Religious Activities and Their Impacts on the Surface Sediments of Two Lakes in Bali, Indonesia: A Case Study from Lake Buyan and Lake Tamblingan

Industrial and agricultural activities, as well as settlements, have changed many lacustrine environments, as recorded in lake sediments. In this study, concentrations of trace metals and rare earth elements (REEs), along with the mass-specific magnetic susceptibility measured at low frequency (470 Hz), termed wLF, were used to elucidate the anthropogenic impacts related to religious activities in 2 lakes (Buyan and Tamblingan) on Bali Island, Indonesia. The predominantly Hindu populations of Bali are known to hold various religious activities in puras, or Balinese temples, in these lakes. There are 8 puras in Lake Tamblingan and only 3 puras in Lake Buyan. In these religious activities, offerings, which might include metallic objects, were thrown into the lakes. Measurements on surface sediments from 20 locations in Lake Buyan and 16 locations in Lake Tamblingan show that wLF 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. The average wLF value of Tamblingan's samples is 492.6 (+/- 282.4) x 10-8 m3/kg, while that of Buyan's samples is 372.0 (+/- 246.6) x 10-8 m3/kg. The average concentrations of Fe and Al in Tamblingan's samples are 5.934 and 4.941 (in % weight), respectively, while those of Buyan's samples are 4.445 and 3.434 (in % weight), respectively. The average concentrations of Ce and Nd in Tamblingan's samples are 2.066 and 1.494 (in ppm), respectively, while those in Buyan's samples are 1.423 and 1.105 (in ppm), respectively. Concentrations of trace metals and REEs, as well as wLF, are higher in locations close to the puras around the lakes. For instance, the wLF of location T11 near Pura Dalem Tamblingan is 975.9 (+/- 99.3) x 10-8 m3/kg, while the concentrations of Fe and Al in this location are 9.790 and 14.090 (in % weight). Furthermore, the Ce and Nd concentrations in T11 are 3.635 and 2.443 ppm, respectively. Based on these results, we can say that the surface sediments of Lakes Buyan and Tamblingan have been changed by human activities, mostly religious ones.