Religious activities and their impacts on the surface sediments of two lakes in Bali, Indonesia: A case study from Lake Buyan and Lake Tamblingan

Lakes could serve as archives of earth and ecosystem history (Cohen, 2003). Lakes have a relatively higher sediment deposition rate when compared to the marine environment. This allows lake sediments to become natural documents that record and provide historical information on changes in lake environmental conditions. Several studies have also been conducted using lake sediments, which prove that each layer of lake sediment can provide information about environmental conditions at the time of deposition, such as the level of weathering of sediments (Zolitschka, 1998; Williams et al., 2015), paleoclimate (Asikainen et al., 2007), as well as sediment sources with lithogenic and anthropogenic components (Jordanova et al., 2004; Blaha et al., 2008; Hissler et al., 2015; Khan et al., 2016; He et al., 2021). However, with population pressure, changes in the lake’s water environment are no longer only caused by natural environmental conditions but also by various human activities. While several previous studies have shown that the anthropogenic component of human activities is the result of agricultural activities (Ma et al., 2015; Yang et al., 2018; Han et al., 2021; Rad et al., 2022), residential waste, industrial waste, mining and petroleum emissions, vehicle emissions, and wastewater from residential and port sources (Yang et al., 2007; Yang et al., 2009; Zhang et al., 2012; Li et al., 2014; Fernandes et al., 2017; Zong et al., 2017; Yunginger et al., 2018; Soroaga et al., 2022; Yang et al., 2022), and tourism (Brtnický et al., 2020; Mikhailenko et al., 2020; Yang et al., 2022). Religious ceremonies and activities, on the other hand, haven’t been studied as much as industrial, residential, and agricultural activities, which are known to put anthropogenic materials into lake sediments.

The island of Bali in Indonesia is an international tourist destination that is well known for its unique nature and culture. According to Statistics Indonesia (2022), the population density of Bali is 755 people per square kilometer. For Balinese people, who are predominantly Hindu (86.8%), the lake is a very sacred or purified place because the lake is a source of water, which is the most important component in Hinduism’s life cycle. Therefore, many religious ceremonies are carried out around the lakes. One of the ceremonies held at the lake shores is the Pakelem ceremony, where the offerings, or yadnya, are sunk into the lake (Arwati, 1991).

Here, we will report the results of a study of surface sediments from 2 lakes located on the island of Bali, namely Lakes Buyan and Tamblingan, that are used, among others, for religious ceremonies. Surface sediments from these 2 lakes were analyzed using magnetic and geochemical methods to identify their anthropogenic components, including those that originated from religious activities. The objective of this study is to identify the extent of pollution (if any) due to religious ceremonies. This study is the first time that religious ceremonies have been looked at as a type of human-made change to the lake environment.

Lake Buyan and Lake Tamblingan are 2 of the 3 lakes in the Bratan Caldera on the island of Bali, Indonesia (Figure 1a and b). These 2 lakes are relatively secluded, unlike neighboring Lake Bratan, the third and largest lake in the caldera, which is bordered by roads, hotels, restaurants, and an amusement park. Lake Buyan and Lake Tamblingan are about similar in altitude (1,214 m above the sea level for Lake Buyan and 1,210 m for Lake Tamblingan). Compared to Lake Tamblingan, Lake Buyan is much larger, with about 460 ha compared to only 137 ha for Lake Tamblingan (Irdhawati et al., 2020). The bathymetry measurements carried out during our survey showed that the deepest point of Lake Tamblingan is 37 m, compared to 66 m for Lake Buyan (Figure 1c). Both lakes do not have rivers as outlets or inlets, so the water comes from precipitation and groundwater circulation (Polkswaska et al., 2015).

Within the Bratan Caldera, there are several volcanic cones that were formed after the caldera’s formation. These volcanic cones are named Mount Lesong, Mount Tapak, Mount Catur, and Mount Pohen (Ryu et al., 2013; Figure 1b). These cones are Holocene in age (Purbo-Hadiwidjojo et al., 1998). Lake Buyan is surrounded by the first 3 cones, while Lake Tamblingan is bordered only by Mount Lesong. Steep walls made of precaldera rocks fringe the northern side of Lake Buyan and the northern and eastern shores of Lake Tamblingan. Alluvial plains surround the eastern side of Lake Buyan and the western and southern shores of Lake Tamblingan. The alluvium is composed of gravel, sand, silt, and clay from lava deposits, tuff, and breccias (Purbo-Hadiwidjojo et al., 1998).

There are temples and places of worship around Lakes Buyan and Tamblingan (see Figure 1d). There are 8 puras (Balinese Hindu temples) around Lake Tamblingan and 3 puras around Lake Buyan (Figure 1c). These puras have different settings and localities. Some of them are on the rock cliffs, while others are on the lakeshore as well as in the woods nearby. The shapes and forms of these puras also vary greatly. Some are simple altars to place offerings, while others have elaborate adobe structures. Some of these puras attracted many visitors from all over Bali. There are human settlements on the southern and northern shores of Lake Buyan, but none at Lake Tamblingan (Figure 1e).

The typical temples at Lake Tamblingan are shown in Figure 2a, while the typical daily offerings, or canang, that might contain kepengs (copper coins) adorned with elaborate designs are shown in Figure 2b. For major ceremonies such as Pakelems, the sunken offerings might be in the form of pripihan, or fish-shaped metals made of iron, copper, and even precious metals (Arwati, 1991).

This study was initiated by a bathymetric survey in both Lake Buyan and Lake Tamblingan in June 2021 using the Garmin Striker Plus 7SV APAC device, where depth measurements were taken at 200-m intervals. This survey was followed by sampling surface sediments in both lakes using a sediment grabber deployed from paddle boats. In total, surface sediments were obtained in 20 locations in Lake Buyan (termed B1–B20) and in 16 locations in Lake Tamblingan (termed T1–T16; Figure 1c). To preserve the metals in the sediments, every 5 L of sediment was given 10 mL of HNO₃ (Siaka et al., 1998).

The sediments were then sieved with a 325-mesh (44 μm) sieve to obtain homogeneous clay and silt particles. The sieved samples were then dried and prepared for magnetic measurements as well as trace metal and rare earth element (REE) measurements. Detailed sample preparation and measurements for magnetic parameters were carried out following Yunginger et al. (2018). In this study, a Bartington MS3 magnetic susceptibility system (Bartington Instruments Ltd., Witney, UK) was used to measure the mass-specific low-frequency magnetic susceptibility (χ_LF) and high-frequency magnetic susceptibility (χ_HF) using a dual-frequency MS2B sensor that operates at both 470 and 4,700 Hz. The value of frequency-dependent magnetic susceptibility (χ_FD%) could be calculated using the following equation: χ_FD% = 100% (χ_LF − χ_HF)/χ_LF. These magnetic measurements were carried out in the laboratory at the Institut Teknologi Bandung, Indonesia. Parameter χ_LF is often used as a proxy indicator of the concentration of magnetic minerals, primarily the ferromagnetic mineral phases like magnetite and hematite (Dearing, 1994; Dunlop and Özdemir, 1997; Dearing, 1999; Solomon et al., 2017; Sudarningsih et al., 2017; Yunginger et al., 2018). Meanwhile, the parameter χ_FD% is often used to determine the concentration of superparamagnetic (SP) fine grains in the sample. Higher χ_FD% values imply higher SP grain concentrations, and vice versa (Dearing, 1999; Bijaksana and Huliselan, 2010; Solomon et al., 2017).

The concentration of heavy and trace elements was determined by X-ray fluorescence (XRF) analyses using an XRF Thermo Scientific type ARL 9900 (Thermo Fisher Scientific, Waltham, MA, USA) at the Central Laboratory of the Indonesian Geological Survey in Bandung, Indonesia. The quality of the data was assured by calibrating the instrument with the following certified reference materials: GBW 7105 (an alkali-olivine basalt), GBW 7103 (a gray medium-grained biotite granite), LKSD 2 (sediment lake), and SY 4 (diorite gneiss). The analytical precision, measured as relative standard deviation, for Si, Fe, Al, Ti, Cu, and Zn was routinely between 0.12% and 0.05%, but about 0.0005 for Cu and Zn. The average analytical standard errors observed with the reported certified materials were below 0.12 for Si, Fe, Al, and Ti and under 0.0005 for Cu and Zn. The concentration of REEs was analyzed by inductively coupled plasma atomic-optical emission spectrometry (ICP-OES) using the Agilent 700 type ICP-OES instrument (Agilent Technologies, Santa Clara, CA, USA) at the Central Laboratory of Universitas Padjadjaran in Sumedang, Indonesia.

The results of magnetic measurements for samples from both Lake Buyan and Lake Tamblingan are summarized in Table 1. The average value of χ_LF in samples from Lake Tamblingan (492.6 ± 282.4 × 10⁻⁸ m³/kg) is higher than that of Lake Buyan (372.0 ± 246.6 × 10⁻⁸ m³/kg). The relatively high values of standard deviation in both Lake Buyan and Lake Tamblingan infer that there are significant differences in the values of χ_LF from one point to another. From the data in Table 1, we interpolated the χ_LF data using the minimum curvature method to obtain Figure 3. As shown in Figure 3, the χ_LF values are higher in several locations near puras. For instance, locations B6, B16, B17, T9, T10, T11, and T12 have significantly higher χ_LF values compared to other locations.

The results of selected trace metal analyses for samples from both Lake Buyan and Lake Tamblingan are listed, respectively, in Tables 2 and 3. In general, samples from Lake Tamblingan have higher average concentrations of oxides, except for ZnO (see Table 2). Similar results were also found when concentrations were expressed in % weight (see Table 3). The average CuO content in both lakes is similar (0.005% weight in Lake Buyan and 0.007% weight in Lake Tamblingan). CuO and ZnO were not detected in bedrock samples from exposed lithologies around the lake that have been reported by Ryu et al. (2013). Interpolated using the minimum curvature method, the distributions of trace metals in both Lake Buyan and Lake Tamblingan are presented in Figure 4. Trace metal concentrations are higher at locations near the temples, except for SiO₂.

The concentration of selected REEs from ICP-OES analyses for samples from both Lake Buyan and Lake Tamblingan is listed in Table 4. Samples from Lake Tamblingan also have a higher average concentration of REEs except for Gd, where the average value in Lake Buyan is higher than that of Lake Tamblingan. Furthermore, based on the data in Table 4, the concentration distribution of REEs in Lakes Buyan and Tamblingan is depicted in Figure 5. As in Figures 3 and 4, the interpolation method that we used was minimum curvature. The concentration of REEs is generally higher close to the puras.

To show the correlation between χ_LF values and trace metal and REE concentrations, their values are plotted in Figure 6. For each plot, the value of the correlation coefficient of linear regression (R) is calculated. As shown in Figure 6, except for Zn and Sm, the R values for other elements are higher in Lake Tamblingan samples compared to those in Lake Buyan. Furthermore, the normality of the data set was tested and found to be nonnormal. Therefore, instead of Pearson’s correlation, Spearmen’s correlation was used. The Spearman rank correlation coefficients (r_s) between χ_LF value and the concentrations of trace metals and REEs in both Lake Buyan and Lake Tamblingan are shown in Table 5. The r_s values between χ_LF value and the concentrations of trace metals in both lakes exceed 0.500 except for Si. Meanwhile, the r_s values between χ_LF values and REEs in samples from Lake Tamblingan exceed 0.500 except for Y. The r_s value for Y is 0.494. In contrast, the r_s values between χ_LF values and REEs in samples from Lake Buyan are less than 0.500 except for Sm. The r_s value for Sm is 0.510.

The average value of χ_LF and concentration of trace metals and REEs in Lake Tamblingan are found to be significantly higher than those in Lake Buyan. While Tamblingan’s samples have an average χ_LF value of 492.6 (±282.4) × 10⁻⁸m³/kg, Buyan’s samples have an average χ_LF value of 372.0 (±246.6) × 10⁻⁸m³/kg. While Tamblingan’s samples have an average Fe and Al concentration of 5.934 and 4.941 (in % weight), respectively, Buyan’s samples had an average Fe and Al concentration of 4.445 and 3.434 (in % weight). The average Ce and Nd concentrations in Tamblingan’s samples are 2.066 and 1.494 (in ppm), whereas they are 1.423 and 1.105 (in ppm) in Buyan’s samples, respectively. As high χ_LF values and high concentrations of trace metals and REEs are found near the location of puras (see Figures 3–5), they suggest that higher values are caused by the higher intensity of religious activities in Lake Tamblingan compared to those in Lake Buyan. In areas near the puras and surrounding the lakes, concentrations of trace metals, REEs, and χ_LF are higher. For instance, the concentrations of Fe and Al at position T11 in Pura Dalem Tamblingan are 9.790 and 14.090 (in % weight), respectively, while χ_LF is 975.9 (±99.3) × 10^–8m³/kg. Additionally, T11 has Ce and Nd concentrations of 3.635 and 2.443, respectively (in ppm). This is the first time that religious activities have been linked to high concentrations of trace metals and REEs in lake sediments.

A previous study by Burton (2002) summarized that there are 8 elements that can indicate the general quality of a lake’s sediments with respect to anthropogenic contamination. These elements are As, Cd, Cr, Cu, Pb, Hg, Ni, and Zn. Of these 8 elements, only 2 are detected in the sediments of Lakes Buyan and Tamblingan (Cu and Zn). Measuring rock samples within the Bratan Caldera, including the post-caldera volcanoes and pre-caldera lava, Ryu et al. (2013) did not detect CuO as well as ZnO (see Table 2). Thus, the CuO and ZnO found in the sediments of Lakes Buyan and Tamblingan are likely due to anthropogenic sources. Compared to the standards cited by Burton (2002), we found that the average Cu content in sediments from both Lake Buyan and Lake Tamblingan is higher than the threshold effect level (TEL) of 35.7 ppm. The average CuO concentration in Tamblingan is 70 ppm, which corresponds to a Cu concentration of 50 ppm. The average CuO and Cu contents in Lake Buyan are slightly lower at 50 ppm and 40 ppm, respectively. Meanwhile, the Zn content in both lakes, Buyan at 80 ppm and Tamblingan at 60 ppm, is below the TEL for Zn of 123 ppm (Burton, 2002). The large amounts of kepengs (copper coins) in the offerings may explain the high concentrations of Cu compared to surrounding bedrock samples and above the TEL.

Although there is no settlement along the shorelines of Lake Tamblingan at present, recent archeological studies conducted in Lake Tamblingan have unearthed artifacts that include pottery, beads, kepengs, furnaces, and blades (Keling, 2021). The high concentration of trace metals in this lake might thus also be due to ancient settlements, especially those near the present puras. These archeological findings described in Bagus (1995) infer that there were ancient settlements around Lake Tamblingan and that some of the settlers were probably blacksmiths. Excavating the shoreline of Lake Tamblingan, Bagus (1995) found 4 groups of artifacts (pieces of iron, potteries, pieces of crucibles, and slags) indicating the presence of metalworks. Such metalworks and other past anthropogenic activities on the shores of Lake Tamblingan might contribute to the relatively high metal contents in the sediments of Lake Tamblingan. The presence of ancient metalworks on the shoreline of Lake Tamblingan is likely the main reason why the trace metal concentration in the sediments of Lake Tamblingan differs significantly from that of Lake Buyan. The main pura in Lake Tamblingan is called Pura Pande Tamblingan (near location T10). The word “pande” in Balinese means blacksmith. Keling (2021) also reported the presence of an inscription made by Raja Bhatara Çri Parameswara in 1384, urging the blacksmith families in Lake Tamblingan to return to their native land after civil unrest. Settlement in Lake Tamblingan, termed karāman i tamblingan had also been mentioned in the IX–XIV century Balinese inscriptions (Wiguna et al., 2021).

One of the reasons why contamination remains detectable for extended periods of time may be due to metal recycling, which can influence metal redistribution in lake sediment records. Based on Percival and Outridge (2013), the stability of some element profiles may possibly be affected by diagenetic changes over time. Such heavy metal elements as Hg and Zn were stable once the sediments were buried below the surface, but other heavy metal elements like Cd, Cu, and Pb underwent a process of redistribution over time. Thus, higher concentrations may be detected longer even after the end of active deposition due to metal recycling. It is very important to determine the presence of metal recycling at Lakes Buyan and Tamblingan because high concentrations of Cu compared to surrounding bedrock samples and above the TEL may also be caused by this metal recycling process. Redox-indicator elements such as Fe, Mn, Mo, and U can be used to determine whether there is metal recycling in a lake based on the state of the water column oxygenation of the lake (Boyle, 2001; El Bilali et al., 2002; Percival and Outridge, 2013). However, for Lakes Buyan and Tamblingan, there is no water column oxygenation data yet. Thus, metal recycling for the 2 lakes cannot be determined. Further studies are needed regarding water column oxygenation to prove the existence of metal recycling, which causes metal redistribution in both lakes. Note that a lake’s redox state (and hence metal recycling in the lake) may not be constant over time. Hence, even if the water column is fully oxygenated today, it may have been anoxic in the past.

Another anthropogenic factor that mainly affects Lake Buyan can be seen in the higher concentration of Gd. It is possible that this is related to the human settlement on the southern shore of Lake Buyan. The same thing was also found by Yunginger et al. (2018), where high Gd content around Lake Limboto is associated with residential areas and hospital waste. Apart from anthropogenic factors, there are also lithogenic factors, one of which can be characterized by the distribution of SiO₂ around Lake Buyan and Lake Tamblingan. The distribution patterns of SiO₂ resemble the bathymetric profiles of the lakes, with high concentrations in shallow water and low concentrations in deep samples. This agrees with the fact that the SiO₂ is a product of the weathering of the surrounding volcanic rocks.

Despite having a higher Cu content than TEL, the overall quality of sediments in both Lakes Buyan and Tamblingan is relatively uncontaminated, as evidenced by the absence of the 6 elements used as sediment quality indicators and the low Zn content. These findings agree with those of Irdhawati et al. (2020), based on fewer sediment samples taken from certain locations in Lakes Buyan and Tamblingan. Nevertheless, the quality of the sediments should be monitored on a regular basis. The correlation between χ_LF values, trace metals, and REEs, especially in Lake Tamblingan (see Table 5), offers an opportunity to use χ_LF as a proxy indicator for sediment quality. The measurement of χ_LF is much easier and more economical compared to geochemical analyses. Different results found in Lakes Tamblingan and Buyan suggest this approach should be taken cautiously. Samples from both lakes show a significant correlation between χ_LF value and the concentration of trace metals. However, a significant correlation between χ_LF value and REE concentrations was found only in samples from Lake Tamblingan but not in those from Lake Buyan. The results of this study might be used and tested in future studies.

The parameter χ_LF has been proposed as a proxy indicator for sediment quality by Boyko et al. (2004), Paradelo et al. (2009), and Pan et al. (2019). Monitoring sediment quality is important as lake sediments, due to their accumulative nature, might record traces of religious activities more effectively than lake waters do. Sukmawati et al. (2020) reported that the water quality in Lakes Buyan and Tamblingan is generally good as the heavy metal contents in the waters, including Cu, Cd, and Pb, are very low and within the range acceptable as potable water.

We have used measurements χ_LF as well as concentrations of trace metals and REEs from surface sediments in Lakes Buyan and Tamblingan to identify anthropogenic traces, particularly religious activities, in these lakes. Despite its smaller size, Lake Tamblingan has more puras, or temples, than Lake Buyan. High χ_LF values and concentrations of trace metals and REEs were observed in locations that are close to puras or temples. The correlations between χ_LF values and the concentrations of trace metals and REEs are also stronger in the sediments of Lake Tamblingan compared to those in Lake Buyan. These findings, along with the presence of CuO and ZnO, which were not detected in the surrounding rocks, imply that the surface sediments of Lakes Buyan and Tamblingan recorded remnants associated with religious activities. Higher trace metal concentrations in Lake Tamblingan might also be associated with artifacts from ancient metalworks as well as metal recycling. These presumptions need to be studied further. The results also show that χ_LF values and concentrations of trace metals and REEs are higher in the sediments of Lake Tamblingan compared to those of Lake Buyan, except for concentrations of SiO₂ and Gd. Although the sediment quality in Lakes Buyan and Tamblingan is relatively uncontaminated, regular monitoring is recommended. The measurement of χ_LF is recommended as an alternative or a complement to geochemical analyses.

All data generated in this study are included in this article and listed at http://tiny.cc/Suandayani_et_al_2023.

The permission to conduct field research at Batur volcano was given by the BKSDA (Balai Konservasi Sumber Daya Alam or Natural Resource Conservation Centre) of Bali, Indonesia.

Financial support for this research was provided by Ministry of Education, Culture, Research, and Technology of the Republic of Indonesia through World Class Research grant to SB though a project entitled “Paleolimnological studies on Indonesian lakes.” The grant number was 008/E4.1/AK.04.PT/2021.

The authors have declared that no competing interests exist.

Contributed to conception and design: NKTS, SB, DD, II.

Contributed to acquisition of data: NKTS, UH, SJF, PBS, SB.

Contributed to analysis and interpretation of data: NKTS, UH, SJF, PBS, KI, SB, DD.

Drafted and/or revised the article: NKTS, UH, SJF, PBS, KI, SB, DD, II.

Approved the submitted version for publication: NKTS, UH, SJF, PBS, KI, SB, DD, II.

How to cite this article: Suandayani, NKT, Harlianti, U, Fajar, SJ, Suryanata, PB, Ibrahim, K, Bijaksana, S, Dahrin, D, Iskandar, I. 2023. Religious activities and their impacts on the surface sediments of two lakes in Bali, Indonesia: A case study from Lake Buyan and Lake Tamblingan. Elementa: Science of the Anthropocene 11(1). DOI: https://doi.org/10.1525/elementa.2022.00140

Domain Editor-in-Chief: Steven Allison, University of California Irvine, Irvine, CA, USA

Associate Editor: Stephen Porder, Brown University, Providence, RI, USA

Knowledge Domain: Ecology and Earth Systems