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Open AccessArticle

The Origin of Major Ions of Groundwater in a Loess Aquifer

by Tianming Huang 1,2,* and Baoqiang Ma 3
1
Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
3
College of Geoscience and Surveying Engineering, China University of Mining & Technology, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Water 2019, 11(12), 2464; https://doi.org/10.3390/w11122464
Received: 26 October 2019 / Revised: 21 November 2019 / Accepted: 22 November 2019 / Published: 23 November 2019
(This article belongs to the Section Hydrology)
When groundwater represents most of the world’s reserves of unfrozen freshwaters, water quality is commonly a limiting factor in quantifying usable fresh groundwater storage, especially in arid and semi-arid areas. Tracing the origin of major ions is important to the understanding of hydrochemical evolution and water–rock interaction. Strontium (Sr) and calcium (Ca) are geochemically similar in terms of ionic radius and ionic charge and can substitute for each other in mineral lattices such as in carbonates and silicates. This allows the use of Sr as a proxy for Ca during water–rock interaction. Geochemical mass balance and carbon and strontium isotope techniques were employed to study the origin of each major ion (such as Na, Ca, Mg, Cl, and SO4) in a loess aquifer (there is 10% to 20% of carbonate in the loess). Geochemical mass balance between atmospheric deposition and groundwater shows that the Cl (average concentration of 5.5 mg/L) and SO4 (average concentration of 6.8 mg/L) in groundwater originated from atmospheric deposition. The dissolution of loess using acetic acid was used to analyze the 87Sr/86Sr ratio of carbonate (mainly deposited in continental environments). Groundwater 87Sr/86Sr ratios (0.710677 to 0.712319) are consistent with 87Sr/86Sr ratios in carbonate (0.710329 to 0.711085) but are significantly lower than the whole-rock (0.715136 to 0.717155) and residue (0.719091 to 0.720438), suggesting that Ca and Mg mainly originated from the dissolution of carbonate in the loess aquifer. However, Na originated from the dissolution of albite, suggesting saturation controls the ability of dissolution. There are cation exchanges between Ca + Mg and Na, resulting in Na concentration increases and Ca + Mg concentration decreases in groundwater. This study is important to the understand of the origin of the major ions in groundwater and the geochemical processes in silicate-carbonate aquifers. View Full-Text
Keywords: groundwater; strontium; isotopes; geochemical methods; acetic acid; 87Sr/86Sr groundwater; strontium; isotopes; geochemical methods; acetic acid; 87Sr/86Sr
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Huang, T.; Ma, B. The Origin of Major Ions of Groundwater in a Loess Aquifer. Water 2019, 11, 2464.

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