REY‐Th‐U Solute Dynamics in the Critical Zone: Combined Influence of Chemical Weathering, Atmospheric Deposit Leaching, and Vegetation Cycling (Mule Hole Watershed, South India)

Abstract

The source and proportion of REY, Th, and U exported by groundwater and by the ephemeral stream along with the elemental proportions passing through vegetation have been assessed in the subhumid tropical forested CZO of Mule Hole, Southern India. The study relies on a pluriannual hydrogeochemical monitoring combined with a hydrological model. The significant difference between the soil input (SI) and output (SO) solute fluxes (mmol/km2/yr) of LREE (SI‐SO = 13,250‐1,500), HREE (1,930‐235), Th (64‐12), and U (63‐25) indicates a strong uptake by roots carried by canopy and forest floor processes. The contribution of atmospheric dust leaching can reach about 60% of LREE and 80% of HREE. At the watershed scale, the U solute flux exported by groundwater (180 mmol/km2/yr) mainly originates from the breakdown of primary U‐bearing accessory minerals and dominates by a factor of 25 the stream flux. The precipitation of authigenic U‐bearing phases and adsorption onto Fe‐oxides and oxyhydroxides play a significant role for limiting the U mobility. In the groundwater, the plagioclase chemical weathering is efficiently traced by the positive Eu‐anomaly. The very low (REY) to nil (Th) contents are explained by the precipitation of authigenic phases. In the stream flow, dominated by the overland flow (87% of the yearly stream flow), the solute exports (in mmol/km2/yr) of REY (1,080 for LREE and 160 for HREE) and of Th (14) dominate those by groundwater. Their mobility is enhanced by chelation with organic ligands produced by forest floor and canopy processes.

REY‐Th‐U Dynamics in the Critical Zone: Combined Influence of Reactive Bedrock Accessory Minerals, Authigenic Phases, and Hydrological Sorting (Mule Hole Watershed, South India)

“…We can also consider that a large part of the U pool mobilized from the breakdown of titanite (78% of the parent gneiss U reservoir) is leached in the groundwater at the weathering front prior to the formation of the soil horizons. The significant groundwater solute U flux, which is 25 times more than for the stream, supports this assumption (Braun et al, 2017).…”

“…The significant variation on the open‐system mass fraction transport functions ( ) is due to the heterogeneity in the accessory mineral distribution within the parent gneiss. Overall, the REY losses are not significant in the saprolite and fall mostly in the ±20% error range attested by the very low exports of LREE and HREE by the groundwater at the weathering front (Braun et al, ).…”

“…Moreover, REE are not available from the exchangeable pool and would not be easily available for the plant uptake by ion exchange. The plagioclase breakdown can be foreseen in groundwater by both the magnitude of the average Eu‐anomaly (Eu/Eu* = 5; Braun et al, ) and the significant Na fluxes in the different piezometers (Riotte et al, b).…”

“…The soil reservoir is mostly controlled by the well‐crystallized authigenic oxides and oxyhydroxides pool (60%) and by the residual pool (30%). The solute export of Th by the groundwater is extremely low (Braun et al, ). Thus, we can hypothesize that Th from the breakdown of bastnesite is precipitated as pH‐dependent thorianite, occluded in iron oxides and oxyhydroxides.…”

“…Schematic view of the REY-Th-U fractionation and redistribution from primary bearing to authigenic phases in the Mule Hole regolith and related solid exports (this study) and solute exports in the stream and groundwater (fromBraun et al, 2017). The mineral abbreviations are: Mzn, monazite; Xtn, xenotime; Aln, allanite; Bst, bastnesite; Ttn, titanite; Ox-Fe, iron oxides and oxyhydroxides which can adsorb or occlude REY-Th-U; Nm, not measured.…”