Residence Time Of 210Pb Research Articles

The Atmospheric Input of Dissolvable Pb Based on the Radioactive 210Pb Budget in the Equatorial Western Indian Ocean

To estimate the atmospheric deposition flux of 210Pb in the equatorial western Indian Ocean, we determined the dissolved (<0.45 μm) and particulate 210Pb (>0.45 μm) in the water column. In addition, we calculated the atmosphere-derived dissolvable Pb in seawater using the budget of 210Pb. The dissolved 210Pb and total 210Pb were higher in the surface layer and, overall, showed a decreasing distribution with depth. In particular, radioactive 210Pb activities in the surface-to-upper layer (<1000 m depth) were 1.5 to 2 times higher than those reported in the 1970s (in nearby regions), suggesting that there has been additional 210Pb input in recent years. Based on the mass balance of the total 210Pb budget in the water column, we estimated the atmospheric deposition flux of 210Pb and the residence time of Pb for the first time in this region. The atmospheric deposition flux of 210Pb was estimated to be 0.1–0.5 dpm cm−2 yr−1, and these values agreed with the general global estimations for the major oceans (0.1–0.7 dpm cm−2 yr−1). Considering the residence time of 210Pb (29–41 years) in the water column (estimated from the 210Pb inventory and 234Th-based Pb scavenging rate), the atmospheric input of seawater-dissolvable Pb was quantified to be 0.08–0.1 nmol cm−2 yr−1, which is about eight times higher than the estimated input in the early 1990s in the region. Therefore, these results imply that radioactive 210Pb could be a useful tracer for quantifying Pb flux in seawater.

210Po and 210Pb as tracers for particle cycling in a shallow semi-enclosed bay of Taiwan Strait

Naturally-occurring 210Po and 210Pb are good tracers for particle cycling in the estuarine and coastal environment. Activities of 210Po and 210Pb were measured in samples of bulk (wet + dry) atmospheric deposition, river water and surface seawater collected in the Zhangjiang Estuary-Dongshan Bay to investigate the geochemical scavenging rates (or residence times) of these particle-reactive nuclides. The spatial distribution of 210Po and 210Pb were strongly influenced by suspended particulate matter (SPM) concentrations and in almost all stations, 210Po and 210Pb were mainly in particulate form (51.7%–92.2% of the total for 210Po and from 44.3% to 90.3% for 210Pb). Due to the very high SPM concentration, enhanced scavenging of 210Po and 210Pb was observed in the maximum turbidity zone of Zhangjiang River, with a marked decrease of dissolved nuclides’ activities. We built a mass balance model and we found out that atmospheric inputs are the major source of these radionuclides comparing with other sources (e.g., river inputs or ingrowth production) in the study area. Residence times of 210Pb were calculated to be 2.2–9.8 and 0.5–8.9 days for dissolved and particulate 210Pb, respectively. And residence times of dissolved and particulate 210Po were 6.3–31.7 and 2.2–13.9 days, respectively. In the turbidity zone, we found an unexpected result that particulate 210Po and 210Pb had the longest residence times (about two weeks), suggesting that SPMs in the turbidity zone would not be easy to sink to the sea floor. We also found that different SPMs showed significantly different 210Po/210Pb activity ratios, which increased in the following order: atmospheric deposition in dry season < SPMs in river < SPMs in the Zhangjiang Eastuary-Dongshan Bay < biogenic particles (including detritus, fecal pellets, cells of phytoplankton and zooplankton). This phenomenon implied that 210Po/210Pb activity ratio in the particles has the potential application in distinguishing SPM sources in the coastal seas.

226Ra, 210Po and lead isotopes in a pit lake water profile in Sweden

A pit lake arises as a consequence of anthropogenic activities in opencast mining areas. These water bodies may be enriched in hazardous stable contaminants and/or in naturally occurring radionuclides depending on the local geological conditions. Mining legacy in Sweden produced hundreds of these pit lakes and most of them are used for recreational purposes in the southern part of the country. In this paper, one pit lake was selected for having enhanced levels of natural radionuclides. Physico-chemical parameters (temperature, pH, oxidation-reduction potential, dissolved oxygen and depth), elemental composition (via Inductive Coupled Plasma Mass Spectrometry) and radiometric characterization (via alpha spectrometry of 226Ra, 210Po and 210Pb) were carried along the depth of a 60 m depth pit lake, with the main aim to describe how natural radionuclides and elements behaves with depth in a non-uraniferous pit lake.Based on observed changes in physico-chemical parameters, a thermocline and a chemocline region were identified at around 10 and 30 m depth respectively. Concerning radionuclides, 226Ra ranged from 75 ± 3 up to 360 ± 12 mBq/kg while 210Po ranged from 11 ± 1 up to 71 ± 3 mBq/kg. 210Pb distribution with depth was also determined via secular equilibrium with 210Po after 2 years and also stable Pb was measured. Disequilibrium 226Ra-210Pb was found and the residence time of 210Pb in the water column was assessed. Additionally, different vertical distributions between 210Pb and Pb were found which points out different sources for different lead isotopes in the water body.

Inventory, fluxes and residence times from the depth profiles of naturally occurring 210Pb in marine sediments of Mumbai Harbor Bay

Measurements of the vertical concentration profile of deposited 210Pb and its parent (226Ra) in cores of recent marine sediment of Mumbai Harbor Bay (MHB) were made to determine the various depth distribution parameters (sedimentation rate, age of sediment, total inventory, fluxes, residence times, scavenging rate, linear attenuation coefficient and relaxation mass depth). Overall, the profile distributions of 226Ra and 210Pb showed a decreasing trend with increasing the depth. The mean of sedimentation rate in the studied area was obtained to be about 0.76 cm year−1 which corresponds to average age of sediment of about 56 years. The average total deposited 210Pb for the studied sediment profile was 9.68 k Bq m−2 and correspondingly the average value of fluxes of total 210Pb and excess 210Pb (atmospheric flux) were 0.59 and 0.32 k Bq m−2 year−1, respectively. The residence time of 210Pb was in the range of 26–226 years throughout the sediment profile and showed generally an increasing trend from top to bottom. The mean relaxation mass depth (h0) derived from the depth profile of 210Pb at the studied area was about 190 kg m−2. The negative association between clay content of cores and the h0 of 210Pb indicates that as the clay content increases, the penetration depth of deposited 210Pb in sediment profile decreases.

Importance of lateral transport processes to 210Pb budget in the eastern Chukchi Sea during summer 2003

Activity concentrations of dissolved and particulate 210Pb and 226Ra in the water column were measured in the eastern Chukchi Sea during summer 2003. 234Th/238U disequilibria were used to estimate the scavenging fluxes of 210Pb from the water column to the underlying sediments. Our results showed that concentrations of 210Pb and its distributions were mainly influenced by mixing processes of water masses and sediment resuspension. The residence times of 210Pb in the eastern Chukchi Sea ranged from 5 to 103d. Short residence times were mostly observed at the shelf stations, indicating a more effective particle scavenging in the shelf region. A mass balance model was constructed to evaluate the contribution of lateral transport to 210Pb budget in the water column. The lateral transport fluxes of 210Pb ranged from 17 to 177Bq/m2/a, comprising up to 63–94% of the total supply of 210Pb in the eastern Chukchi Sea. We hypothesize that the accumulative removal of 210Pb in the Pacific inflow waters during their transport across the Chukchi Sea and the import of 210Pb from sea ice rafted sediments are the two major lateral transport pathways for the import of 210Pb to the eastern Chukchi Sea. Our results highlight the importance of lateral transport processes to the geographical distribution of particle-reactive elements and their biogeochemical cycles in the Arctic Ocean.

Geochemical behavior of 210Pb and 210Po in the nearshore waters off western Taiwan

Dissolved and particulate 210Pb and 210Po were determined at 15 stations along the coastline off western Taiwan in April 2007. The 210Pb activities in dissolved and particulate phases fell within a relatively small range of 2.4–5.2dpm 100L−1 and 1.0–3.2dpm 100L−1, respectively. The dissolved and particulate 210Po activities also fell within a small range of 0.8–3.4dpm 100L−1 and 1.1–2.9dpm 100L−1, respectively. The correlation of the distribution coefficients (Kd) of 210Pb and 210Po with particle concentration in turbid waters are not as evident as in the open ocean. The mass balance calculation shows that the residence times of 210Pb and 210Po with respect to particle removal from the nearshore waters ranges from 3 to 15days and from 14 to 125days, respectively. The flux of particulate organic carbon was estimated by 210Po proxy and ranged from 4.8 to 33.7mmol-Cm−2d−1.

Geochronology of Pb and Hg pollution in a coastal marine environment using global fallout 137Cs

Global fallout 137Cs was used for dating sediment cores and estimation of recent sedimentation rates (up to 1 cm/y) in the Thane Creek, which lies in the southern part of the Deccan belt of India. The residence time of 210Pb in the Thane Creek water was calculated to be 0.7 years. Further, the concentrations of Pb (up to 70 μg/g) and Hg (up to 10 μg/g) in sediment profiles were measured to assess the anthropogenic input of contaminants due to large-scale industrialization, which has taken place in this area over the last two decades. The depth-wise concentration profile of Hg shows positive evidence of continued fresh input into the Creek.

Balance and residence times of 210Pb and 210Po in surface waters of the northwestern Mediterranean Sea

Concentrations of the naturally occurring radionuclides 210Pb and 210Po were determined in both the dissolved (<0.2 μm) and particulate (>0.2 μm) fractions in surface waters of the northwestern Mediterranean Sea. About 4–8% of the 210Pb activity was observed to be associated to particles, which were determined as being mostly biogenic. The 210Po content in the particulate fraction was generally greater than that of 210Pb, accounting for about 20% of the total activity. Total activities of 210Pb and 210Po were found to be in equilibrium. 226Ra concentrations were also determined in an effort to investigate its occurrence in Mediterranean waters. 226Ra was in excess to both 210Pb and 210Po, with a mean concentration of 1.72±0.03 Bq m −3. Steady-state balance equations of the three radionuclides permit calculation of residence times for 210Pb and 210Po in surface waters (0.8 and 3.0 yr, respectively). 210Po residence times were almost one order of magnitude higher than mostly reported values. Polonium is known to manifest higher affinity and different binding mechanisms (by entering the organic cycle) to biogenic material than 210Pb. Several processes, such as 210Po uptake by buoyant particles and recycling of organic matter in the surface layer, could lead to such long residence times of 210Po and, therefore, of organic particles in surface waters. Larger 210Po atmospheric fluxes than those here considered and inputs from rivers and/or fine-grained sediment resuspension could also help to explain the observation, although presumably to a lesser extent.

210Pb, 226Ra and 32Si in Pavin lake (Massif Central, France)

Measurements of cosmogenic 32Si and the U-decay series' nuclides 210Ph and 226Ra in waters and sediments of lake Pavin are reported. Both 210Pb and 226Ra are enriched in the anoxic deep waters compared to the oxic surface waters, respectively by a factor of 4 and 10, whereas 32Si is depleted by a factor of ∼ 2. Redox conditions in the lake appear to have no marked effect on the 32Si. Using a steady-state box model it is shown that the deep-water 32Si concentration is controlled by the underground lacustrine springs. The residence times of 210Pb, 32Si and 226Ra are ∼ 1, ∼ 10 and ∼ 80 a, respectively. In the case of 32Si where more data are available, the assessed inventory data from the overhead atmospheric fallout and that measured in the sediments agree very well as expected. The 210Pb, and 32Si-based deposition rates during the past ∼ 100 a ranged from 0.8 to 1.9 mm a −1, earlier these were a factor of ∼ 3–5 faster. The geochemistry of 32Si and 210Pb, in lake Pavin in many ways resembles that in the ocean, only the time scales of the processes involved are faster.

210Pb, 226Ra and 32Si in Pavin lake (Massif Central, France)

Measurements of cosmogenic 32Si and the U-decay series' nuclides 210Pb and 226Ra in waters and sediments of lake Pavin are reported. Both 210Pb and 226Ra are enriched in the anoxic deep waters compared to the oxic surface waters, respectively by a factor of 4 and 10, whereas 32Si is depleted by a factor of2. Redox conditions in the lake appear to have no marked effect on the 32Si. Using a steady-state box model it is shown that the deep-water 32Si concentration is controlled by the underground lacustrine springs. The residence times of 210Pb, 32Si and 226Ra are1,10 and80 a, respectively. In the case of 32Si, where more data are available, the assessed inventory data from the overhead atmospheric fallout and that measured in the sediments agree very well as expected. The 210Pb- and 32Si-based deposition rates during the past100 a ranged from 0.8 to 1.9 mm a −1, earlier these were a factor of3–5 faster. The geochemistry of 32Si and 210Pb in lake Pavin in many ways resembles that in the ocean, only the time scales of the processes involved are faster.

A multitracer study of radionuclides in Lake Zurich, Switzerland: 2. Residence times, removal processes, and sediment focusing

Residence and settling times of particles and particle‐reactive nuclides evaluated from in situ tracer studies can be used as diagnostic indicators of trace element pathways in lakes. Natural (e.g., 210Pb, 7Be, and 10Be ) and artificial (i.e., Chernobyl 137Cs) radionuclide fluxes through Lake Zurich (at 50 and 130 m depth) from 1983 to 1987, atmospheric fluxes of the same nuclides and fluxes of 10Be during 1987 allowed the calculation of nuclide residence times as well as particle settling and transit velocities in Lake Zurich. The residence time of 210Pb in the lake is approximately 1 month and, hence, of the same order as the residence times of particles and stable Pb. Steady state residence times for 7Be were calculated as 150–170 days, while non steady state, instantaneous removal residence times ranged from 50 to 800 days. Polonium 210 was removed from Lake Zurich with removal times of 10–26, months indicating slow removal processes or efficient recycling in the lake water. Transit velocities of particles, calculated from the attenuation of Chernobyl 137Cs fluxes at 50 m and 130 m depth during early May 1986, were 17 m d−1, while 2–4.5 m d−1 was calculated as an effective settling velocity from the attenuation of 7Be fluxes at 130 m depth as compared to 50 m depth. Model calculations reveal that the overall removal process of atmospherically deposited 7Be from the water column to the sediments is mainly controlled by the adsorption/coagulation step and not by the rate of particle settling. Extra inputs of 10Be, 210Pb, or 137Cs into the lower trap (i.e., “rebound flux”), originating from episodic lateral inputs of fine particles with a high content of 210Pb, 10Be, and 137s, were observed during the lake stratification period (i.e., summer). This novel observation of summertime radionuclide and sediment focusing is different from previously described wind‐generated resuspension and focusing effects caused by river plumes and lake sediments during the stagnation period. Boundary and focusing effects for radionuclide‐bearing particles are proposed to explain observed summertime enhancements of longer‐lived nuclide fluxes at 130 m depth as well as the decrease in calculated effective settling velocities.

Comparative behavior of 210Pb and Ra isotopes in the waters of Tokyo Bay and Osaka Bay mixing zones.

Measurements of 228Ra, 226Ra and 210Pb in the waters of Tokyo Bay and Osaka Bay mixing zones are presented. 228Ra concentrations and the activity ratios of 228Ra/226Ra are considerably higher in Osaka Bay than in Tokyo Bay. This probably reflects the different riverine/estuarine input of 228Ra from the two geologically different drainage zones of Tokyo Bay and Osaka Bay areas (volcanic versus granitic), because of the way in which 228Ra, a daughter of 232Th, is leached and transported by rivers. The Ra isotope distributions follow the trend of conservative mixing with open ocean water in which Ra isotope concentrations are low. Using a simple box-model, we estimate a value of 1.4±0.2 months for the mean residence time of Osaka Bay water and a value of 0.25 dpm cm-2y-1 for the 228Ra flux from Tokyo Bay sediments. In contrast to 228Ra, 210Pb concentrations in Tokyo Bay and Osaka Bay waters are the same and nearly constant at 34 dpm/103l, a value considerably lower than 200 dpm/103l in the Kuroshio surface water. The non-conservative behavior of 210Pb during mixing demonstrates that 210Pb is rapidly removed near the front of Kuroshio Current far outside the bays, and hence there is no significant lateral input of 210Pb from offshore to the two bays. The mean residence time of 210Pb in seawater is also calculated to be 2 weeks for Tokyo Bay and 3 weeks for Osaka Bay.

The geochemistries of 210Po and 210Pb in waters overlying and within the Orca Basin, Gulf of Mexico

Vertical profiles of dissolved 210Po, 210Pb, and 226Ra were obtained from waters overlying and within the Orca Basin; an anoxic, hypersaline and virtually non-sulfide bearing basin in the Gulf of Mexico. The station had a depth of 2410 m. The seawater-brine interface was located at 2230 m. In oxic waters, near-surface (0–250 m) concentrations of dissolved 210Po and 210Pb ranged from 6 to 8 dpm/100 kg and decreased to about 3 dpm/100 kg at 740 m. Below this depth concentrations remained relatively constant. Near-surface 226Ra concentrations varied from about 7 to 9 dpm/100 kg, and then increased with depth to a relatively uniform deep-water value of approximately 13 dpm/100 kg, in agreement with previously reported data for the Gulf of Mexico. The slight deficiency of 210Pb in the surface waters may result from mixing with inner shelf waters that are enriched in 226Ra. At the seawater-brine interface, sharp maxima were observed in the profiles of 210Po, 210Pb, and 226Ra, where concentrations reached 56.0, 28.3, and 840 dpm/100 kg, respectively. These are among the highest values reported for the marine environment to date. Below the interface, concentrations decreased to about 9, 6, and 210 dpm/100 kg for 210Po, 210Pb, and 226Ra, respectively. These sharp maxima may be caused by a combination of the accumulation of particles in the strong pycnocline, the subsequent dissolution of particulate hydrous Mn and Fe oxides and hydroxides, and the decomposition of biogenic particulate material. The residence time of 210Pb in the brine is about one year, a value similar to that reported for the anoxic waters of the Cariaco Trench even though the concentration of 210Pb in that basin is much lower.