) shows the possibility of a strong upwelling phenomenon in the area. Many studies conducted along the east coast of Peninsular Malaysia have shown that the Ekman dynamic formation from southerly-southwesterly wind has resulted in the existence of upwelling systems in this region such as a recent research by (Kok et al. 2017) reveals that the Ekman pump plays a major role in the upwelling formation in any month in the southwest monsoon season along the east coast of Peninsular Malaysia if compared to Ekman transport. However, the study found that both Ekman transport and Ekman pump were similarly common in southern coastal areas and thus might cause stronger upwelling activities in the region. In previous studies, suspended particle flux was found to be high in proximity to the upwelling region (Hong et al. 1999; Lutz et al. 2002; Thunell 1998). The behaviour of 210Pb that can be scavenged and removed from the water column to the bottom of the seafloor by the sinking and settling of suspended particles consequently revealed the relationship between the upwelling process and high radioactivity levels in that particular region.
The three stations showing low radioactivity level of 210Pb in the Straits of Malacca, M55, M58 and M59 is likely due to its sediment particle size which classified as coarse sandy sediments. It can be clearly proven with their lower porosity compared to the average porosity of all the sediments sample at Straits of Melaka which calculated as 63%. Porosity is known to have an inversely proportional relationship with particle size. Analysis of Pearson (r) bivariate product moment correlation coefficient was conducted and the results found that sediment porosity showed weak positive correlation r = 0.197 (p > 0.05) with the activity of 210Pb in the sediment. Thus, we can conclude here that the radioactivity of 210Pb in the sediment will also influenced by the sediment particle size or its porosity. This is in agreement with study by (Huang et al. 2013) which shows sediments of silt and clay are always recorded to have high radionuclide readings compared to sandy type sediments.
4.2 Different input sources of 210Pb and the roles of insitu parameter in water column
The eastern maritime region of sSCS (sSCS1) which shows different 210Pb pattern from the other zones by having a higher 210Pb activity on the surface of the seawater compared to its bottom layer strongly shows that the region is mainly receive 210Pb input from the atmosphere and therefore proves that seawater in this region is more influenced by wind-borne sources from airborne 210Pb or gaseous 222Rn in the atmosphere. Depleting activity when going down through the water column indicates that scavenging may occur between the water-sediment medium.
In the other hand, the higher 210Pb at the bottom layer of the water column at the southern maritime regions of sSCS (sSCS2) and the Straits of Malacca is said to occur in the event of a resuspension on surface sediment or the occurrence of lead dissolution from the bottom seafloor (Tanaka et al. 1983). These increasing trends of vertical 210Pb towards the bottom layer of water column strongly support the discussion in Sect. 4.1; the influence of in situ seafloor sediment processes in the distribution of 210Pb is attributed to the dynamic deposition environment, involving hydrodynamics that might increase sediment resuspension rates or complicated geologic compositions of the sediment because of complex origin sources. These complex origins include the fact that numerous rivers provide input from bordering land masses in both Sumatra and Peninsular Malaysia. The result that shows highest 210Pb values in the bottom layer of the water column compared to its surface at the sSCS2 stations might also support upwelling transport of radionuclides in this region during the southwest monsoon. This is because the upwelling current can also trigger sediment transport at the seafloor and might lead to the high rate of sediment resuspension in the area.
The result from this study further demonstrates the great potential of the SCS especially in the area of the eastern maritime zone of sSCS to be selected as a better station for sedimentology research focusing on the impact of monsoonal changes as it is proven to be dominated by wind-driven radionuclides compared to the Malacca Strait. The unique geological characteristic of Malacca Strait that has a very high 210Pb radioactivity level which influenced by various input sources will eventually complicates the process of source identification. Based on (Keller and Richards 1967), wind-driven sediments actually exist in the Malacca Strait in the form of volcanic ash and pumice stone, but it is very small and does not indicate significant existence when compared to the quantity of river-borne materials deposited on the seafloor. Consequently, most of the ash found in the Malacca Strait is considered detrital rather than wind-borne (Keller and Richards 1967). Therefore, although the Malacca Strait is a part of the geographical area where the monsoon affects climatic and oceanology conditions, the direct impact of the seasonal wind is very difficult to see in this region because it is protected by archipelago land masses that overshadow the monsoonal effect with their massive terrestrial processes.
Results from Table 3 prove that the variation of surface sea conductivity and salinity can significantly affect the behaviour and distribution of 210Pb radioactivity on the surface layer of the ocean. It is widely recognised that salinity and conductivity are interconnected. Often, conductivity will increase with increasing salinity due to an increase in soluble ions in the seawater. This can be proven from the correlation test in this study which shows a very strong Pearson correlation between conductivity and salinity at the surface of seawater.
A study by (Zinabu et al. 2002) also shows a good correlation between conductivity and cations such as Na+, K+, Ca2, Mg2+. According to (Beneš and Černík 1992), kinetic behavior of the radionuclides in the environmental medium is controlled by important parameters such as pH value. (Carvalho 1995) has shown that adsorb-desorption behavior of radionuclide like 210Pb on the environmental medium can be affected with the precipitation of iron-manganese hydroxides which is known to have the ability to affect the pH parameter in the water system. (Carvalho 1995) has proven that the increased concentration of both 210Pb and 210Po in particulate matter in the studied area is possibly because of this mechanism of unselective co-precipitation. Therefore, it can be summarized that the increase in conductivity and salinity will potentially contribute to high 210Pb radioactivity levels at the surface of the seawater especially in shallow water regions such as the waters of Peninsular Malaysia.
4.3 Impact of excess 210Pb in coastal surrounding Peninsular Malaysia
In Table 1 and Table 2, the excess 210Pb is much higher compared to 226Ra in the deltaic region, where there was a difference of 55.5 % and 32.1 % in Kelantan and Pahang River respectively. Meanwhile, the higher value of 226Ra exhibits in both coastal waters of Malacca Straits and sSCS, where there was a difference of 53.6 % present in the sSCS while about 19.4 % difference in Malacca Straits comparing to the excess 210Pb within same coastal waters. This illustrates the high concentration occurred within the river mouth before being diluted and mobilized towards the coastal region in sSCS. On the other hand, the shallow depth of Malacca Straits may lead to the continuous physical activities (i.e., resuspension, absorption, desorption, diffusion), which lead to higher removal between water-sediment interface (von Brömssen et al. 2008). Earlier hypothesis would suggest the continuous supply of the Kelantan and Pahang Rivers may affecting the total load present in the sSCS (Rahim et al. 2021), while the Malacca straits may affect to the external inputs such as Andaman Sea and Martaban Bay influencing the total flux present in the Malacca Straits (Ramaswamy et al. 2004). For the deltaic region, the higher concentration of excess 210Pb was contributed by the wet and dry atmospheric deposition where monsoonal seasons was responsible to the periodic precipitation occurred in both Kelantan and Pahang deltaic region (Rahim et al. 2021). Eventually, this would lead to the erosion within the riverbanks, thus increasing the excess 210Pb in the river channel. Furthermore, the terrestrial input contributed by the anthropogenic inputs (i.e., municipal wastewater, husbandry operation and sand mining) may lead to the increase excess 210Pb compared to the 226Ra (San Miguel et al. 2004). On top of that, the in-situ decay within terrestrial and mobilized via runoff may increasing the total concentration of excess 210Pb present in the deltaic region.
Low concentration of 226Ra present in the deltaic region would suggest the present of mobilization and dilution of suspended particulate phase. Basically, the runoff present in both Kelantan and Pahang Rivers was originated from multiple terrestrial sources, thus releasing a large amount of suspended particulate matter (SPM) towards the deltaic region. On top of that, the effect of desorption and absorption while co-existed with oxyhydroxides such as Fe, Al, and Mn under the co-precipitation reaction in the sediment -water interface (Nirdosh et al. 1990). As mobilization occurred, the 226Ra which having a high affinity towards the SPM was decaying to 210Pb, thus leading to the freshly-deposited 210Pb within the sSCS region. Furthermore, (Zakaria 1975) stated that the high discharge rates on both Kelantan and Pahang Rivers contributed to the excess radium, as the mobilization of radium was being carried out as active dilution, thus leading to higher volume of radium towards the sSCS.
On the other hand, similar occurrence of sSCS occurred where higher value of 226Ra present within the Malacca straits. This may suggest the geologic existence of surrounding island and archipelago which contributed to the higher 226Ra concentration. Similarly, the presence of natural 238U was found by (Khandaker et al. 2018) where presence of black sands along the Langkawi Island contributing the 238U increment towards the straits. Several known content of black sand such as ilmenite, zircon, monazite, magnetite, garnet, rutile and allanite, leading to the elevated 238U and its daughters. Furthermore, the presence of granitic rocks also contributed to the increment of 226Ra in the Malacca Straits. This can be obtained through Langkawi Island and its surrounding cluster of islands where the granitic rock become a viable source for the Malacca Straits (Leman 2010). The main composition of granite consists of SiO2, as it’s displaying a uranium affinity within the igneous rocks and exhaling radon onto surface horizon from weathered source (Przylibski 2004). Furthermore, the oversaturation of the 226Ra within the straits suggesting the boundary scavenging process contributed to the elevated concentration of 226Ra. Mobilization of weathered product occurred within the Irrawaddy River, leading to the longshore transport along the Martaban Bay towards the southern Thailand (Ramaswamy et al. 2004). This eventually leads to excess amount of 226Ra through lateral transport via wave current and Ekman transport which carry high load of nuclides from other source towards the Malacca Straits (Bruland et al. 1974).