Abstract In a groundwater flow system, the age of groundwater should gradually increase from the recharge zone to the discharge zone within the same streamline. However, it is occasionally observed that the groundwater age becomes younger in the discharge zone in the piedmont alluvial plain, and the oldest age often appears in the middle of the plain. A new set of groundwater chemistry and isotopes was employed to reassess the groundwater 14C ages from the discharge zone in the North China Plain (NCP). Carbonate precipitation, organic matter oxidation and cross-flow mixing in the groundwater from the recharge zone to the discharge zone are recognized according to the corresponding changes of HCO3− (or DIC) and δ13C in the same streamline of the third aquifer of the NCP. The effects of carbonate precipitation and organic matter oxidation are calibrated with a 13C mixing model and DIC correction, but these corrected 14C ages seem unreasonable because they grow younger from the middle plain to the discharge zone in the NCP. The relationship of Cl− content and the recharge distance is used to estimate the expected Cl− content in the discharge zone, and ln(a14C)/Cl is proposed to correct the a14C in groundwater for the effect of cross-flow mixing. The 14C ages were reassessed with the corrected a14C due to the cross-flow mixing varying from 1.25 to 30.58 ka, and the groundwater becomes older gradually from the recharge zone to the discharge zone. The results suggest that the reassessed 14C ages are more reasonable for the groundwater from the discharge zone due to cross-flow mixing.

中文翻译：

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深承压含水层错流混合排放区地下水的14 C年龄再评价

摘要 在地下水流系统中，同一流线内的地下水年龄应从补给区向排泄区逐渐增加。但在山前冲积平原的泄流带偶尔会观察到地下水年龄变年轻，而最古老的往往出现在平原中部。一套新的地下水化学和同位素被用来重新评估来自华北平原 (NCP) 排放带的地下水 14C 年龄。根据NCP第三含水层同一流线中HCO3−（或DIC）和δ13C的相应变化，识别出补给区到排放区地下水中的碳酸盐沉淀、有机质氧化和错流混合。碳酸盐沉淀和有机物氧化的影响用 13C 混合模型和 DIC 校正进行校准，但这些校正的 14C 年龄似乎不合理，因为它们从中部平原到 NCP 的排放带变得更年轻。Cl-含量与补给距离的关系用于估计排放区的预期Cl-含量，并提出ln(a14C)/Cl来校正地下水中的a14C以应对错流混合的影响。由于错流混合从 1.25 到 30.58 ka，并且地下水从补给区到排放区逐渐变老，因此重新评估了 14C 年龄，并使用校正的 a14C。结果表明，由于错流混合，重新评估的 14C 年龄对于来自排放区的地下水更合理。