The hydro-chemical characteristics of shallow groundwater in loess tableland and its implication to carbon cycle
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Abstract:
The hydro-chemical composition, Dissolved Inorganic Carbon (DIC), Dissolved Organic Carbon (DOC), Particulate Organic Carbon (POC) and δ13CDIC, δ13CPOC in spring water discharging from loess section in Qiushe village, Dudian town, Lingtai county, Gansu province of Northwestern China were determined, and CO2 consumption of carbonate weathering during rainfall recharge was estimated in this study. The results show that the chemical type of groundwater in study area is HCO3-Ca?Mg type. The Saturation Index of calcite (SIc) in spring water (above 0) displays the springs are saturated, however, the spring hardly appears the phenomenon of massive precipitation. The DIC concentration in groundwater of loess area is 5.25~5.45mmol/L, and the DOC concentration is 0.59 ~ 0.62 mg/L, lower than that in surface water. While POC concentration is high due to the mixture of loess particle matter. The δ13C of DIC in springs and wells ranged from -9.19‰ to -8.90‰. The positive distribution of δ13CDIC may be attributed to the repeated exchange of C isotope during the process of carbonate weathering and precipitation. The δ13CPOC ranged from -19.99‰ ~ -18.87‰, consistent with the isotope ratio of the organic carbon in loess. The HCO3-, Ca2+ and Mg2+ in groundwater mainly derive from the weathering of carbonate in loess. According to the ion balance of carbonate weathering reaction, the CO2 sink by carbonate weathering is calculated up to 2.82mmol/L. In other words, the 2.82mmol CO2 can be absorbed by carbonate weathering when 1 L rainfall can infiltrate into the loess until below the zero flux plane.
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