Study Site
The study area forms a part of Toposheet 58 A/11 situated in the Nilgiri district of Tamil Nadu. It falls between Latitude 11°27'37.96"N to 11°24'17.45"N and Longitude 76°36'21.20"E to 76°34'16.79"E, located about 19 kilometers from Ooty, Tamil Nadu (Fig. 1). The Pykara River is viewed as extremely consecrated by the Todas. It ascends at the Mukurthi crest, streams northwards, and afterward swing toward the west after achieving the edge of the level. The waterway moves through Murkurti, Pykara, and Glenmorgan dams, and structures some portion of an imperative hydroelectric power venture. Pykara flaunts very much secured, fenced sholas, Toda settlements, vast green knolls, and great natural life living space. The Pykara Dam falls and the repository pulls in numerous visitors. There were no stringent laws and regulations in place to protect and save India's most pristine lakes in ancient times. As we all know, the majority of India's high-altitude lakes are used for tourism. As a result, there could have been a risk in the past of more people coming to the lake site from the surrounding area, influencing anthropogenic activities and exceeding the sediment concentration. Previous studies suggest that the Lake Lykara water is enriched with heavy metals like Cu, Zn, Mn, Ni, Cr, Al, Li, and K (Anusiya Devi et al., 2015). The study area experiences a subtropical monsoon climate and the temperature fluctuates from 10 to 25 0C throughout the year. The mean annual precipitation is 1,920.80 mm and gets rainfall from both monsoons. Furthermore, the climatic conditions in the study area are favorable for rock weathering.
Core sampling and analysis
The sediment core was retrieved from Lake Pykara in January 2019. The core was collected using a gravity corer and the core was recovered for 48 cm. The samples were carried to the research facility lab and stored in a freezer at 4°C before being sliced at 1 cm intervals. Water content (%) and dry bulk density (DBD, g cm-3) of the samples were determined by weighing the 1 cm slices of the sediment core before and after drying at 60°C overnight.
Estimation of dry bulk density (ρd) and porosity (Φ)
The dry bulk density and porosity were measured from the natural moisture and grain specific gravity esteems utilizing the conditions of soil mechanics;
Where, ρd, Wd, V, ρp, and Φ are bulk density, dry weight (g), volume (cc), particle density, and porosity of the sediment, respectively. “Inorganic sediment particle density (ρp) is within the range of 2.6–2.7 and is routinely taken as 2.65 g cm− 3” (Blake and Hartge, 1986; Boyd, 1995; Avnimelech et al., 2001).
Radioactivity measurements
Following Sanchez-Cabeza (1998), 2 gm of dried ground sediment samples were taken to determine the 210Pb concentration utilizing 210Po alpha emitter assuming secular equilibrium with 210Pb. Then the ground samples were treated with HNO3, HF, and HCl. The radionuclides were deposited on silver coins after the conversion of Fe3+ to Fe2+ with the addition of C6H8O6 (Ascorbic acid). “After that, the silver coins were positioned amid ZnS (Ag) phosphor discs, and each side of the discs was measured at alpha energy of 5.30 MeV utilizing 209Po (4.88 MeV alpha emission) as the internal tracer by alpha spectrometry (ORTEC, OCTAT) with 13% efficiency. Chemical yields utilizing a 209Po tracer varied from 87 to 92%. The activity was calculated by the ratio of counts per second (alpha emission of 210Po) with the sample mass. Intercalibration practices were additionally performed utilizing standards samples. The supported 210Pb-specific activity was subtracted from the total 210Pb-specific activity to determine the excess (unsupported) 210Pb” (Kumar et al., 2015).
The 137Cs concentration was estimated by gamma spectrometry utilizing a cylindrical NaI (Tl) detector. “The size of the NaI (Tl) crystal is 4"*4" with a well of 1" diameter * 2" height. The 137Cs esteem was calculated by measuring the gamma peak at 661.62 keV with an 85% branching ratio. The energy calibration of the instrument was done with a mixture of 137Cs and 60Co source, while the efficiency calibration was done with reference standard soil (IAEA-326)” (Singhal et al., 2012).
Sedimentation models
Assessment of sedimentation rates from the 210Pbex depth conveyances reported for the study requires the utilization of a model to build up the chronology or age-depth relationship for the core. The distribution pattern doesn't follow an exponential decline, which demonstrates a variable sedimentation rate. “The (CIC) model couldn't be applied since it assumes a consistent sedimentation rate with a monotonously decreasing excess of 210Pb” (Alhajji et al., 2014). Consequently, the (CRS) model was utilized for evaluating the rate of sedimentation and the chronology of the sediment layers. The CRS model presumes a consistent 210Pb flux however allocates the sediment supply to fluctuate. Accordingly, this method is applied to many sedimentary basins where the sediment furnish might fluctuate in response to climatic or anthropogenic alterations. The CRS dating method is communicated as follows:
“Where, At = cumulative 210Pbex below the level representing time t, λ = decay constant of 210Pb (0.03114 y− 1), A0 = total cumulative 210Pbex inventory (Bq m− 2) at the point where the 210Pbtot activity reaches radioactive equilibrium with the supporting 226Ra” (Gharibreza et al., 2013)..
Where ρi = dry sediment bulk density (kg m–3) of the ith depth interval, hi = thickness of the ith depth interval (m), and Ai = 210Pbex (Bq kg–1) (Gharibreza et al., 2013). Besides, 210Pb flux (Bq m–2 y–1) can be measured by the accompanying equation:
210 Pb flux = A0* λ …. (5)
The age of the sediment at any depth can be calculated by the following equation:
The rate of sedimentation was estimated as per the accompanying condition:
$$\mathbf{S}= \frac{\mathbf{h}}{\mathbf{t}}$$
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Where S = rate of sedimentation (cm yr-1); h = depth of sediment deposition (cm), and t = estimated time (yr).
For 137Cs estimation, the rate of sedimentation was determined as per the 137Cs maximum layer, which communicates to the 1963- time indicator. The period of different layers was acquired depend on the rate of sedimentation of the recognized marker. “Even though there may be an upward or downward diffusion of the 137Cs peaks, it would not affect the position of the 137Cs peaks in the sedimentary profiles or the utilization of the 137Cs peak as time markers” (Cheng et al., 2019). The 137Cs inferred mean rate of sedimentation for a sample was determined utilizing the following conditions:
\({\mathbf{S}\mathbf{R}}_{1}= \frac{{\mathbf{H}}_{1}}{(\mathbf{n}-1963)}\) & \({ \mathbf{S}\mathbf{R}}_{2}= \frac{{\mathbf{H}}_{2}}{(\mathbf{n}-1986)}\)… (8)
Where SR1 and SR2 are the sedimentation rates (cm yr-1) for this sample, H1 and H2 are the depths (cm) of 137Cs peaks for the 1963 and 1986-time markers and n is the year of sampling. The mean rate of sedimentation of the core sample is given as the average esteem of SR1 and SR2. In this manner, the age for the layers deposited over the 1963-time indicator depth can be communicated as:
$${\mathbf{T}}_{\mathbf{n}}=\mathbf{A}+ \frac{\left({\mathbf{H}- \mathbf{h}}_{\mathbf{n}}\right)}{\mathbf{r}}$$
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Where r is the sedimentation rate, Tn = age (yr), and hn = depth (cm) for this layer, A = The time markers (1963 or 1986). For the layers deposited underneath the 1963-time indicator depth, the age was determined as follows:
$${\mathbf{T}}_{0}=\mathbf{A}- \frac{({\mathbf{h}}_{0}-\mathbf{H})}{\mathbf{r}}$$
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Where T0 = age (yr) and h0 depth (cm) for this layer. 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, and the Chernobyl accident (1986).
Computation of lake life
The calculation of lake life gives a thought regarding the timeframe after which the lake won't be valuable for water-related activities. The limit of the lakes is diminished due to sedimentation up to a degree that it is outrageous to anticipate to fulfill out the water necessitates. This activity ought to be done periodically to comprehend the lake condition and to design appropriate estimates convenient for lake reclamation if the reduction of lake capacity is found at higher rates. The anticipated useful life of the lake is calculated by the ratio of the mean depth of the lake by a weighted average sedimentation rate. The useful life of the Lake Pykara is determined as;
$${\mathbf{L}}_{\mathbf{u}}= {\mathbf{D}}_{\mathbf{m}}\times \frac{100}{{\mathbf{R}}_{\mathbf{s}}}$$
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Where LU = useful life of the lake (yr), Dm = mean depth of the lake (m), Rs = sedimentation rate (cm yr− 1).