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.

Groundwater residence time estimates obscured by anthropogenic carbonate.

Groundwater is an important source of drinking and irrigation water. Dating groundwater informs its vulnerability to contamination and aids in calibrating flow models. Here, we report measurements of multiple age tracers (14C, 3H, 39Ar, and 85Kr) and parameters relevant to dissolved inorganic carbon (DIC) from 17 wells in California's San Joaquin Valley (SJV), an agricultural region that is heavily reliant on groundwater. We find evidence for a major mid-20th century shift in groundwater DIC input from mostly closed- to mostly open-system carbonate dissolution, which we suggest is driven by input of anthropogenic carbonate soil amendments. Crucially, enhanced open-system dissolution, in which DIC equilibrates with soil CO2, fundamentally affects the initial 14C activity of recently recharged groundwater. Conventional 14C dating of deeper SJV groundwater, assuming an open system, substantially overestimates residence time and thereby underestimates susceptibility to modern contamination. Because carbonate soil amendments are ubiquitous, other groundwater-reliant agricultural regions may be similarly affected.

Controls on the 36Cl/Cl input ratio of paleo-groundwater in arid environments: New evidence from 81Kr/Kr data.

Measurements of the long-lived 81Kr and 36Cl radioisotopes in groundwater from the Negev Desert (Israel) were used to assess the 36Cl/Cl input ratios and Cl- contents for paleorecharge into the Nubian Sandstone Aquifer (NSA). The reconstructed Cl- content of the recharge flux was on the order of 300-400 mg/L. An initial 36Cl/Cl ratio of 50 × 10-15 was assessed for the groundwater replenishment in the Negev Desert since the late Pleistocene, in agreement with the 36Cl/Cl ratios in recent local rainwater. This is despite possible changes in the climatic conditions and the 36Cl production rates in the atmosphere over this timeframe. This similarity in values is explained by the major role played by the erosion and weathering of near-surface materials in the desert environment that dominate the hydrochemistry of rains, floods, and the consequent groundwater recharge. Spatial variation in the reconstructed initial 36Cl/Cl ratio is accounted for by the differences in the mineral aerosol sources for specific recharge areas of the NSA. Accordingly, regional variations in the 36Cl/Cl input in groundwater reservoirs surrounding the Mediterranean Sea indicate various processes that govern the 36Cl/Cl system. Finally, the results of this study highlight the great advantage of integrating 81Kr age information in evaluating the initial 36Cl/Cl and Cl- input, which is essential for the calibration of 36Cl radioisotope as an available long-term dating tool for a given basin.

Radiokrypton unveils dual moisture sources of a deep desert aquifer.

In arid regions, groundwater is a vital resource that can also provide a long-term record of the regional water cycle. However, the use of groundwater as a paleoclimate proxy has been limited by the complex hydrology and the lack of appropriate chronometers to determine the recharge time without complication. Applying 81Kr, a long-lived radioisotope tracer, we investigate the paleohydroclimate and subsurface water storage properties of the Nubian Sandstone Aquifer in the Negev Desert, Israel. Based on the spatial distributions of stable isotopes and the abundance of 81Kr, we resolve subsurface mixing and identify two distinct moisture sources of the recharge: one recent (<38 ky ago) from the Mediterranean and the other 361 ± 30 ky ago from the tropical Atlantic, both of which occurred under conditions of low orbital eccentricity comparable to that of the present. The recent recharge provided by the moisture from Mediterranean cyclones can be attributed to the southward shift of the storm track during the Last Glacial Maximum, and the earlier recharge can be attributed to moisture from the Atlantic delivered as tropical plumes under a climate colder than the present. Furthermore, the residence time of the latter reveals that tectonically active terrain can store groundwater for an unexpectedly long period, likely due to strongly attenuated groundwater flow across the fault zones. With this tracer, groundwater can now serve as a direct record of paleoprecipitation over land and of subsurface water storage from the mid-Pleistocene and onward.