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Keywords: 210 Pb, sedimentation rate, Sorsogon Bay Sedimentation Patterns in Sorsogon Bay, Philippines Using 210 Pb Department of Science and Technology – Philippine Nuclear Research Institute (DOST-PNRI) Commonwealth Avenue, Diliman, Quezon City 1101 Philippines Efren J. Sta. Maria*, Jordan F. Madrid, Ryan Joseph Aniago, Anie Day DC. Asa, Jennyvi P. Dayaon, Adelina DM. Bulos, and Elvira Z. Sombrito 210 Pb 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 210 Pb-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. Philippine Journal of Science 148 (S2): 43-51, Special Issue on Nuclear S&T ISSN 0031 - 7683 Date Received: 17 Jun 2019 *Corresponding Author: [email protected] INTRODUCTION The 210 Pb dating technique is a well-established method in studying marine and freshwater environments. It is a very useful technique for reconstructing the depositional history of various pollutants and other materials associated with sediments (Brugam 1978, Al-Masri et al. 2002). A number of investigators have successfully utilized the technique for sedimentation rate determination and hence in dating purposes (Goldberg 1963, Krishnaswamy et al. 1971, Biford and Brenner 1986, Berger et al. 1987, Anderson et al. 1987, Al-Masri et al. 2002), especially for dating the chemical changes and pollution that have occurred during the past 100 years in lake sediments (Robbins and Edgington 1975, Longmore and Olearry 1983, Gunten et al. 1987, Allan et al. 1993). In the Philippines, the use of 210 Pb dating method was first applied in Manila Bay (Furio et al. 1996) and recently employed by Sombrito et al. (2004) for sedimentation rates determination in Malampaya Sound, Palawan and Manila Bay in a study related to harmful algal blooms. 210 Pb is a member of the 238 U decay series. It has a half- life of 22.3 years and reaches an accumulating material in two pathways: in situ decay of 226 Ra (termed ‘supported’ 210 Pb) and atmospheric fallout (termed ‘unsupported’ 210 Pb, excess 210 Pb, or simply 210 Pb Ex ). The 210 Pb that is being measured in the sediment sample is called the 43

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Keywords: 210Pb, sedimentation rate, Sorsogon Bay

Sedimentation Patterns in Sorsogon Bay, Philippines Using 210Pb

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

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

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.

Philippine Journal of Science148 (S2): 43-51, Special Issue on Nuclear S&TISSN 0031 - 7683Date Received: 17 Jun 2019

*Corresponding Author: [email protected]

INTRODUCTIONThe 210Pb dating technique is a well-established method in studying marine and freshwater environments. It is a very useful technique for reconstructing the depositional history of various pollutants and other materials associated with sediments (Brugam 1978, Al-Masri et al. 2002). A number of investigators have successfully utilized the technique for sedimentation rate determination and hence in dating purposes (Goldberg 1963, Krishnaswamy et al. 1971, Biford and Brenner 1986, Berger et al. 1987, Anderson et al. 1987, Al-Masri et al. 2002), especially for dating the chemical changes and pollution that have

occurred during the past 100 years in lake sediments (Robbins and Edgington 1975, Longmore and Olearry 1983, Gunten et al. 1987, Allan et al. 1993). In the Philippines, the use of 210Pb dating method was first applied in Manila Bay (Furio et al. 1996) and recently employed by Sombrito et al. (2004) for sedimentation rates determination in Malampaya Sound, Palawan and Manila Bay in a study related to harmful algal blooms.

210Pb is a member of the 238U decay series. It has a half-life of 22.3 years and reaches an accumulating material in two pathways: in situ decay of 226Ra (termed ‘supported’ 210Pb) and atmospheric fallout (termed ‘unsupported’ 210Pb, excess 210Pb, or simply 210PbEx). The 210Pb that is being measured in the sediment sample is called the

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total 210Pb (Total 210Pb = ‘supported’ 210Pb + 210PbEx). It is but essential that the total 210Pb within a sediment sample be separated into these two components since it is assumed that the ‘unsupported’ component (210PbEx), once incorporated in the sediment, it decays exponentially with time according to its half-life (and thus the only component being used in the sedimentation rate calculations).

137Cs is produced by nuclear fission and has been released into the environment as a result of nuclear weapon testing during 1950–1970 (with a maximum atmospheric input in 1963) and the Chernobyl accident in 1986. However, nuclear weapon derived 137Cs inputs were shown to be significantly lower in the southern hemisphere than in the northern hemisphere; also, inputs in the equatorial areas were less than those in the mid-latitude areas of Europe and North America (Zhang and Walling 2005).

In this study, seven sediment cores have been collected in Sorsogon Bay to investigate and document the sedimentation patterns using 210Pb and possible factors affecting the spatial variation will be inferred. By estimating the sedimentation rates and the possible factors affecting the spatial variation, better management of land-based activities can be instituted to prevent further deterioration of Sorsogon Bay – an important fishing ground of Sorsogon province.

MATERIALS AND METHODS

Study AreaSorsogon Bay is one of the important fishing grounds in the Philippines. It is located on the south-eastern coast of Luzon Island (12.80°–13.00 °N, 123.73°–124.05 °E) with an approximate area of about 220 km2 and a shallow water depth of ~ 5 m (Figure 1). The Municipality of Castilla lies on the west of the bay, while it is bounded by the Municipalities of Magallanes and Juban on the south and Municipality of Casiguran on the east. Castilla is part of the northeastern range bordering Sorsogon’s north and serves as its watershed covered mainly by secondary forest growth, which is composed mainly of coconut, abaca, and fruit trees. The bay has been experiencing long shellfish bans in the past due to recurring harmful algal blooms.

SamplingField samplings were conducted from 01 Oct 2009 to 01 Sep 2011 to obtain sediment core samples. Sediment cores with approximately 150 cm length and 7.5 cm diameter – coded SO-01, SO-02, SO-03, and SO-04 –were collected using a fabricated gravity corer of the University of the Philippines – Marine Science Institute (Geological Oceans 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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Laboratory AnalysisMoisture Content and Dry Bulk Density of Samples. One and a half (0.5) mL wet sediments were dried to constant weight at 70 °C for dry bulk density determination and at 110 °C for moisture content analysis. Moisture content is measured by weight loss and dry bulk density is calculated as dry weight divided by wet volume.

210Pb and 137Cs Determination. 210Pb was determined by measurement of its daughter nuclide, 210Po, which decays 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 the 210Pb measurements of sediment samples (Sta. Maria et al. 2009). The International Atomic Energy Agency (IAEA) 300 reference material was used to standardize and calibrate laboratory measurements.

Determination of 210Po was performed on 1 g of the dried, homogenized sediments. After adding 208Po as a tracer (tracer activity = 0.1 Bq) for chemical yield measurement and n-octanol, the sediment was digested by adding concentrated HNO3 and HF. The polonium isotopes were then spontaneously plated onto silver disc from 0.5 M HCl solution. Addition of ascorbic acid (Blanchard 1966) and hydroxylamine hydrochloride (Flynn 1968) are used during autodeposition to reduce the effect of competing ions that are present in the sample. 210Po and 208Po activities were measured on a Tennelec alpha spectrometry system with surface-barrier Si detector for a minimum of 24 h. 230Th radionuclide was used for energy calibration.

210Pb chronologies and sedimentation rate calculations were made using the Constant Initial Concentration model. This was originally developed by Goldberg in 1963, although its first application to lake sediments was

by Krishnaswamy et al. (1971). This model assumes that at each stage in accumulation, the initial concentration of 210Pb in the sediment is constant. In undisturbed cores, 210Pb activity concentration values must decline monotonically with depth (Goldberg 1963, Robbins and Edgington 1975). The sedimentation rates were determined from the slope of the least-squares fit for excess 210Pb values plotted versus sediment core depth on a semi-logarithmic scale. The amount of supported 210Pb estimated from the lower portion of the core where the 210Pb activity attained an almost constant value (Schelske et al. 1994). The activity concentrations were corrected for decay of 210Pb and 210Po. Measurements of the radionuclides were standardized using IAEA Sediment Reference Material IAEA-300.

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 crystal dimension, 56.9% relative efficiency, and 0.02 Bq/kg minimum detectable activity; and GEM30s with 58.7 x 57.8 mm crystal dimension, 35% relative efficiency, and 0.03 Bq/kg minimum detectable activity. The detectors are shielded with interlocking Pb bricks of 10 cm thickness. A weighed amount (~ 5 g) of the freeze-dried and homogenized sediment core sample was placed in a plastic container and counted for at least 24 h on the HPGe detector. The efficiency of the detector was determined using the IAEA-300 Sediment Reference Material.

RESULTS AND DISCUSSION

Moisture Content and Dry Bulk Density of Sediment CoresWater content, dry bulk density, and porosity are important properties of bottom sediments. These three properties have an intrinsic dependence in the absence of trapped air in the sediments. On average, the water content of all cores is the same (45.79–71.79%). Water content or porosity generally decreases while dry bulk density generally increases with sediment depth and this is observed for cores SO-01, SO-02, SO-03, SO-04, and SO-05 (Figure 2). However, with reference to the more recently deposited sediments at the top of the core, sediment cores SO-06 and SO-07 exhibited a sudden decrease in moisture content and a sudden increase in dry bulk density with sediment depth at around 55–65 cm and 40–50 cm, respectively (Figure 3). Using the 210PbEx derived sedimentation rate for SO-06, which is 1.8 cm yr–1, this core section corresponds to around year 1975 while a sedimentation rate of 1.3 cm yr–1 for SO-07 signifies that the core section corresponds to around year 1970. Since the top layers correspond to younger

Table 1. Sediment core locations and water depth in Sorsogon Bay.

Sediment core code Coordinates Water depth (m)

SO-01 12° 56' 34.8"123° 53' 30.4" 6.0

SO-02 12° 55' 0.9"123° 55' 2.9" 8.5

SO-03 12° 51' 59.7"123° 55' 4.0" 4.5

SO-04 12° 55' 45.2"123° 58' 59.4" 6.0

SO-05 12° 55' 26.9"123° 56' 29.1" 8.0

SO-06 12° 57' 30.0"123° 54' 31.5" 6.4

SO-07 12° 57' 00.8"124° 00' 01.8" 4.4

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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.

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sediments, the sudden increase in bulk density of the core sediments possibly reflects the lower organic content of the sediments deposited to the bay after 1970. According to the Regional Development Council of Bicol (http://www.neda5.net/rpfp/urbanization_trend.htm), the number of municipalities with urban agglomerations increases by almost 50% from 1970 to 1980. The rapid urbanization and increase in population growth rate from 1975 to 1980 might have triggered the shift in organic matter content in the sediments deposited to the bay because most of the lands were converted to residential areas to serve the growing population; due to this, the lands mostly used for field farming decreased in number and the organic-rich agricultural soil coming from these areas that are carried to the bay are also reduced.

All seven cores exhibited non-uniform down-core values of dry bulk density and moisture content. Cores SO-03, SO-06, and SO-07 showed an almost similar range of dry bulk density of about 0.40–0.80 g cm–3 – while SO-01, SO-02, SO-04, and SO-05 showed lower values at approximately 0.30–0.60 g cm–3. Similarly, SO-03, SO-06, and SO-07 have almost the same average dry bulk density of approximately 0.55 g cm–3 – higher than that of SO-01, SO-02, SO-04, and SO-05 with an average dry bulk density of 0.45–0.50 g cm–3.

Studies have shown that dry bulk density is inversely related to organic carbon (Avnimelech et al. 2001). Assuming that the empirical relationship also holds true for the Sorsogon cores, the organic matter content of SO-01 is higher than that of SO-03 and SO-07; this assumption can be verified by directly measuring the organic matter content of the cores. This observation is supported by the 210Po content of the upper layers of the cores. Due to its chemistry, polonium is expected to be highly correlated with sulfur, protein, or other sulfur seeking metals in natural samples, much as it is in marine plants and animals (Stewart et al. 2007). Proteinaceous material makes up a major fraction of particulate organic matter in the ocean and, because Castilla (SO-01) is closer to the sea than it is to Sorsogon City and Cadacan River, it is expected to have higher amount of particulate organic matter from marine plants and animals resulting to lower bulk density and higher 210Po content of the upper layers of the core. Besides organic matter content, the water content and dry bulk density of sediment samples can be related to 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 dating profile. The information on silt/clay to sand and gravel ratios of the sediment cores could also help explain the observed 210Pb values. However, PNRI does not have the experimental set up to conduct grain size analysis.

Visually, the cores generally have a dark greenish-gray color with no observed distinct sand/gravel layers.

210Pb ChronologyIn attempting to establish a 210PbEx-based sediment chronology, it is important to have an idea of the extent of mixing of the sediment particles and the associated radionuclides. Results of 210Pb analysis of the core and

210PbEx measurement are shown in Figures 4, 5, and 6. Cores SO-02, SO-03, SO-04, SO-05, and SO-07 have maximum 210PbEx at the topmost layer, while SO-01 and SO-06 have maximum 210PbEx activity at 10 cm but statistically within the range of the value of the top layer. With this, little or negligible mixing of the layers can be assumed. However, it is not possible to address the mixing without a second particle tracer. It should be noted that the depth of the radionuclide penetration is a function of the analytical sensitivity to detect it above the parent supported level. Even if there is 20% of the 210PbEx at the surface (61.37–80.17 Bq/kg) introduced to depth by biological and/or physical mixing process, it would be difficult to detect since 210Pbsupported concentrations range from 11.97 Bq/kg to 16.19 Bq/kg. If ever a maximum 210PbEx is detected below the top layers, then the calculated sedimentation rates represent an upper limit (Baskaran and Naidu 1995).

The calculated sedimentation rates for the seven cores are given in Table 2. The high sedimentation rates of SO-03 and SO-07 may be attributed to the activities near the rivers. The sampling station for SO-03 is near Cadacan River which, according to the Agricultural Officer at Sorsogon, carries volcanic matter and debris from Mount Bulusan.

The sampling station for SO-07 is near Sorsogon City, and it is expected to exhibit enhanced erosion due to urbanization and human activities such as the conversion of areas covered by trees and plants to residential/industrial areas. There are already a number of studies that illustrate clearly that urbanization can create significant changes in

Table 2. 210Pb-derived sedimentation rates of sediment cores.

Sediment core code

Estimated sedimentation rate (cm/yr)

Water depth(m)

SO-01 0.8 6.0

SO-02 1.0 8.5

SO-03 1.8 4.5

SO-04 0.9 6.0

SO-05 0.5 8.0

SO-06 1.8 6.4

SO-07 1.3 4.4

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sedimentation rates (Goudie 2006). In addition, there are more field farms at areas near Sorsogon City and Cadacan River than at areas around Castilla; hence, more soil erosion due to land tilling and a large amount of exposed ground is expected and this might have contributed to the enhanced sedimentation rate observed at these areas.

The sampling site where SO-06 was taken is near an open dumpsite (located in between the municipal boundaries of Buenavista and Rizal) where possible materials (wastes, debris, leachates, etc.) could be carried offshore. However, the lower layers (~ 45–60 cm sub-depth) could have been experiencing a lower sedimentation rate of ~ 0.3 cm/yr (r2 = 0.88). Furthermore, the higher sedimentation rates

Figure 4. 210PbTotal and 210PbEx profile of a) SO-01, b) SO-02 and c) SO-03.

calculated for SO-03, SO-06, and SO-07 compared to the sedimentation rates calculated from other cores reflect the water depth of the locations where the cores were obtained; higher sedimentation rates at SO-03, SO-06, and SO-07 result to faster input of sediments that makes these areas shallower compared to the area where the other cores were acquired.

The 210PbEx surface activities of the collected cores are given in Table 3. The 210Pbsupported of the cores are of the same magnitude so the large difference in 210PbEx surface activity between cores SO-01 and SO-02 compared to other cores might be due to the higher organic content of SO-01 as deduced from the bulk density data; polonium

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

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

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Table 3. 210PbEx surface activity of sediment cores.

Sediment core code210PbEx surface activity

(Bq kg–1)

SO-01 118.96 + 2.42

SO-02 128.10 + 11.35

SO-03 64.36 + 7.43

SO-04 59.14 + 3.94

SO-05 56.68 + 6.22

SO-06 80.17 + 8.99

SO-07 61.37 + 7.15

has a higher affinity to organic matter particularly sulfur-containing compounds (Stewart et al. 2007).

Susceptibility profiles of the sediment cores could help explain the observed differential sedimentation rates at the study site. Unfortunately, PNRI does not have the facility to do susceptibility measurements on the sediment cores.

137Cs ActivityThe values of 137Cs activity of the cores show that most of the segments have activities below the lower limit of detection of the instrument; hence, a conclusive chronology cannot be obtained using 137Cs profiles of the cores to verify the 210Pb derived sedimentation rate. The low 137Cs inventories associated with these areas of reduced receipt of fallout introduce measurement problems in terms of both detection limits and the long count times required to obtain results with an acceptable limit, making it difficult to make definite conclusions regarding variations in calculated values, unless the variations are large enough to overcome statistical errors (Moungsrijun et al. 2010).

CONCLUSIONIn this study, 210Pb is used as a tracer for investigating the sedimentation process occurring in Sorsogon Bay. Sedimentation rates vary across Sorsogon Bay. A sedimentation rate of 1 cm/yr could be estimated for some areas of Sorsogon Bay with the areas near Sorsogon City (SO-07), Cadacan River (SO-03), and the offshore areas of Buenavista and Rizal (SO-06) experiencing enhanced sediment deposition; this 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, etc,) could be carried offshore respectively. The sedimentation rates seem to be correlated with the

water depth from which the sediment cores were obtained. Higher sedimentation rates correspond to shallower sediment beds.

ACKNOWLEDGMENTSThis research work was funded by the DOST – Grants-in-Aid, PNRI, and the IAEA.

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