Radium isotopes to trace uranium redox anomalies in anoxic groundwater

Autor: Eva Lena Tullborg, Juhani Suksi, Don Porcelli, Luis Auqué-Sanz, Lindsay Krall, Giada Trezzi, Jordi Garcia-Orellana, Per Andersson
Rok vydání: 2020
Předmět:
Zdroj: Chemical Geology. 531:119296
ISSN: 0009-2541
Popis: 223Ra,224Ra,226Ra, and228Ra isotopes have been measured in groundwaters from depths ranging 50–900m in fractured crystalline bedrock (Forsmark, Sweden) to understand the reason for elevated (up to 150μg/L) aqueous uranium (Uaq) at 400–650m depth. Ra isotope data is interpreted alongside previously reported222Rn,234U, and238U data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [223Ra/226Ra]GW-based approach (where brackets and “GW” subscript refer to expression of an activity ratio measured from groundwater) to groundwater residence time estimation shows that elevated [Uaq] is most common in Holocene-age groundwaters of marine origin. Although these groundwaters are geochemically reducing, the [223Ra/228Ra]corr(where “corr” subscript refers to a correction applied to compare [223Ra/228Ra]GWto the more commonly reported [226Ra/228Ra]GW) suggest that they interact with U-rich pegmatites containing Proterozoic- and Palaeozoic-age Ca-U(VI)-silicate minerals, which are undersaturated in the present groundwaters. Local aqueous U(VI) can be stabilized in Ca2UO2CO30complexes at pe-values as low as −4.5 but is susceptible to reduction after a modest decrease in pe-value, alkalinity, or Ca concentration. The [223Ra/228Ra]corrand [224Ra/228Ra]GWalso suggest that U(VI)aqprecipitates as UO2+Xat the interface between marine and non-marine groundwaters. From these data, local [Uaq] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals.
Databáze: OpenAIRE
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