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

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.

Extreme Copper Isotope Fractionation Driven by Redox Oscillation During Gleysols Weathering in Mun River Basin, Northeast Thailand

“…There are two sources of heavy Cu isotope enrichment in topsoil: (a) Cu deposition from the atmosphere to the soil surface (i.e., mineral dust and anthropogenic particulate matters); and (b) Cu exchange from groundwater to surface water during the dry seasons as Cu (II) is more soluble than Cu (I) and has greater leaching loss to groundwater (Vance et al., 2016; Wang et al., 2022). Given that the precipitation in the dry season of MRB is negligible compared with the weathering of element cycling (S. Zhang et al., 2021), and that the enrichment of Fe, Mn, and Cu is low in the topsoil of the S1 and S2 profiles (Figure 3), the atmospheric deposition could not result in heavy isotope pools.…”

Geochemical and Seasonal Characteristics of Dissolved Iron Isotopes in the Mun River, Northeast Thailand

“…The pH in the dry season ranged from 6.1 to 7.7, with an average of 7.0 [45], and the pH in the midstream (6.6) was lower than upstream (7.1) and downstream (7.1). The pH in the wet season varied from 6.4 to 7.7, with an average of 6.9 [29], and the pH in the midstream (6.6) was lower than upstream (7.4) and downstream (6.7).…”

“…Figure 2. (a) Dissolved oxygen (DO) and (b) Dissolved organic carbon (DOC) in the upstream, midstream and downstream of Mun River (MR) during dry and wet season; data are from[29,45].…”

Water, Salt, and Ion Transport and Its Response to Water-Saving Irrigation in the Hetao Irrigation District Based on the SWAT-Salt Model

“…The results are similar to the findings of other scholars. However, the concentrations of Na + and Cl − are underestimated in the ionic content [43][44][45], which may be related to the fact that the fraction of NaCl content was not recorded in the soil data survey. In addition, as can be seen in Figure 7C,E,H, the concentrations of Mg 2+ , K + , and HCO 3 − show an increasing trend from year to year.…”