Using 81Kr and isotopic tracers to characterise old groundwater in the Bangkok metropolitan and vicinity areas.

Krypton-81 was applied to investigate the age of groundwater in the aquifer system in the Bangkok metropolitan and vicinity areas. Stable (2H, 18O and 13C) and radioactive (3H, 85Kr and 14C) isotopes and noble gases were applied in parallel. Low levels of 14C and significant radiogenic 4He confirm that groundwater in the deep aquifers is older than 30 ka. 81Kr analysis identified groundwater with ages ranging from 17 to 300 ka. At some sites, large age discrepancies between 81Kr and 14C indicated that inter-aquifer mixing is likely occurring. The interpretation of the noble gases suggests that groundwaters in the deeper aquifers, with apparent ages of 300 to 10 ka, have recharged in slightly colder and wetter climates than those found in the upper aquifers with apparent ages < 10 ka. Degradation of water quality from seawater intrusion was identified in the upper four aquifers. This was also evidenced by higher δ18O and δ2H values, typical of seawater. The four deeper aquifers contain high quality water characterised by less enriched 18O and 2H. This work presents new findings of very old groundwater in the Bangkok aquifer system.

Distribution and source of plutonium in sediments from the southern Gulf of Mexico.

Here, we report on new data (75 analyses) of plutonium (Pu) isotopes to elucidate activity concentrations, inventories, sources, and their transport from the ocean surface to the seafloor from a collection of six deep-sea sediment cores (depths ranging from 257 to 3739 m) in the southern Gulf of Mexico. Sediment cores collected from the continental shelf and upper slope region of the Gulf of Mexico showed 240Pu/239Pu ratios of 0.15 to 0.26, and 239+240Pu-inventories ranging from 14.7 to 33.0 Bq m-2. Inventories and ratios are consistent with global fallout Pu for this tropical region. In contrast, sediment cores collected from the lower slope region and abyssal plain showed low 240Pu/239Pu ratios of 0.07 to 0.13 and much lower 239+240Pu inventories below 6.8 Bq m-2. This implies that only a small fraction of the expected global fallout Pu has reached the deep-sea sediments. The low 240Pu/239Pu isotope ratios indicate that fallout from the Nevada testing site was an important source of Pu in deep-sea sediments, and that this Pu was likely more efficiently scavenged from the water column than Pu from global fallout. We estimated that up to 44% of the total inventory of 239+240Pu in deep-sea sediments is due to the Nevada source. Low values and a progressive decrease of 240Pu/239Pu ratios and 239+240Pu inventories with increasing water depth have been previously reported for the Gulf of Mexico. Analysis of Pu isotopes in two sediment traps from the upper slope regions shows 240Pu/239Pu ratios comparable to those observed in global fallout. These results indicate that global fallout Pu is currently the main source of Pu in sinking particles in the water column. Therefore, a significant fraction of global fallout Pu must still be present, either in a dissolved phase, or as biologically recycled material in the water column, or scavenged on the shelf and shelf break. Our results bring to light important questions on the application of Pu isotopes to establish sediment chronologies in deep-sea sediments, since global fallout features such as the 1963 maximum are not available.

Assessment of residual radionuclide levels at the Bokak and Bikar Atolls in the northern Marshall Islands.

The Bikar and Bokak Atolls, located in the northern Marshall Islands, are extremely isolated and consist of pristine marine and terrestrial ecosystems. Both atolls may have experienced significant radioactive deposition following the nuclear weapon testing conducted at Bikini and Enewetak proving grounds. Here we report activity concentrations of artificial radionuclides (239Pu, 240Pu, 241Pu, 241Am, 137Cs and 90Sr) in marine and terrestrial samples collected from Bikar and Bokak Atolls. Artificial radionuclides in soil from the Majuro Atoll are also reported and form a radiological baseline against which the levels at the other atolls can be compared. We observed low levels of artificial radionuclides in soil from Majuro and Bokak, but significantly higher levels in soil from Bikar. The residual radioactivity in the Bikar environment is comparable to the levels previously reported for other nearby atolls, including Taka and Utrik, but lower than for Rongerik, Rongelap, Bikini and Enewetak. An analysis of 240Pu/239Pu isotope ratios and estimations of the dates of contamination from 241Am/241Pu activity ratios both indicated that the Bikar Atoll was contaminated mainly by radioactive fallout from the Castle Bravo test in 1954. We compare the results of our measurements at Bikar and Bokak to data from other atolls in the Marshall Islands and to regions of the world affected by both global and regional fallout from atmospheric nuclear weapons testing and nuclear accidents.

Determination of Plutonium and Uranium Radionuclides in Glacier Ice Samples by MC-ICP-MS.

A radiochemical procedure for the determination of plutonium (Pu) and uranium (U) radionuclides in ice samples by multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) is presented. Pu and U radionuclides are preconcentrated by coprecipitation and then separated by extraction chromatography. The purified Pu and U fractions are analyzed by MC-ICP-MS. Detection limits of 2 × 10 -3 and 3 × 10-6 mBq kg-1 were achieved for 239Pu and 236U, respectively. Surface ice samples collected from the Gauli glacier (Switzerland) were analyzed by this method. The surface of the Gauli Glacier retains historical records of 239Pu, 240Pu and 236U from the nuclear weapon testing (NWT) period. Pu and U radionuclides were found to be consistent in terms of pattern, showing two peaks possibly related to the two main periods of the NWTs (1954-1958 and 1961-1963). ³H measurements, also released by the NWT, further confirmed the Pu and U results. The 240Pu/239Pu ratio ranged from 0.14 to 0.25, and 236U/ 239Pu ranged from 0.14 to 0.81. The Pu atom ratios ranged within the limits of global fallout in the most intensive period of NWT (1952 to 1962).

Calibration and use of well-type germanium detectors for low-level gamma-ray spectrometry of sediments using a semi-empirical method.

Gamma-ray spectrometry is a widely used technique to quantify the presence of numerous radionuclides in environmental samples. In this work, we describe a methodology for efficiency calibration of four well-type germanium detectors and their use for the determination of low-level activities of gamma emitters in sediment samples. An experimental efficiency calibration for each detector was built with three materials for 17 energies, ranging from 46.5 keV to 1460 keV. For efficiency transfer to different geometries and sample types, we used the effective solid angle approach (ET-Ω method). Final calibrations were calculated for all detectors, several counting geometries, and elemental composition of selected sample types. Calibrations were validated with six reference materials. This methodology allowed to reliably analyze nine gamma emitters (210Pb, 241Am, 234Th, 228Ac, 214Pb, 208Tl, 137Cs, 134Cs and 40K) in sediment samples. Using these calibrations, gamma emitter profiles of sediment cores from contrasting aquatic systems (lake, intertidal, marine and deep-sea areas) provided reliable profiles of 210Pb and artificial radionuclides useful for dating and stratigraphic interpretation. A protocol to implement this methodology is also presented.

Quality and management status of the drinking water supplies in a semiarid region of Northeastern Brazil.

Surface and groundwater resources in the Seridó Region (Brazilian semiarid) were investigated to evaluate their current quality conditions and suitability for domestic use. The water was characterized in terms of physical, chemical, and radiological parameters; including those required by the Brazilian Drinking Water Quality Standard (DWQS). Information about major and trace elements and radiological aspects of the water are reported for the first time. Salinization was confirmed as a key problem in the region, driven natural and anthropogenic. Overall, water has poor organoleptic characteristics. The concentration of most trace elements was below the recommended level, except for uranium and selenium in groundwater. Gross alpha and beta activities higher than the recommended levels were also recorded in several water samples, mostly from the investigated aquifers. In these samples, a detailed radionuclide analysis is required to estimate the effective dose received by the local population. Overall, the results show that water from the investigated region is not suitable for human consumption unless proper treatment is applied. Water requires proper treatment to decrease the content of dissolved salts, toxic elements, and radionuclides responsible for the high gross alpha and beta activities.

Application of isotope techniques to study groundwater resources in the unconsolidated aquifers along the Ping River (Thailand).

Severe droughts during the dry season and floodings during the rainy season are among the major problems encountered in the valleys along the Ping River in Thailand. Improving our understanding of the groundwater resources in this agricultural area is an important issue for the sustainable development of the region. Hence, in order to gain understanding on the groundwater flow dynamics in the unconsolidated shallow aquifers along the Ping River, stable (13C, 18O, 2H, noble gases) and radioactive (3H, 14C) isotope techniques were combined with hydrogeochemical tools. The comprehensive interpretation of the chemical and isotope data consistently showed different origins for groundwater in the northern and southern areas of the investigated aquifers. Groundwaters in the northern part have younger 3H/3He ages, are less mineralized, and have suffered a stronger evaporation than groundwaters in the southern part of the aquifer. Overall, our results are consistent with the hydrogeological situation of the investigated area, namely shallow groundwaters and a spatially extended recharge. 3H/3He apparent ages indicated that young groundwater (<50 years) is present in all the investigated wells. Elevated concentrations of some pollutants (nitrate and phosphate) in parts of the aquifer evidenced a relatively high vulnerability of the aquifer to surface pollution.