Introduction

Wetlands/Lakes contribute to water requirements for local residences and agricultural and industrial activities worldwide, and they are considered the kidneys of the landscapes. River, rainwater, brackish, and glacial meltwater are essential lake water sources (Fairbridge 1968). The wetland accumulates with the sediments regularly via streams, rivers, and entering overland flow. Aggregation of the sediments in the plain region of the lake’s environs depends on the slope, vegetation cover, and geology of the lake catchments. On the lake floor, the sediment deposits happen slowly as a natural process, reducing storage capacity light penetration, encouraging biotic growth, and affecting the ecosystem’s functioning. Elevated anthropogenic activities in recent years have affected the hydrological regime of lakes in the country (Singh et al. 2008). Almost 12% of the global human population lives in the mountain regions (Das and Vasudevan 2021). At altitudes above 3500 m Mean Sea Level (MSL), the wetlands formed due to the glacial meltwater and are defined as “glacial wetlands” (WWF 2005). The wetland in the high altitude regions experiences high input of sediment loads due to the steep inclines and transportation of dissolved materials due to water, occasional variation in the diverse plant canopy, and later formative exercises (Das et al. 1994; Diwate et al. 2020). At high altitudes, Himalayan wetlands indicate an enormous input of sediments that diminish the lake environments. The higher sediment inputs and lithological vulnerability are mainly worth noticing in the variation of high altitude Himalayan Lake (Kothari 1996; Kumar 1999a, 1999b; Rai 2007b; Singh 2007; Singh et al. 2008; Humane et al. 2016). Many researchers have investigated the wetlands sedimentation rate and deposition characteristics (Kusumgar et al. 1989; Das et al. 1994; Kumar et al. 1999c; Das and Vasudevan 2021; Singh and Vasudevan 2021; Rout and Vasudevan 2022; Palani et al. 2023; Guan et al. 2023; 2024). In the wetlands, the enormous input of the sediments deciphers the death of the lake environments (Mondal et al. 2012; Nautiyal et al. 2012; Chandrakiran and Kuldeep 2013; Khadka and Ramanathan 2013; Bhat and Pandit 2014). The insights of the present study will support erosion control, sediment impediment, and future lake management strategies.

The radioisotopes from natural and artificial sources can be found in lake sediments. To estimate the sedimentation rate, the radioisotopes 210Pb (Lead-210) and 137Cs (Cesium-137) have been particularly helpful in dating the sediment deposit in lakes and reservoirs. Out of all the radioisotopes, the 210Pb and 137Cs activities occur naturally and artificially in the aquatic environment (Ritchie and McHenry 1990; Edington et al. 1991; Kumar et al. 2007; Semertzidou et al. 2019; Singh and Vasudevan 2021). Natural radionuclide 210Pb and anthropogenic 137Cs were used to estimate the current sedimentation rate in lake environs (Edington et al. 1991; Putyrskaya et al. 2020; Strakhovenko et al. 2020). It is worth noticing the 210Pb activities for evaluating wetlands sedimentation rates and unveiling the chronological age of the lakes. The half-life of the 210Pb indicates 22.20 ± 0.17 years (Goldberg 1963) due to 226Ra (Radium-226) decay in the sediments, resulting from the 238U (Uranium-238) decay chains. Calculate the sedimentation rates over 120–150 years using the 210Pb radionuclide sources (Corcoran et al. 2018). A significant quantity of the artificial radionuclide 137Cs was presented in 1963, primarily due to atmospheric fallout from thermonuclear weapon tests during the 1950s and 1960s (UNSCEAR 2000). Several incidents, such as the Chornobyl Accident (CA) disaster on April 26, 1986 (Appleby 2001; Shah et al. 2020), and the Fukushima Daiichi Nuclear Power Plant (FNPP) incident on March 11, 2011 (Butler 2011; Chino et al. 2011), have led to the enrichment of 137Cs radionuclides in the environment. The artificial radionuclide 137Cs served as a validation tool for analyzing 210Pb natural radionuclides in establishing temporal sedimentation rates and age models for wetland environments (Appleby 2001; Guo et al. 2020). This present study focuses on evaluating the status of the sedimentation rate and age model for Perumal Lake along with life computation, which can identify the influences of the rate of depositions on appropriate management of the wetland environments.

Description of study area

The present study investigates Lake Perumal, near the Neyveli lignite mines in the Cuddalore district. Intriguingly, the lake describes narrow elongated southeast slopes, and it is an artificial lake roost at an altitude above the 24.38 m MSL. The profound study of the lake argues that it is a closed basin with admissibly 13.24 sq. km and is situated east of the Neyveli lignite mines. It has significant input from the Neyveli lignite mining regions through the Paravanar River and lies between north latitudes 11° 30’ to 11° 45’ N and East longitudes 79°30’ to 79°47′30" E (Fig. 1). The physiography of Lake Perumal demonstrates the area is almost a plain coastal area, and river alluviums exist on most of its part with slightly high ground of Tertiary sediments followed by geographic features of the study reveal Gadilam and Ponnaiyar rivers in the north, and Vellar and Coleroon river in the south drains to the lake. Subsequently, the morphometric characteristics of the lake represent the undulate bottom floor with a maximum water depth of 5.6 m recorded in the northeastern part of the surveyed area of the water body (Table 1).

Fig. 1
figure 1

Sample location map of core sediment of the Perumal Lake

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Table 1 Data represents the morphometric parameters of Lake Perumal
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Materials and methods

Bathymetry forecast

Evaluating the bathymetry of a lake basin is crucial for assessing the lake’s condition and comprehending its management prospects and stipulations. A Garmin GPS Map 178 (Chartplotter/Sounder) was equipped with a dual-frequency 50/200 kHz transducer, providing a 10-cm vertical accuracy within the 120-degree and 450-degree cone angles. The transducer is fitted on the back of the boat, the Global Navigation Satellite System (GNSS) receiver external antenna is mounted above the transducer, and the displaying unit is positioned on the front side of the boat (Szarlowicz et al. 2017). Forty-eight bathymetric data points were rigorously collected, along with their corresponding paths and locations. The bathymetric points were accurately acquired using a GNSS receiver to determine the lake’s depth and then imported into ArcGIS 10.8.2 software for analysis. In the bathymetric modeling process, inverse distance weighting (IDW) interpolation was employed to anticipate measured depths from adjacent locations to estimate depths at unknown points (Bedient and Huber 1992; Burrough and McDonnell 1998).

Core sampling and analysis

On March 2023, the one core sediment was reclaimed at a depth of 5.0 m and was chosen based on a bathymetry map from the center part of the closed basin at the geo-coordinates (latitude  – 11°34′30" N & longitude  – 79°40′20" E) of the Perumal Lake employs the gravity coring method. Temporal core sediment recovered with a diameter of 58 mm, followed by a length of 51 cm. The retrieved core was securely packed and kept in a vertical orientation during sampling to prevent any disturbance to the sediment within the coring tube. To avoid sediment compaction during transportation, proper precautions include keeping the containers stable and upright, avoiding excessive shaking or jolting during transport, using appropriate cushioning material to minimize movement, and securing the samples to maintain their original structure. The core sediment was safely transported to the laboratory for further analysis.

The core sediments were stored in the freezer at + 4 °C for seven days before sub-sampling into 1 cm intervals (Gharibreza et al. 2013). To remove the moisture content, the sediment samples were subjected to a hot-air oven to dry at 70 °C for 12 h (Sert et al. 2016). The quantification of dried samples was determined to concede the bulk density, and the dry weight sample helps us achieve the water content based on the calculated weight reduction of dry samples subtracted from the wet samples. Sediment samples were dried before calculating the percentage volume of water in the pores relative to the total volume of sliced core sediment samples (Das and Vasudevan 2021; Palani et al. 2023). The sediment samples were again heated at 60 °C for about 24 h to remove the moisture content in the hot air oven. Dry sediment samples were gently crushed by using agate mortar at a grain size < 200 µm and were homogenous. The sediment samples were utilized for subsequent analyses, including assessments of grain size distribution and identifying radioactive nuclides as part of further investigations.

The grain size analysis was performed with the Horbia LA-300 Scattering particle size analyzer. The sediment samples were preprocessed by the double distilled water + hydrogen peroxide (H2O2) with a ratio of 1:1 and Double distilled water + hydrochloric acid (HCl) with a ratio of 2:1 to decrease the effects of soluble salts, organic matters, and carbon contents. Additionally, at the time of analysis, a pinch of Sodium hexametaphosphate (NaPO3) was added to separate the fine sediments from the bonding of the particles (Singh and Vasudevan 2021; Palani et al. 2021, 2023). The anticipated outcomes were standardized using the Wentworth scale method to determine the sediment fractions (Wentworth 1922).

To appraise the 210Pb and 137Cs radioactivity using the α and γ spectrum analyzing system assists in the Analytical Chemistry Division, Bhabha Atomic Research Centre (BARC), Trombay, Mumbai, India. From the 01 g of powdered sediment samples, the radiochemical separation of 210Po was digested using a closed microwave digested system (Milestone Srl, Model Ethos, and Italy) pricked with 209Po tracers, and HF: HNO3 (1:1) as well as hydrogen peroxide (H2O2) in Teflon beakers as per the guidelines in the manual from the manufacturer. To reduce the Fe, add ascorbic acid to convert Fe3+ to Fe2+ with the aid of 210Po and 209Po mounted in the silver planchets along with 0.5 N HCl solutions at 90 °C for three hours. 210Po activity was estimated with alpha energy of 5.30 MeV using 209Po (4.88 MeV alpha emission) as the internal tracer by alpha spectrometry (ORTEC, OCTAT) with 13% efficiency, 20 keV resolutions, and 300 mm2 areas with a depletion depth of 100 nm. Intercalibration exercises were also performed using standard samples (IAEA-414). Blanks were run through the same analytical procedures as samples to ensure no contamination from analytical reagents. Afterward, meticulous observation was carried out on 210Pb activities were determined from its granddaughter (210Po) with alpha spectrometry during the secular equilibrium stage (Wood et al. 1997; Sanchez-Cabeza et al. 1998; Henderson et al. 1999; Al-Masri et al. 2003; San Miguel et al. 2004). Each segmented samples were counted ranging from 6 × 104 to 8 × 104 s, although the background value was accounted for and calculated for every five samples for the time interval of 2 × 104 to 3 × 104 s. The formula adopted to evaluate the specific activity (Bq/g) of the measured 210Po radionuclide through its marked ROI (Region of Interest) and activity counts. The formula has been mentioned below as,

$${\text{Specific}} \, {\text{activity}} \left( {\frac{{{\text{Bq}}}}{g}} \right) = \left( {\frac{{{\text{Counts}}}}{{{\text{Time}} \left( s \right)}} - \frac{{{\text{Bkg}} \, {\text{Counts}}}}{{{\text{Time}} \left( s \right)}}} \right) \times \frac{100}{{{\text{Efficiency}} \left( \% \right)}} \times \frac{1000}{{{\text{Sample Weight}} \left( g \right)}}$$
(1)

where Bkg stands for background counts of polonium measured as 0.025cps, which is 2000 counts in 80,000 s; 13% efficiency, 2 g of sediment weight. The 210Pbsup (210Pb—supported) activity was assessed as the mean of assorted constant 210Pbexc (210Pb—unsupported) determination in the bottom sediment layers (beneath the zone of 210Pbexc exponential decline) (Zaborska et al. 2007). The 210Pbexc was estimated by subtracting the total 210Pb activity and 210Pbsup, which are in secular equilibrium (Appleby and Oldfield 1992).

To manage the 137Cs activities using gamma spectroscopy aided with a well-type NaI (Tl) crystal detector with the size of 4ʺ × 4ʺ a well of 1ʺ diameter × 2ʺ height. Each sample was counted for 6 × 104 to 8 × 104 s for gamma counting. For a period of 2 × 104 to 3 × 104 s, the background (Bkg) values were recorded using intervals of 10 samples. The concentration of 137Cs was dictated by estimating the gamma peak at 661.62 keV with an 85% branching ratio. The lower limitation of detection, with 95% confidence, was 0.3 Bq for 24 h of measuring time. The energy calibration of the instrument was done with a mixture of 137Cs and 60Co sources, and the efficiency calibration was done with reference standard soil (IAEA-326) (Singhal et al. 2012) for the quality control of the gamma spectrometer. Reference and repeated samples showed 137Cs values in an acceptable range of activity (Kumar et al. 2008; Gharibreza et al. 2013). The formula was adopted to evaluate the specific activity (Bq/kg) of the measured 137Cs radionuclide of each sample with the aid of gamma counting.

$${\text{Specific activity}} \left( \frac{Bq}{g} \right) = \left( {\frac{{{\text{Counts}}}}{{{\text{Time}} \left( s \right)}} - \frac{{{\text{Bkg}} {\text{Counts}}}}{{{\text{Time}} \left( s \right)}}} \right) \times \frac{100}{{{\text{Gamma}} \, {\text{Abundance}}}} \times \frac{100}{{{\text{Efficiency}} \left( \% \right)}} \times \frac{1000}{{{\text{Sample}} \, {\text{Weight}} \left( g \right)}}$$
(2)

where Gamma abundance is 85.1, efficiency is 31.94%, and the background value in cps was 0.006 (480 counts in 80,000 s).

Chronology of age-depth sedimentation model

210Pb radionuclide models

The sedimentation model shows positive results in building the chronological or age depth of the sediment deposition in the wetlands. The sedimentation model has two dating approaches to decipher the chronological or age-depth. Two models are named Constant Rate Supply (CRS) and Constant Initial Concentration (CIC) (Appleby and Oldfield 1978; Robbins et al. 1978). The (CIC) model is unsuitable for this study’s criteria due to the constant rate of accumulation accompanied by a repeatedly diminishing abundance of 210Pb (Alhajji et al. 2014).

According to Krishnaswamy et al. (1971) put forth the CRS model to assess the sedimentation rate and age-depth of the core sediments with the steady 210Pb fallout from the atmosphere to the ocean or lake water at the consistent rate of supply of 210Pb to the sediments. The CRS model addresses an oscillation in sediment supply while adopting a 210Pb flux configuration. The outcome of the CRS model indicates that many sedimentary basins and sediments fluctuate due to climate or human-induced alterations. The cumulative residual 210Pbexc, Ad, beneath deposits of age t varies concerning the formula.

$$\begin{gathered} A_{d} = A_{0} e^{ - \lambda t} \hfill \\ {\text{where, }}t{ = }x/s \hfill \\ \end{gathered}$$
(3)

So,

$$s \, = \, x/t$$
(4)

(Liang Kangkang et al. 2014).


where the Ad defines unsupported 210Pb in the temporal sediments below the depth "d," and A0 is the entire unsupported 210Pb below the mud/water interface (Gharibreza et al. 2013), S is the rate of sedimentation, x indicates the depth of the sediment deposits, and t infers estimated time (year).

The sediment’s age at any depth can be calculated using the below-given formulae,

$$t = \frac{1}{\lambda }\left( {\ln A \left( 0 \right)/\left( d \right)} \right)$$
(5)

Using a temporal sediment sample from Perumal wetlands, the model’s applicability was entirely investigated.

137Cs radionuclide model

Notably, the 137Cs activity spiked in 1963, which is considered a key marker year (Albrecht et al. 1998) due to the Nuclear Test Ban Treaty. In the following years, the 137Cs spike rate decreased sharply and is scarcely accountable because of lower atmospheric 137Cs concentration. Additionally, the 137Cs peaked in 1986 may be due to the effect of the Chernobyl accident, also considered a key marker year (Palani et al. 2023; Rout and Vasudevan 2022; Das and Vasudevan 2021; Singh and Vasudevan 2021). The profound study of 137Cs activity peak variations (i.e., increase or decrease) in the depth profile did not affect the heightened area of the concentration or time marker zone (Zapata 2002; Zhang et al. 2012; Cheng et al. 2019). The formula to derive the sedimentation rate is estimated as follows,

$$r = \frac{H}{{\left( {n - Y} \right)}}$$
(6)

where r value unveils the sedimentation rate (cm/year) for this samples, H is denoted as the depth (cm) of the Key marker year Y (1963 or 1986) of 137Cs activity peak, and followed by n infers the sampling year i.e., 2023. As well as the age of the excavated core sediments above deposited respective Key marker year (1963 or 1986) depth can be defined as,

$$T_{n} = Y + \left( {\left( {H - h_{n} } \right)|r} \right)$$
(7)

Above the marker year (Cheng et al. 2019).

The formula for the sediment layer deposited below the respective key marker year (1963 or 1986) is calculated as,

$$T_{o} = Y - \left( {\left( {h_{0} - H} \right)|r} \right)$$
(8)

Below the marker year (Cheng et al. 2019).


where, Y is the marker year; Tn and T0 are the age (year) above and below the key marker year; and hn and h0 are denoted as depth (cm) for these layers, respectively. The accuracy of 210Pb dates via the CRS model is substantiated by reference to the well-determined peaks of 137Cs at individual horizons. 137Cs horizons incorporate the first appearance in sediment columns (1952–1954), the fallout maximum (1963–1964) from atmospheric testing of nuclear bombs, the Chernobyl accident (1986), and the Fukushima Daiichi Nuclear Power Plant incident (2011).

Computation of wetland life

The seasonal wetland, ephemeral wetland, or intermittent wetland diminish seasonally; the wetlands were put forth to investigate the age of the wetland, and it is no longer useful for water activities. The diminishing activity of the wetland is due to the alarming sedimentation rate, and it cannot fulfill the water essential. The wetlands should be monitored, and suitable restoration and preservation controls should be planned (Singh et al. 2008). Wetland life was determined by considering the average sedimentation rate in conjunction with the ratio of the mean lake depth in its surroundings. The formulae adopted to determine the lifespan of Perumal lake are given as follows,

$$L_{U} = D_{m} \times \frac{100}{{S_{R} }}$$
(9)

where LU is the useful life of the wetland (in years); Dm is the mean depth of the wetland (in m); SR is the rate of sedimentation by 210Pb and 137Cs (in cm/year).

Result and discussion

Bulk density, porosity, and water content

The bulk density in the core sediment ranges from 1.04 to 2.19 g/cm3, averaging 1.52 g/cm3 and the porosity of the sediments expresses from 2.76% to 53.97%, with a mean value of 32.47%, and followed by water content varies from 0.26% to 4.57%, with the average of 1.61% as shown in Fig. 2; Table 2. From the findings, the porosity and bulk density have been inversely proportional, indicating that the porosity increases eventually as the bulk density decreases (Fig. 2). In the sediment cores from Perumal Lake, it is notable that bulk density steadily increases with depth, attributed to the gradual compaction of overlying sediments, as observed in the study by Aja et al. (2015). Subsequently, the water content of the core sediments argues there is no significant relationship with the porosity of the core sediments.

Fig. 2
figure 2

Vertical profile of sand %, silt %, and clay % in the core sediments of the Perumal Lake

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Table 2 Bulk density (g/cm3), water content (%), and porosity (%) in the core sediments of Perumal Lake
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Analysis of the Characteristics of sediment deposition environments

The sediment textures in the Perumal wetland can be classified into three categories: sand (> 63 µm), silt (> 2 and < 63 µm), and clay (< 2 µm). The composition includes sand ranging from 0 to 5% with an average of 2%, silt varying from 0 to 8% with an average of 3%, and clay fluctuating from 87 to 100% with an average of 95% across the core profile. The pronounced predominance of clay content suggests a ‘low-energy environment’ (Table 3 and Fig. 3).

Table 3 Percentage of sand, silt, and clay in the core sediments of Perumal Lake
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Fig. 3
figure 3

Vertical profile of Bulk density (in g/cc), water content (in %), and porosity (in %) in the core sediments of the Perumal Lake

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Analysis of the effect of grain size on 210Pbexc activity

The investigation on the effects of grain size in the 210Pbexc activities possessing the clay content attains more importance to absorption of the 210Pbexc in the sediments supplied from the atmospheric input and river or meltwater. Several researchers suggest clay is the main controlling factor for the 210Pbexc concentration in smaller particles with larger specific surface areas (He and Walling 1996). Sun et al. (2017) proposed that the 210Pb activity of the clay is 24.7 times that of the sand and 3.2 times more that of the silt composition. Correlation analysis of the sediment composition shows the relationship between depth and 210Pbexc activities. The strong positive relationship of core sediments between the finer fraction (Clay) and depth unveils the higher content of 210Pb in the vertical core profile.

Analysis of the vertical profiling of 210Pb

According to the CRS model, the 210Pbexc dating is significantly assigned to conduct the sediment age and accumulation rates (Sanchez-Cabeza et al. 1999). Measurements of 210Pb radionuclide, the core sediment were sliced into 1 cm intervals, and the intermittent samples were undergone for analysis. The value of 210Pbexc ranges from 5.44 ± 0.43 (Bq/kg) to 80.12 ± 6.40 (Bq/kg), with a mean value of 27.27 ± 2.09 (Bq/kg), as shown in Table 4. The oldest layer of the vertical core profile at a depth of 51 cm represents the age of 1944 AD. The mean accumulation rate (kg/m2/year) and sedimentation rate (cm/year) of core sediments implies 0.57 ± 0.05 (kg/m2/year) and 0.84 ± 0.07 (cm/year) dated from 1944 to 2023 AD, and to discuss the increase and decreasing criteria in the sediment deposits within the wetland supervened by various reasons.

Table 4 Age dating and sedimentation rate based on the CRS method of 210Pb radionuclide in the core sediments of Perumal Lake
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Notably, the accumulation rate of core sediments has increased towards the top layers compared to the bottom layers. Additionally, the 210Pbexc sedimentation rate demonstrates peak values beneath the top layers, indicating that the uppermost layer experiences disturbance primarily from river inputs, as highlighted in the study by Hancock and Hunter (1999). Conspicuously, the core sediment findings imply the maximum sedimentation rate has been noted as 1.37 ± 0.11 (cm/year), and the rate of accumulation is 2.38 ± 0.19 (kg/m2/year) at a depth of 2 cm dated since 2022AD. Furthermore, another maximum sedimentation rate of 1.36 ± 0.11 (cm/year) accumulation rate is 5.29 ± 0.42 (kg/m2/year) at a depth of 1 cm dated as 2023AD. From 1944 to 1992AD, the sedimentation rate gradually increased at the depth of 51 cm to 33 cm, with a mean value of 0.62 ± 0.05 (cm/year), as shown in Fig. 4.

Fig. 4
figure 4

210Pbexc activity shows in the temporal profile (a) and sedimentation rate (cm/y) and Accumulation rate (kg/m2/y) shows in (b) of the core sediments

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Additionally, the sedimentation rate fluctuation in the core sediments may be due to the nuclear tests in 1970AD, considered as the key marker for the auxiliary value of 137Cs in the atmospheric depositions, and the industrial revolution 1960s and Chernobyl accident in 1986AD tends to increase the heat radiation in the atmosphere throughout the world unveils the rapid retreat the glaciers to increases the sediment transportation and deposition in the high altitude wetlands (Das and Vasudevan 2021; Rout and Vasudevan 2022).

Many researchers have investigated the sediment accumulation rate in the different parts of the Himalayan and Eastern Ghats wetland regions to compare it with that of Perumal Lake. The increased sediment accumulation rate in recent years can be attributed to silicate weathering and carbonate weathering, as well as anthropogenic influences from mining activities, tourism, residents, and ongoing construction activities. The sediment accumulation rate of the other wetlands, such as Bhimtal (0.22 kg/m2/year; Das et al. 1994), Nainital (0.86 kg/m2/year), Naukuchiatal (0.42 kg/m2/year), Sattal (0.3 kg/m2/year) (Kumar et al. 1999c), Mansar (0.89 kg/m2/year; Rai et al. 2007a), Satopanth (0.31 kg/m2/year; Das and Vasudevan 2021), Pykara (0.26 kg/m2/year; Singh and Vasudevan 2021), Chandratal (0.63 kg/m2/year; Rout and Vasudevan 2022), and Kodaikanal (1.00 kg/m2/year; Palani et al. 2023). The Nainital wetland in the Himalayan region shows an accumulation rate due to anthropogenic activities. On the other hand, the wetlands were dominated by the weathering of carbonate rocks in the regions (Rai et al. 2007a). The accumulation rate of Mansar wetland possesses more sediments due to the fine-grained sandstones alternating with siltstone, mudstone, and clay eroded by the Siwalik region of the lesser Himalayas. The elevated activity of 210Pb in the sediments can be accounted as atmospheric fallouts to rivers and lakes; heavy precipitation plays a vital role in transporting the fine particles to the lake environs and, followed by the radionuclide trapped in the mud contents because of deforestation, agricultural works, industrial activities and tourism in the Kodaikanal Lake. The sedimentation rate of some other wetlands examined by the researchers noted that the variation in the sedimentation rate does not depend on similar climatic conditions in the areas. To save the wetland, the necessary action must be taken to avoid the enormous sedimentation rate in the wetlands (Sarkar et al. 2016).

Analysis of vertical profiling of 137Cs dating

Despite knowing that 137Cs is a marker of sedimentation processes that tightly bind predominantly to the clay lattice, including silt, with a long half-life (30.2 ± 0.1 years) (Francis and Brinkley 1976), it can also be considered to be exchangeable by the sediment (Smith and Comans 1996). There are two approaches to examining the 137Cs: the horizon and peak methods. The sedimentation rate of the 137Cs method was augmented by adopting 1963 as the key marker to construct the age profile for Perumal Lake (1937AD to 2021AD), as shown in Table 5.

Table 5 Age dating and mean sedimentation rate of 137Cs radionuclide in the core sediments of Perumal Lake
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In the study area, the core sediment concerning the 137Cs was used to examine the sedimentation rate and age of the core profile in the wetland. The value of 137Cs ranges from 1.1 ± 0.07 (Bq/kg) to 44.6 ± 2.81 (Bq/kg), with a mean value of 12.78 ± 0.81 (Bq/kg), as shown in Table 5. The most significant spikes of 137Cs in the core profile possessing at 1963AD and 2011AD at a depth of 34 cm and 7 cm indicate the nuclear test year of 1963AD inferred as 44.6 ± 2.81 (Bq/kg), and the Fukushima Daiichi 2011 indicates 33 ± 2.08 (Bq/kg). The 137Cs activity remained relatively stable below 1963, reflecting minimal cesium input in the sediments. However, a notable secondary peak of 24.2 ± 1.52 (Bq/kg) at 28th cm depth emerged post-1963, representing the peak fallout from nuclear bomb testing. Furthermore, another peak at 16th cm depth displaying 27.4 ± 1.73 (Bq/kg) indicates increased 137Cs activity post the Chornobyl incident 1986, as depicted in Fig. 5. Various studies have investigated the 137Cs activity in core sediments, revealing different ranges such as in Kodaikanal Lake 3.5 ± 0.1 to 22.04 ± 1.4 Bq/kg (Palani et al. 2023), Satopanth Tal 2.48 ± 0.1 to 7.24 ± 0.1 Bq/kg (Das and Vasudevan 2021), Chandratal 2 ± 0.2 to 165 ± 0.3 Bq/kg (Rout and Vasudevan 2022), and Pykara Lake 1.21 ± 0.1 to 17.15 ± 1.7 Bq/kg (Singh and Vasudevan 2021).

Fig. 5
figure 5

Temporal profile of 137Cs (Bq/kg) activity represented in a age and b depth profile in the core sediments of the Perumal Lake

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The Mean sedimentation rate of the core for pivotal marker years 1963 and 1986 represents 0.57 ± 0.04 (cm/year) and 0.43 ± 0.03 (cm/year), respectively. This should happen because of a year difference between the marker times or instrumentation ambiguity. Regional aspects such as human activity, regional catchment/limnological processes, and sediment mixing may be accountable for this phenomenon. The overall age of the 137Cs core sediment from Perumal Lake has been calculated to be 86 years.

Analysis of the life of the wetlands

To decode the life computation, the Perumal Lake accounted for the sedimentation rate in the single zone, volume of the wetland, and mean depth (Singh et al. 2008). From the valuable findings, the Lake lifespan is around 369.05 years and 543.86 years concerning 210Pb and 137Cs, respectively. In this context, radionuclide dating, such as 210Pb and 137Cs, is considered to identify the sedimentation rate and the recent age module to assess the life of the lake.

Conclusion

The comprehensive analysis conducted in March 2023 on core sediments from Perumal Lake, Cuddalore district, Tamil Nadu, utilizing 210Pb and 137Cs radionuclide activity, has provided valuable insights into the sedimentation patterns and age profiles. Several significant findings have emerged from our meticulous investigation. The bathymetric study has revealed key morphometric characteristics, establishing the lake’s dimensions at 11.54 km in length with an average depth of 3.1 m. The temporal relationship between bulk density and porosity suggests an inverse correlation, indicating the sediment’s behavior. Granulometric analysis has revealed a predominance of a low-energy environment within the lake, characterized by a 100% clay composition. The association between clay content and 210Pb activity underscores the efficient trapping of 210Pb in the core sediments. Furthermore, the accumulation rate is notably higher in the upper sections of the core sediments compared to the lower sections. The heightened 210Pb activity in the sediments is attributed to atmospheric fallout transported by heavy precipitation, exacerbated by deforestation, local human activities, mining, agriculture, and industry. Our study underscores the critical influence of dynamic sediment accumulation rates on the wetland’s longevity.

The calculated sedimentation rates using 210Pb and 137Cs radionuclides indicate an estimated lifespan of 369.05 years and 543.86 years, respectively. This suggests relative stability in the current environmental conditions. However, proactive measures are imperative to address and regulate the elevated sedimentation rates in Perumal Lake and safeguard the wetland ecosystem from rapid degradation. Future research endeavors should consider employing numerical simulations to delve deeper into the responses of different wetland areas to 210Pb and 137Cs activities, offering more profound insights for effective wetland management strategies.