T12 Using Geologic Context to Improve Hydrogeologic Characterizations

Sunday, 22 September 2024
8:00 AM - 12:00 PM
209B

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

Paper number: T12-13
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

  • Xiao-Wei Jiang Presenting Author
    China University of Geosciences, Beijing, Beijing
  • Tao-Tao Ji
    China University of Geosciences, Beijing, Beijing
  • Rui Wang
    China University of Geosciences, Beijing, Beijing
  • Ying Tan
    China University of Geosciences, Beijing, Beijing
  • Hong Zhang
    China University of Geosciences, Beijing, Beijing

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-405414

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