Philippine Journal of Science 149 (S1): 43-51, Special Issue on Nuclear S&T ISSN 0031 - 7683 Date Received: 17 Jun 2019

Sedimentation Patterns in Sorsogon Bay, Philippines Using 210Pb

Efren J. Sta. Maria*, Jordan F. Madrid, Ryan Joseph Aniago, Anie Day DC. Asa, Jennyvi P. Dayaon, Adelina DM. Bulos, and Elvira Z. Sombrito

Department of Science and Technology – Philippine Nuclear Research Institute (DOST-PNRI) Commonwealth Avenue, Diliman, Quezon City 1101 Philippines

210Pb has been used as a tracer to give an insight into the sedimentation process occurring in Sorsogon Bay. The sedimentation process can provide the possible sources, movement, pathways, and sinks of sediments and sediment-associated materials (e.g., chemicals, pollutants) – which are important for a better understanding of the changes happening in the bay and its watershed. The information that will be derived can be used as an input parameter by other researchers doing modeling work (material/water/pollutant flow, movement, dispersion, and residence time dynamics) at Sorsogon Bay. Seven sediment cores have been collected in Sorsogon Bay to determine the 210Pb-derived sedimentation rate estimates across the bay. A sedimentation rate of 1 cm/yr could be estimated for the eastern, central, and western areas of Sorsogon Bay. Areas near Sorsogon City (SO-07), Cadacan River (SO-03), and the area offshore of Buenavista and Rizal (SO-06) have enhanced sediment deposition, which could be due to an area where enhanced erosion from human activities is apparent, proximity to a river system that drains/carries volcanic material and debris from Mount Bulusan, and near an open dumpsite where possible materials (wastes, debris, leachates) could be carried offshore respectively. The sedimentation rates are shown to be higher in the shallower areas of the bay.

Keywords: 210Pb, sedimentation rate, Sorsogon Bay

INTRODUCTION occurred during the past 100 years in lake sediments (Robbins and Edgington 1975, Longmore and Olearry 210 The Pb dating technique is a well-established method 1983, Gunten et al. 1987, Allan et al. 1993). In the in studying marine and freshwater environments. It is a Philippines, the use of 210Pb dating method was first very useful technique for reconstructing the depositional applied in Manila Bay (Furio et al. 1996) and recently history of various pollutants and other materials associated employed by Sombrito et al. (2004) for sedimentation with sediments (Brugam 1978, Al-Masri et al. 2002). A rates determination in Malampaya Sound, Palawan and number of investigators have successfully utilized the Manila Bay in a study related to harmful algal blooms. technique for sedimentation rate determination and hence in dating purposes (Goldberg 1963, Krishnaswamy et 210Pb is a member of the 238U decay series. It has a half- al. 1971, Biford and Brenner 1986, Berger et al. 1987, life of 22.3 years and reaches an accumulating material in Anderson et al. 1987, Al-Masri et al. 2002), especially two pathways: in situ decay of 226Ra (termed ‘supported’ for dating the chemical changes and pollution that have 210Pb) and atmospheric fallout (termed ‘unsupported’ 210 210 210 210 Pb, excess Pb, or simply PbEx). The Pb that *Corresponding Author: [email protected] is being measured in the sediment sample is called the

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210 210 210 210 total Pb (Total Pb = ‘supported’ Pb + PbEx). MATERIALS AND METHODS It is but essential that the total 210Pb within a sediment sample be separated into these two components since it is Study Area assumed that the ‘unsupported’ component (210Pb ), once Ex Sorsogon Bay is one of the important fishing grounds in incorporated in the sediment, it decays exponentially with the Philippines. It is located on the south-eastern coast of time according to its half-life (and thus the only component Luzon Island (12.80°–13.00 °N, 123.73°–124.05 °E) with being used in the sedimentation rate calculations). an approximate area of about 220 km2 and a shallow water 137Cs is produced by nuclear fission and has been released depth of ~ 5 m (Figure 1). The Municipality of Castilla into the environment as a result of nuclear weapon testing lies on the west of the bay, while it is bounded by the during 1950–1970 (with a maximum atmospheric input Municipalities of Magallanes and Juban on the south and in 1963) and the Chernobyl accident in 1986. However, Municipality of Casiguran on the east. Castilla is part of nuclear weapon derived 137Cs inputs were shown to be the northeastern range bordering Sorsogon’s north and significantly lower in the southern hemisphere than in the serves as its watershed covered mainly by secondary forest northern hemisphere; also, inputs in the equatorial areas growth, which is composed mainly of coconut, abaca, and were less than those in the mid-latitude areas of Europe fruit trees. The bay has been experiencing long shellfish and North America (Zhang and Walling 2005). bans in the past due to recurring harmful algal blooms. In this study, seven sediment cores have been collected in Sorsogon Bay to investigate and document the Sampling sedimentation patterns using 210Pb and possible factors Field samplings were conducted from 01 Oct 2009 to 01 affecting the spatial variation will be inferred. By Sep 2011 to obtain sediment core samples. Sediment cores estimating the sedimentation rates and the possible factors with approximately 150 cm length and 7.5 cm diameter – affecting the spatial variation, better management of coded SO-01, SO-02, SO-03, and SO-04 –were collected land-based activities can be instituted to prevent further using a fabricated gravity corer of the University of the deterioration of Sorsogon Bay – an important fishing Philippines – Marine Science Institute (Geological Oceans ground of Sorsogon province. Laboratory) on 01 Oct 2009, while sediment cores coded SO-06 and SO-07 were obtained using the same method on 30 Nov 2009. Another core, coded SO-05, was obtained on 01 Sep 2011 using a gravity corer fabricated at the DOST-PNRI. The sample locations are given in Figure 1 and Table 1.

Figure 1. Map of Sorsogon Bay and locations of sediment core samples.

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Table 1. Sediment core locations and water depth in Sorsogon Bay. by Krishnaswamy et al. (1971). This model assumes that at each stage in accumulation, the initial concentration Sediment core code Coordinates Water depth (m) of 210Pb in the sediment is constant. In undisturbed 12° 56' 34.8" SO-01 6.0 cores, 210Pb activity concentration values must decline 123° 53' 30.4" monotonically with depth (Goldberg 1963, Robbins 12° 55' 0.9" SO-02 8.5 and Edgington 1975). The sedimentation rates were 123° 55' 2.9" determined from the slope of the least-squares fit for 12° 51' 59.7" SO-03 4.5 excess 210Pb values plotted versus sediment core depth 123° 55' 4.0" on a semi-logarithmic scale. The amount of supported 12° 55' 45.2" SO-04 6.0 210Pb estimated from the lower portion of the core where 123° 58' 59.4" the 210Pb activity attained an almost constant value 12° 55' 26.9" SO-05 8.0 (Schelske et al. 1994). The activity concentrations were 123° 56' 29.1" corrected for decay of 210Pb and 210Po. Measurements of 12° 57' 30.0" SO-06 6.4 the radionuclides were standardized using IAEA Sediment 123° 54' 31.5" Reference Material IAEA-300. 12° 57' 00.8" SO-07 4.4 124° 00' 01.8" 137Cs was measured by gamma spectrometry using ORTEC co-axial vertical high-purity germanium (HPGe) detector systems (GEM-FX8530 with 85.0 x 32.7 mm Laboratory Analysis crystal dimension, 56.9% relative efficiency, and 0.02 Bq/ Moisture Content and Dry Bulk Density of Samples. One kg minimum detectable activity; and GEM30s with 58.7 x and a half (0.5) mL wet sediments were dried to constant 57.8 mm crystal dimension, 35% relative efficiency, and weight at 70 °C for dry bulk density determination and at 0.03 Bq/kg minimum detectable activity. The detectors 110 °C for moisture content analysis. Moisture content is are shielded with interlocking Pb bricks of 10 cm measured by weight loss and dry bulk density is calculated thickness. A weighed amount (~ 5 g) of the freeze-dried as dry weight divided by wet volume. and homogenized sediment core sample was placed in a plastic container and counted for at least 24 h on the HPGe 210Pb and 137Cs Determination. 210Pb was determined by detector. The efficiency of the detector was determined measurement of its daughter nuclide, 210Po, which decays using the IAEA-300 Sediment Reference Material. by alpha particle emission. The activity is calculated by assuming secular equilibrium between 210Pb and 210Po. 210Pb analysis was conducted at the DOST-PNRI. A modified procedure of Smith and Ellis (1998) was used for RESULTS AND DISCUSSION the 210Pb measurements of sediment samples (Sta. Maria et al. 2009). The International Atomic Energy Agency Moisture Content and Dry Bulk Density of Sediment (IAEA) 300 reference material was used to standardize Cores and calibrate laboratory measurements. Water content, dry bulk density, and porosity are Determination of 210Po was performed on 1 g of the dried, important properties of bottom sediments. These three homogenized sediments. After adding 208Po as a tracer properties have an intrinsic dependence in the absence (tracer activity = 0.1 Bq) for chemical yield measurement of trapped air in the sediments. On average, the water and n-octanol, the sediment was digested by adding content of all cores is the same (45.79–71.79%). Water content or porosity generally decreases while dry bulk concentrated HNO3 and HF. The polonium isotopes were then spontaneously plated onto silver disc from 0.5 M density generally increases with sediment depth and HCl solution. Addition of ascorbic acid (Blanchard 1966) this is observed for cores SO-01, SO-02, SO-03, SO-04, and hydroxylamine hydrochloride (Flynn 1968) are used and SO-05 (Figure 2). However, with reference to the during autodeposition to reduce the effect of competing more recently deposited sediments at the top of the core, ions that are present in the sample. 210Po and 208Po sediment cores SO-06 and SO-07 exhibited a sudden activities were measured on a Tennelec alpha spectrometry decrease in moisture content and a sudden increase in system with surface-barrier Si detector for a minimum of dry bulk density with sediment depth at around 55–65 24 h. 230Th radionuclide was used for energy calibration. cm and 40–50 cm, respectively (Figure 3). Using the 210 PbEx derived sedimentation rate for SO-06, which is 210Pb chronologies and sedimentation rate calculations 1.8 cm yr–1, this core section corresponds to around year were made using the Constant Initial Concentration 1975 while a sedimentation rate of 1.3 cm yr–1 for SO- model. This was originally developed by Goldberg in 07 signifies that the core section corresponds to around 1963, although its first application to lake sediments was year 1970. Since the top layers correspond to younger

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Figure 2. Moisture content and dry bulk density of sediment cores SO-01, SO-02, SO-03, SO-04, and SO-05.

Figure 3. Moisture content and dry bulk density of sediment cores SO-06 and SO-07.

46 Special Issue on Nuclear Science and Technology Sta. Maria et al.: Sedimentation patterns in Sorsogon Bay, Philippines Using 210Pb sediments, the sudden increase in bulk density of the core Visually, the cores generally have a dark greenish-gray sediments possibly reflects the lower organic content of color with no observed distinct sand/gravel layers. the sediments deposited to the bay after 1970. According to the Regional Development Council of Bicol (http:// 210Pb Chronology www.neda5.net/rpfp/urbanization_trend.htm), the number In attempting to establish a 210Pb -based sediment of municipalities with urban agglomerations increases by Ex chronology, it is important to have an idea of the extent almost 50% from 1970 to 1980. The rapid urbanization of mixing of the sediment particles and the associated and increase in population growth rate from 1975 to 1980 radionuclides. Results of 210Pb analysis of the core and might have triggered the shift in organic matter content 210Pb measurement are shown in Figures 4, 5, and 6. in the sediments deposited to the bay because most of Ex Cores SO-02, SO-03, SO-04, SO-05, and SO-07 have the lands were converted to residential areas to serve the maximum 210Pb at the topmost layer, while SO-01 growing population; due to this, the lands mostly used for Ex and SO-06 have maximum 210Pb activity at 10 cm field farming decreased in number and the organic-rich Ex but statistically within the range of the value of the top agricultural soil coming from these areas that are carried layer. With this, little or negligible mixing of the layers to the bay are also reduced. can be assumed. However, it is not possible to address All seven cores exhibited non-uniform down-core values the mixing without a second particle tracer. It should be of dry bulk density and moisture content. Cores SO-03, noted that the depth of the radionuclide penetration is a SO-06, and SO-07 showed an almost similar range of function of the analytical sensitivity to detect it above the 210 dry bulk density of about 0.40–0.80 g cm–3 – while SO- parent supported level. Even if there is 20% of the PbEx 01, SO-02, SO-04, and SO-05 showed lower values at at the surface (61.37–80.17 Bq/kg) introduced to depth approximately 0.30–0.60 g cm–3. Similarly, SO-03, SO- by biological and/or physical mixing process, it would be 210 06, and SO-07 have almost the same average dry bulk difficult to detect since Pbsupported concentrations range density of approximately 0.55 g cm–3 – higher than that from 11.97 Bq/kg to 16.19 Bq/kg. If ever a maximum 210 of SO-01, SO-02, SO-04, and SO-05 with an average dry PbEx is detected below the top layers, then the calculated bulk density of 0.45–0.50 g cm–3. sedimentation rates represent an upper limit (Baskaran and Naidu 1995). Studies have shown that dry bulk density is inversely related to organic carbon (Avnimelech et al. 2001). The calculated sedimentation rates for the seven cores are Assuming that the empirical relationship also holds true given in Table 2. The high sedimentation rates of SO-03 for the Sorsogon cores, the organic matter content of SO- and SO-07 may be attributed to the activities near the 01 is higher than that of SO-03 and SO-07; this assumption rivers. The sampling station for SO-03 is near Cadacan can be verified by directly measuring the organic matter River which, according to the Agricultural Officer at content of the cores. This observation is supported by the Sorsogon, carries volcanic matter and debris from Mount 210Po content of the upper layers of the cores. Due to its Bulusan. chemistry, polonium is expected to be highly correlated with sulfur, protein, or other sulfur seeking metals in Table 2. 210Pb-derived sedimentation rates of sediment cores. natural samples, much as it is in marine plants and animals Sediment core Estimated sedimentation Water depth (Stewart et al. 2007). Proteinaceous material makes up a code rate (cm/yr) (m) major fraction of particulate organic matter in the ocean SO-01 0.8 6.0 and, because Castilla (SO-01) is closer to the sea than it SO-02 1.0 8.5 is to Sorsogon City and Cadacan River, it is expected to have higher amount of particulate organic matter from SO-03 1.8 4.5 marine plants and animals resulting to lower bulk density SO-04 0.9 6.0 and higher 210Po content of the upper layers of the core. SO-05 0.5 8.0 Besides organic matter content, the water content and SO-06 1.8 6.4 dry bulk density of sediment samples can be related to SO-07 1.3 4.4 other physical and chemical properties, which could give an insight into the properties of the bottom sediments. Furthermore, the moisture and dry bulk density profile of the cores can facilitate the interpretation of the 210Pb The sampling station for SO-07 is near Sorsogon City, dating profile. The information on silt/clay to sand and and it is expected to exhibit enhanced erosion due to gravel ratios of the sediment cores could also help explain urbanization and human activities such as the conversion the observed 210Pb values. However, PNRI does not have of areas covered by trees and plants to residential/industrial the experimental set up to conduct grain size analysis. areas. There are already a number of studies that illustrate clearly that urbanization can create significant changes in

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210 210 Figure 4. PbTotal and PbEx profile of a) SO-01, b) SO-02 and c) SO-03. sedimentation rates (Goudie 2006). In addition, there are calculated for SO-03, SO-06, and SO-07 compared to the more field farms at areas near Sorsogon City and Cadacan sedimentation rates calculated from other cores reflect River than at areas around Castilla; hence, more soil the water depth of the locations where the cores were erosion due to land tilling and a large amount of exposed obtained; higher sedimentation rates at SO-03, SO-06, ground is expected and this might have contributed to and SO-07 result to faster input of sediments that makes the enhanced sedimentation rate observed at these areas. these areas shallower compared to the area where the other cores were acquired. The sampling site where SO-06 was taken is near an open 210 dumpsite (located in between the municipal boundaries of The PbEx surface activities of the collected cores are 210 Buenavista and Rizal) where possible materials (wastes, given in Table 3. The Pbsupported of the cores are of the 210 debris, leachates, etc.) could be carried offshore. However, same magnitude so the large difference in PbEx surface the lower layers (~ 45–60 cm sub-depth) could have been activity between cores SO-01 and SO-02 compared to experiencing a lower sedimentation rate of ~ 0.3 cm/yr other cores might be due to the higher organic content of (r2 = 0.88). Furthermore, the higher sedimentation rates SO-01 as deduced from the bulk density data; polonium

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210 210 Figure 5. PbTotal and PbEx profile of a) SO-04, b) SO-05 and c) SO-06.

210 210 Figure 6. PbTotal and PbEx profile of SO-07.

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210 water depth from which the sediment cores were obtained. Table 3. PbEx surface activity of sediment cores. 210 Higher sedimentation rates correspond to shallower PbEx surface activity Sediment core code sediment beds. (Bq kg–1) SO-01 118.96 + 2.42 SO-02 128.10 + 11.35 ACKNOWLEDGMENTS SO-03 64.36 + 7.43 SO-04 59.14 + 3.94 This research work was funded by the DOST – Grants- SO-05 56.68 + 6.22 in-Aid, PNRI, and the IAEA. SO-06 80.17 + 8.99 SO-07 61.37 + 7.15 REFERENCES

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