A Strontium and Hydro-Geochemical Perspective on Human Impacted Tributary of the Mekong River Basin: Sources Identification, Fluxes, and CO2 Consumption

Shitong Zhang,Guilin Han,Fairda Malem,Jie Zeng,Xuhuan Xiao

doi:10.3390/w13213137

Abstract

As the largest and most representative tributary of the Mekong River, the Mun River Basin (MRB) provides critical understanding of regional hydro-geochemical features and rock weathering processes on a basin scale. The present study measured strontium (Sr) isotopes with hydro-geochemistry data of 56 water samples in detail in the MRB in northeast Thailand. The dissolved Sr contents and 87Sr/86Sr isotopic ratios were reported to be 8.7–344.6 μg/L (average 126.9 μg/L) and 0.7085–0.7281 (average 0.7156), respectively. The concentrations of dissolved Sr in the mainstream slightly decreased from upstream to downstream, while the variation trend of 87Sr/86Sr was on the contrary. Correlation analysis showed that Na+ strongly correlated with Cl− (0.995, p < 0.01), while Ca2+ exhibited weak relationships with SO42− (0.356, p < 0.01). Samples of the MRB exhibited lower Mg2+/Na+, Ca2+/Na+, HCO3−/Na+ and 1000Sr/Na ratios, and gathered around the end-member of evaporite dissolution, with slight shift to silicate weathering end-member, demonstrating the dominant contribution of evaporite dissolution and silicate weathering on dissolved loads. Comparing with data of major world rivers from previous research, our results remained consistency with rivers draining through similar geological conditions. The dissolved Sr flux to the adjacent Mekong River was estimated to be 20.7 tons/year. In accordance with the forward model, silicate weathering rate and CO2 consumption rate during dry season were calculated to be 0.73 tons/km2/year and 1.94 × 104 mol/km2/year, and may get underestimated due to intense water consumption by extensive agricultural activities. The superimposed effect of anthropogenic impacts on the water environment could enhance chemical weathering, and thus should be taken into account in regional ion cycles and carbon budgets. These findings highlight the coupling analysis of Sr isotopes and hydro-geochemistry in Earth surface processes and provide basic investigation for sustainable regional water treatment mechanisms in the pan basin of the Mekong River.

Highlights

This study provides basic analysis of dissolved loads on such a Mekong tributary through Sr isotope geochemistry, which benefits the management of local water resources
The elevation decreases from west to east, since mountains are encircled in the southwest of the Mun River Basin (MRB), and the middle and lower regions are distributed by the plains
This study focused on source identification and distribution characteristics of riverine Sr and hydro-geochemistry in the Mun River under natural and anthropogenic impacts

Summary

Introduction

Chemical weathering serves as one major process of continental CO2 consumption and makes a significant contribution to the biogeochemical carbon cycle [1,2,3]. This terrestrial process provides sufficient insights in monitoring short-time atmospheric CO2 variation and in estimating regional climate change in the long run [4,5]. Anthropogenic activities and atmospheric inputs could interfere with the ionic composition of rivers, making it more complicated to discriminate the controlling factors of chemical weathering processes only through the analysis of water chemistry [12,14]. Source characterization through the use of dissolved Sr and its radiogenic isotopes has shown to be effective in revealing weathering contribution and types to riverine solutes, especially with the correlative studies on hydrochemistry [3,12,18,19]

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Results

Conclusion