Application of Constant Rate of Supply Model for the Determination of Sediment Accumulation Rates and Chronology of Bukit Merah Reservoir Perak, Malaysia

Abstract


Introduction
Concentrations of radionuclides and chemical compositions in the bottom sediment core of aquatic ecosystems provide crucial insights into the chemical and physical processes occurring in the aquatic environment (Parrinello and Kondolf, 2021;Saadawi et al., 2023).Specifically, the assessment of 210 Pb concentrations and other contaminants in bottom sediment cores enables the tracking of the origins and historical pathways of various contaminants in aquatic environments (Al-Hamdany et al., 2023;Akogwu et al., 2023;Yakovlev et al., 2021).
The isotope 210 Pb, with a half-life (t 1 /2 = 22.3 years), is a decay product of the 238 U series (t 1 /2 = 4.5 x 10 9 years), predominantly found in landscape soils.It undergoes decay to the gaseous product 222 Rn (t 1 /2 = 4.5 x 10 9 years) and ultimately transforms into the short-lived product 210 Pb through a series of decay processes (Mabit et al., 2014).The 210 Pb formed in soil is termed 'supported,' while atmospheric 210 Pb is termed "excess" 210 Pbxs, which is utilised in sedimentation studies (Appleby, 2008).During sedimentary processes, 210 Pbxs maintains constant radioactive equilibrium with its long-lived precursor, 226 Ra.Thus, the activities of 210 Pbxs reaching the sediment core's base can be computed as the difference between the activities of 210 Pb and 226 Ra radionuclides (Appleby, 2008).
Other radionuclides like 137 Cs and 214 Am, derived from anthropogenic sources, act as markers for anthropogenic activities in 210 Pb dating techniques (Hughes et al., 2009;Ran et al., 2021).Activities of 241 Am and 137 Cs can corroborate major historical nuclear test events (Wang et al., 2018), such as the nuclear re-release of 1952, the nuclear peak testing period of 1964, the Chernobyl nuclear disaster of 1986, and the Tohoku earthquake and tsunami nuclear accident of 2011 (Hughes et al., 2009;Ivanov et al., 2021).
The excess activity of lead-210 within sediment layers decays over time during the sedimentary process, adhering to the principle of radioactive decay (Natalia et al., 2020).This principle serves as the basis for determining the age of accumulated sediments over a period given that the early activity of lead-210 that was deposited on the sediment bed of the aquatic ecosystem can be estimated (Plater et al., 1992;Lubis, 2006).Assuming a steady sedimentation rate due to stable erosion in the watershed area, each sediment deposit is expected to have a similar initial excess activity of lead-210, leading to a significant decay with the depth of the sediment core (Al-Masri et al., 2002;Lubis, 2006).Lead-210 activity can then be calculated from the gradient of the exponential excess lead-210 against increasing mass or depth using the constant flux-constant sedimentation rate (CF: CS) model (Baxter et al., 1995).
Sedimentation rates and erosion have increased significantly in the last 150 years due to intensifying human activities in aquatic catchment areas (Appleby, 2008;Balaky et al., 2023).Consequently, the 210 Pb sediment concentration profile may not be linear.Therefore, mathematical models, including the constant rate of supply (CRS) model and the constant initial concentration (CIC) model, have been developed to calculate lead-210 dates under changing sediment accumulation rates (Hancock et al., 2011;Sanchez-Cabeza et al., 2012).The CIC model operates under the assumption that a rise in the sediment particle flux within the water column will result in the removal of an equivalent quantity of lead-210 from the water column to the sediments.As a result, the excess activity of lead-210 is expected to change with depth, as per the principle proposed by Pennington et al. (1976) expressed as below: where C(o) represents the concentration of excess lead-210 at the interface between sediment and water.The age (t) of a sediment deposit with a concentration of lead-210 is given as: , (2) The constant rate of supply model assumes a steady fallout of atmospheric lead-210 into the aquatic environment, resulting in a constant rate of supply of lead-210 to the sediment, regardless of changes in sedimentation rates (Sanchez-Cabeza et al., 2012).Proposed by Robbins et al. (1993), this model introduces the concept of excess lead-210 cumulative residue, denoted as A, in the bottom sediments at age t, t which varies as expressed in the formula: where A(o) represents the excess lead-210 overall residual in the sediment core and k represents the decay constant for radioactive lead-210, where k = ln (2)/T 1 /2.The values of A and A(O) are computed by mathematical integration of the lead-210 profile.Subsequently, the age of the sediment at depth x is then presented as: , The sediment accumulation rate is given by the formula , ( 5) The various models employed to establish the 210 Pb-specific profile of sediment cores, considering sedimentation rates, have been discussed earlier.However, the Constant Rate of Supply (CRS) model for excess 210 Pb is widely used in reservoirs and lakes, primarily due to urbanisation and constant change in land use activities in the catchments of these aquatic ecosystems, resulting in high sedimentation rates (Ritchie and McHenry, 1990).Furthermore, the CRS model accommodates variations in sediment accumulation rates with depth and successful application of the CRS model has been used to understand past environmental changes in the catchment area of aquatic ecosystems (Appleby, 2008).
Bukit Merah Reservoir has faced severe siltation and sedimentation challenges in recent decades due to inconsistent land-use policies and increasing agricultural activities in the catchment area (Ismail et al., 2011).Although some quantitative methods have been applied to study the sedimentation process due to human impacts in the reservoir (Hasan et al., 2011;Talib et al., 2012).However, critical information about past sedimentation rates, sources, and patterns, due to anthropogenic activities in the catchment area of the reservoir is lacking.Therefore, this study aims to utilize the CRS model to determine the age and sediment accumulation rates in relationship to time and depth of the sediment core from BMR.It is the first research to use the CRS model for studying the sedimentary process of BMR, and the results are anticipated to provide essential information on the lake's past sedimentation pattern, aiding in the management of the impacts caused by sediment loads into the reservoir.

Study Area
The Bukit Merah Reservoir is located approximately 65 km south of Penang, Ipoh State at Latitude: 05° 01' 35.42"N, Longitude: 100° 39' 42.92" E (Fig. 1).It was established in 1902 with a storage capacity of about 70 million m 3 .The reservoir boasts an average depth of 2.5 m and covers a surface area of 33 km 3 .It is physically divided into southern and northern sections by a railway line on a bridge and causeway spanning 4.7 km (Sharip et al., 2018).Integral to the Kerian Irrigation Scheme, BMR supports the cultivation of paddy rice fields of about 24,000 ha and serves as a crucial domestic water source for the residents of Larut Matang and Kerian districts (Ismail and Najib, 2011).Moreover, the reservoir plays multifaceted roles including flood control and drought mitigation, while also serving as a tourist destination since 1990s, after the establishment of hotels and resorts in the catchment area.Notably, the reservoir's outlet supplies water to Arowa fish farming sites (Hidzrami, 2010;Zakeyuddin et al., 2016).
Encompassing an area of 408 km 2 (Ismail and Najib, 2011), the reservoir catchment is characterized by various land uses, including oil palm plantations (117.28 km 2 ), rubber plantations (37.92 km 2 ), mixed agriculture (1.55 km 2 ), fruit orchards (1.63 km 2 ), and rice cultivation (15.55 km 2 ) (Fadhullah et al., 2020).Additionally, there are small and medium-scale industries, such as oil palm mills, wood production, and rubber processing industries (Fadhullah et al., 2020).Population growth and the continuous expansion of economic activities within the catchment area have significantly impacted the water quality of the reservoir.Furthermore, water diversion from catchment areas and lake reclamation for oil palm and rubber plantations pose substantial threats to the biodiversity and ecosystem services of the lake.The continuous excavation of sands in the Sungai Kurah catchment further disrupts the natural water flow in BMR, affecting both water quality and ecosystem dynamics (Zakeyuddin et al., 2016).

Sampling of Core
After conducting a bathymetric study in October 2019, a sediment core (BMR1) measuring 25 cm in length was obtained from the Bukit Merah Reservoir (Figure 2) at a depth of 2.6 meters, utilizing a gravity corer featuring an inner diameter of 9 cm.To prevent any potential mixing, the sediment core was held in an upright position and transported to the laboratory for analysis.Subsequently, the core was sliced at an interval of 1 cm and subjected to freeze-drying at -50°C using a 6-liter Labconco freeze-dryer system.After the freeze-drying process, all sediment samples were pulverized using a mortar and pestle and then passed through a sieve with a mesh size of 100 μm, and stored in whirl-pak® bags before undergoing analysis.

Radiometric Dating of Sediment Core
The activities of radioisotopes such as 210 Pb, 137 Cs, and 214 Am were analyzed using an intrinsic germanium detector for 210 Pb gamma dating, following the methods outlined by Appleby et al. (1988).Freeze-dried sediment samples, approximately 0.5 g each, were placed in plastic test tubes sealed with 2-Ton Epoxy® and left for three weeks to ensure radioactive equilibrium before being subjected to gamma counting.Unsupported 210 Pb was calculated by subtracting the radioactivity of 226 Ra (considered as unsupported 210 Pb) from the total radioactivity of 210 Pb.The detector's efficiency was standardized using sediment sources with known activity.The sediment's rate of accumulation and age were determined by applying the Constant Rate of Supply (CRS) sediment dating model (Appleby et al., 1988), employing linear regression between excess 210 Pb and core depth.

210 Pb Concentrations and activities of 137 Cs and 214 Am in the sediments of Bukit Merah Reservoir
Results revealed that the excess 210 Pb and unsupported 210 Pb profiles in the BMR core ranged between 217.68 and 352.47 Bq.kg -1 and 40.28 and 126.86 Bq.kg -1 , with mean mass fluxes rate of 5.47 kg.m -2 (Table 1).Also, the excess 210 Pb and unsupported 210 Pb did not exhibit an exponential decrease with depth, as shown in Figures 2a and 2b.The activity of 137 Cs was not detected, and low 241 Am activities were observed in two isolated samples, though inadequate for use as a marker (Fig. 3).However, the elevated concentration of lead-210 at a depth of 12.5 cm below the surface, along with the non-monotonic activities of 137 Cs and 214 Am in the reservoir's sediment core could be attributed to sediment redistribution and re-suspension processes correlated with the reservoir's aging, as suggested by Najib et al. (2017).Sediment resuspension in aging reservoirs is an important process involving the exchange of nutrients between the overlying water and sediment (Hoess and Geist, 2021).The impacts of nutrient re-suspension and transfer from sediment to the water column become more pronounced in shallow lakes (Adámek and Maršálek, 2013).Moreover, the suspension of organic materials from sediment into the water column can accelerate nutrient regeneration and the mineralization of organic compounds, influencing microbial ecology and nutrient dynamics in shallow reservoirs (Guo et al., 2021).Additionally, turbulent motions produced by wave action, currents, and winds may contribute to sediment particle fluxes in shallow waters (Guo et al., 2021;Cereja et al., 2022).
Conversely, bioturbation associated with burrowing organisms may bring newer sediment down into older sediment deposits (Kiwango et al., 2015).Burrowing activities in sediments can alter the microstructure and composition of the sediment (Ben-Awuah and Eswaran, 2015).These organisms rework sediments, organic matter, and mineral grains, changing the primary fabric of the sedimentary material (Jiang et al., 2010).Thus, influencing the fluxes of nutrients between the water column and sediments (Gosselin and Hare, 2003).Generally, the movement of sediments and significant alteration in sediment composition caused by burrowing organisms are the most widely recognized forms of bioturbation observed across various aquatic settings (Wang et al., 2010).Despite concerns about sediment mobilization and mixing, lead-210 proved to be a reliable tool in the Bukit Merah reservoir for assessing sediment accumulation rates and dating the sediment core.

Radiometric chronology and sedimentation rates of Bukit Merah Reservoir, Malaysia
The sediment age and sedimentation rate of Bukit Merah Reservoir were determined using the CRS sediment dating model (Sanchez-Cabeza et al., 1999) as shown in Figure 4 and Table 3.The CRS model, which assumes a constant rate of 210 Pb supply, dated the oldest sediments (22.5-24.5) to AD 1985 ± 34 years (Figure 4), indicating that the sediment accumulation rate of Bukit Merah Reservoir has undergone a significant increase over the last 34 years, with the minimum and maximum sedimentation rates of 0.14 and 0.37 gcm -2 yr -1 documented in 1985 and 1996, respectively (Table 3).Meanwhile, the mean sedimentation rate of 1.10 cm yr-1 documented in the reservoir surpasses values reported for some freshwater ecosystems worldwide.For instance, Waters et al. (2015) reported mean sedimentation rate values of 0.08-0.30g cm-2 y-1 in Lake Seminole, USA and Baskaran et al. (2015) estimated values of 0.12 to 0.29 g cm-2 y-1 in Lake Union, USA.Agricultural intensification, changing land use patterns, population growth, and urbanization have been major challenges contributing to the high sediment accumulation in most reservoirs in tropical areas (Mizael et al., 2020).However, the elevated sedimentation rate seen in Bukit Merah Reservoir indicated that the 1980s witnessed a surge in agricultural initiatives by the Perak government aligned with the Fourth Malaysian Plan (1981)(1982)(1983)(1984)(1985) (Shevade and Lobodo, 2019).This plan aimed to enhance the well-being of Malaysians by implementing various agricultural projects.Consequently, the watershed area of the reservoir experienced a flourishing of agricultural activities (Shevade and Lobodo, 2019), Common practices included soil tillage and forest clearing to make way for the cultivation of oil palm, rubber, and rice paddy.Notably, large-scale land clearing for replanting by smallholders or estates led to increased surface runoff, siltation, and a decline in water quality (Abdullah and Hezri, 2008).Through the 1990s the tourism sector underwent substantial development with significant investments.Over 1670 acres of government land and an additional 100 acres within the lake were allocated to property developers for the construction of water theme parks, hotels, and residential homes (Wan, 1985).This initiative aimed to boost state land revenue and promote tourism, potentially leading to an augmented sediment load in the reservoir (Wan, 1985).Whereas from 2000 upwards, heightened sedimentation into the reservoir was also influenced by small-scale sand mining activities upstream (Abdullah and Nakagoshi, 2006), Simultaneously, the establishment of small and medium-scale industries, including rubber manufacturing, oil palm mills, and wood industries, coupled with unregulated land use changes resulting from unplanned development activities around the BMR water catchment, further intensified sediment yields in the reservoir (Najib et al., 2017).

Conclusions
CRS model 210 Pb dates were used to establish sediment chronologies in Bukit Merah Reservoir, which also enabled the quantification of sediment deposition rates.Our results reveal discernible human impacts on the reservoir over the past 34 years based on dated sediment layers.Intensification of agricultural activities, population growth, and land-use changes are major factors identified as contributing to the accelerated sedimentation and particle loading into the reservoir, thus affecting the ecological balance of the reservoir.The findings also demonstrate the importance of using 210 Pb and independent markers like 137 Cs and 214 Am to understand past human impacts and temporal ecological changes in aquatic ecosystems, which underscores the need for sustainable land management practices and monitoring at the reservoir to mitigate further human impacts and deterioration of the reservoir.

Fig. 1 .
Fig.1.Map of Bukit Merah Reservoir showing its catchment area

Fig. 3 .
Fig.3.Depth distribution of 214 Am and 137 Cs in sediment core BMR2 from Bukit Merah Reservoir, Malaysia

Fig. 4 .
Fig.4.Depth and sedimentation rate versus age relations for sediment core from Bukit Merah Reservoir, Malaysia

Table 2 .
210Pb chronology of sediment core BMR2 from Bukit Merah Reservoir, Malaysia