黄土塬区浅层地下水化学特征及其碳循环意义

对甘肃省灵台县独店镇秋射村黄土剖面浅层地下水的水化学组成、溶解无机碳(DIC)、溶解有机碳(DOC)、颗粒有机碳(POC)等进行了取样分析,对其δ~(13)CDIC和δ~(13)CPOC的特征及其控制因素进行了探讨,并评估了降雨补给过程中的碳酸盐风化碳汇强度。结果表明,研究区地下水的水化学类型为HCO3-Ca·Mg型,其方解石饱和指数SIC大于0,已经饱和,但尚未达到大规模沉淀的程度。研究区黄土浅层地下水的DIC变化范围为5.25~5.45mmol/L,DOC含量为0.59~0.62mg/L,明显低于地表水体;而POC稍高,这是因为黄土颗粒物的混入造成。泉水和井水的δ~(13)CDIC变化范围在-9.19‰~-8.90‰之间,其较高的δ~(13)C与碳酸盐风化-沉积过程中反复的碳同位素交换有关。而δ~(13)CPOC变化范围在-19.99‰~-18.87‰之间,与黄土有机碳同位素特征基本一致。地下水中的HCO-3、Ca2+和Mg2+主要来源于碳酸盐的化学风化。根据风化反应的离子平衡关系,计算得到研究区的风化碳汇为2.82mmol/L,即每有1L的降水入渗到零通量面以下,就会产生2.82mmol的碳汇。

The hydro-chemical characteristics of shallow groundwater in loess tableland and its implication to carbon cycle

We analyzed the hydro-chemical composition,dissolved inorganic carbon (DIC),dissolved organic carbon (DOC),particulate organic carbon (POC) in the shallow groundwater of a loess profile in Qiushe village,Dudian town,Lingtai county,Gansu province of Northwestern China,discussed the characteristics and controlling factors of the δ13CDIC and δ13CPOC,and assessed the CO2 consumption of carbonate weathering during rainfall recharge.The results showed that the hydro-chemical type of the groundwater in the study area was the HCO3-Ca·Mg type.The saturation index of calcite (SIc) was above 0,indicating full saturation but no massive precipitation yet.The DIC concentration in the shallow groundwater of the loess area was 5.25~5.45 mmol/L and the DOC concentration was 0.59~0.62 mg/L,which were lower than those in surface water.The POC concentration was high due to the mixture of loess particle matter.The δ13C of DIC in spring water and well water ranged from-9.19‰ to-8.90‰.This might be associated with the repeated exchange of C isotope during the process of carbonate weathering and precipitation.The δ13CPOC ranged from-19.99‰ to-18.87‰,which was consistent with the isotope ratio of the organic carbon in loess.The HCO-3,Ca2+ and Mg2+ in groundwater mainly came from the weathering of carbonate in loess.According to the ion balance of carbonate weathering reaction,we calculated the CO2 sink by carbonate weathering in the study area to be 2.82 mmol/L.In other words,2.82 mmol CO2 can be absorbed by carbonate weathering when every 1 L rainfall infiltrates below the zero flux plane.