Determination of²¹⁰Pb in lake sediments and in air samples by direct gamma-ray measurement

High resolution gamma spectrometry is one of the most widely used techniques in the measurements of environmental level ²¹⁰Pb in sediment from coastal and freshwater environments and such measurements are needed to establish ²¹⁰Pb chronology for the past 100–150 years. Precise measurement of ²¹⁰Pb in sediment and soil requires appropriate self-absorption correction for its low-energy (46.5 keV) gamma radiation due to differences in the matrix between the sample and standard used to calibrate the instrument. Here we report a method that involves precise determination of ²¹⁰Pb activity in sediment sample by calibrating the HPGe well detector with RGU-1-IAEA Certified Reference Material for well-defined geometries. A comparison of the ²¹⁰Pb activity obtained from gamma-ray spectrometry with that obtained from alpha spectrometry via ²¹⁰Po, using ²⁰⁹Po as yield tracer, indicates good agreement. We propose an empirical relation between the absolute efficiencies and packing densities of sample in a well-defined geometry (cylindrical counting vial) by affecting the count rate of ²¹⁰Pb and its progenitor, ²²⁶Ra (via ²¹⁴Pb and ²¹⁴Bi). The effects of self-attenuation of 46.5 keV (²¹⁰Pb), for naturally-occurring high-density minerals (apatite, titanite, monazite, and cerite) are evaluated. Specific activity of ²¹⁰Pb on apatite measured by alpha and gamma spectrometry are compared. This study is relevant and useful for precise measurements of gamma-emitting environmental radionuclides such as ²¹⁰Pb, ⁷Be, ¹³⁷Cs as well as ²²⁶Ra.

Lake Dian, the largest lake on South China’s Yunnan-Guizhou Plateau, exhibits two abnormal funnel-like features in the deepest part of the lake. Nevertheless, how and when these funnels formed and their implications and potential environmental and ecological effects remain unclear. To address these questions, we carried out a detailed multi-method survey including underwater topographic monitoring, extraction of sediment records, and geophysical exploration across the funnel areas. Sediment records revealed a significant sedimentation hiatus around these features. For example, sediments with thicknesses of ca. 69.24 cm and 83.56 cm in cores D4 and D6 were eroded based on the calculated of sedimentation rates. Deformed sedimentary structures and a hiatus, together with large-scale funnel-shaped topography, prove that underlying karst collapse may trigger massive water outflow through these funnels, which could result in an erosional hiatus in the local lakebed. Continuous clay deposits in the upper part of the sedimentary column suggest that water leakage may have ceased since the 1930s. However, bathymetric surveys conducted in October 2015 and September 2019 indicated that the maximum lake depth increased by 0.97 m and 0.2 m in the large and small funnels, respectively, over this 4-year period. The corresponding increased volumes were 1520 m³ and 375 m³ in the large and small funnels, respectively. Numerous pits were distributed around the funnels. Cumulatively, these underwater topographic changes reveal that small-scale, slow water leakage has been developing, which could potentially trigger catastrophic water discharge due to increasing hydraulic pressure and/or underlying karstification. If catastrophic water outflow occurs, ~14.05 × 10⁸ m³ of highly polluted water would not only contaminate the upper reaches of the large river and/or the lake itself, but would also endanger regional water security and induce widespread negative ecological impacts.

Reconstructing the history of metal deposition in Singapore lake sediments contributes to understanding the anthropogenic and natural metal deposition in the data-sparse Southeast Asia. To this end, we present a sedimentary record of Pb, Pb isotopes and eleven other metals (Ag, As, Ba, Cd, Co, Cr, Cu, Ni, Tl, U and Zn) from a well-dated sediment core collected near the depocenter of MacRitchie Reservoir in central Singapore. Before the 1900s, the sedimentary Pb concentration was less than 2 mg/kg for both soil and sediment, with a corresponding ²⁰⁶Pb/²⁰⁷Pb of ∼1.20. The Pb concentration increased to 55 mg/kg in the 1990s, and correspondingly the ²⁰⁶Pb/²⁰⁷Pb decreased to less than 1.14. The ²⁰⁶Pb/²⁰⁷Pb in the core top sediment is concordant with the ²⁰⁶Pb/²⁰⁷Pb signal of aerosols in Singapore and other Southeast Asian cities, suggesting that Pb in the reservoir sediment was mainly from atmospheric deposition. Using the Pb concentration in the topmost layer of sediment, the estimated atmospheric Pb flux in Singapore today is ∼1.6 × 10⁻² g/m² yr. The concentrations of eleven other metals preserved in the sediment were also determined. A principal component analysis showed that most of the metals exhibit an increasing trend towards 1990s with a local concentration peak in the mid-20^th century.

This chapter summarizes the use of U- and Th-series nuclides particularly ²²²Rn and its daughter products in the study of scavenging processes in the atmosphere. The chapter also includes progress made in these fields during recent years, as well as perspectives on their use in aquatic systems. ²³⁸U and ²³²Th are primordial radionuclides contained in rocks and minerals. Other sources of U- and Th-series nuclides to the atmosphere include volatile emissions from biogenic sources, and anthropogenic activities such as fossil fuel combustion, depleted uranium from military activities, sewage digestion, landfills, and mining activities. The distribution of U- and Th-series nuclides in the atmosphere is governed by their supply, atmospheric circulation/mixing, removal by scavenging/deposition and radioactive decay. The scavenging of nuclides from the atmosphere and their subsequent deposition on Earth's surface are important processes governing their supply to lakes and ocean surface waters and provides the basis for their applications as tracers to investigate selected processes in these reservoirs. The atmospheric deposition flux of ²¹⁰Pb and other nuclides can be determined directly or by sampling natural collectors. Among the U-series nuclides of atmospheric origin, ²¹⁰Pb has found extensive applications to determine chronologies of a variety of aqueous system deposits and thereby provide a time frame for records stored in them. The generation of ²¹⁰Pb in the atmosphere from its gaseous grandparent ²²²Rn has led to many uses in tracing the transport and scavenging of similarly generated elements. Another application of ²¹⁰Pb has been to track the deposition of trace metals from the atmosphere to the sea surface. However, these applications require a large number of nuclide measurements, specifically high-resolution spatial and temporal measurements of ²²²Rn and its daughters in the atmosphere. Recent studies should enhance the use of U-series nuclides as tracers of atmospheric and their subsequent aquatic processes.

A method of efficiency calibration for ²¹⁰Pb determinations by gamma-ray spectrometry in sediment samples is outlined. This method, developed for a cylindrical sample geometry with coaxial Ge detectors, is based on the gamma transmission method. It accounts for variable sample height and supplies a fundamental advantage: individual self-absorption corrections can be easily determined knowing the apparent densities and the elemental composition of the samples. This calibration has been used for dating sediments of the Huelva estuary (southwest of Spain). The results obtained by the application of the proposed calibration method to superficial sediments shows good agreement with those obtained by alpha-particle spectrometry. This fact validates it. Additionally, this calibration has been used successfully for dating sediment cores through the application of the ²¹⁰Pb method.

This study examines the influence of the Atchafalaya River, a major distributary of the Mississippi River, on stratigraphic evolution of the inner continental shelf in the northern Gulf of Mexico. Sedimentary, geochemical, and shallow acoustic data are used to identify the western limit of the distal Atchafalaya subaqueous delta, and to estimate the proportion of the Atchafalaya River's sediment load that accumulates on the inner shelf seaward of Louisiana's chenier-plain coast. The results demonstrate a link between sedimentary facies distribution on the inner shelf and patterns of shoreline accretion and retreat on the chenier plain. Mudflat progradation on the eastern chenier-plain coast corresponds to the location of deltaic mud accumulation on the inner shelf. On the central chenier-plain shelf, west of the subaqueous delta, relict sediment is exposed that was originally deposited between ∼1200 and 600 years BP during activity of the Lafourche lobe of the Mississippi Delta complex. Mass-balance calculations indicate that the eastern chenier-plain inner shelf and coastal zone form a sink for 7±2% of the sediment load carried by the Atchafalaya River.