HYDROCHEMICAL EVOLUTION OF Ca AND Mg IN A SILICATES-DOMINATED BASIN: EVIDENCE FROM MULTIPLE METAL ISOTOPES

The flow paths of groundwater control groundwater age and determine the spatial distribution of its chemical components. In many basins, young groundwater in the recharge area is rich in Ca²⁺ and Mg²⁺, while groundwater in the discharge area is enriched in Na⁺ and depleted in Ca²⁺and Mg²⁺. It is widely known that the dissolution rates of carbonates far exceed those of silicates, leading to the assumption that silicates' contribution to Ca²⁺ and Mg²⁺ in groundwater is negligible. Conversely, studies focus on the would river water argue that silicate weathering significantly contributes to Ca²⁺ and Mg²⁺. Therefore, quantifying contribution from silicate weathering to Ca²⁺ and Mg²⁺ in groundwater is essential.

In the Ordos Basin in northwestern China, groundwater age varies from 10 years in the recharge area to about 10³ and 10⁴ years in shallow and deep discharge areas, respectively. Using Mg isotopes and various metal isotopes, this study identifies the processes controlling the release and removal of Ca²⁺ and Mg²⁺ during groundwater evolution. Groundwater in the recharge area exhibits δ⁷Li and δ⁴¹K values higher than those of the aquifer sandstone, indicating silicate weathering. A three end-member mixing model using Sr isotopes was established to calculate the contribution of silicate weathering to Ca²⁺ and Mg²⁺ release, showing it accounts for 45% of Ca²⁺ and 91% of Mg²⁺ release in young groundwater.

Mg isotopes reveal that in young groundwater in the recharge area (~10 yrs), Mg²⁺ was removed by clay formation, with a Mg removal rate of 53.77%. In the shallow discharge area (~10³ yrs), Mg removal is primarily controlled by adsorption, while in the deep discharge area (~10⁴ yrs), both adsorption and cation exchange control Mg removal, with rates of 22.26% and 10.17%, respectively. Thus, a three-stage Mg evolution, including clay formation, adsorption, and ion exchange, cumulatively remove nearly 90% of Mg. We also find that Ca²⁺ is removed by adsorption, ion exchange, and calcite precipitation during long-term groundwater circulation, with removal rates of 81.97%, 13.33%, and 0.80%, respectively. Our study enhances the understanding of hydrochemical evolution and elemental cycling in silicate-dominated aquifers.