Dating groundwater in the Bohemian Cretaceous Basin: Understanding tracer variations in the subsurface
Dating groundwater in the Bohemian Cretaceous Basin: Understanding tracer variations in the subsurface
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DOI：
10.1016/j.apgeochem.2012.11.014
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发表时间：
2013-02-01
影响因子：
3.4
通讯作者：
Purtschert, R.
中科院分区：
文献类型：
--
作者：
Alvarado, J. A. Corcho;Paces, T.;Purtschert, R.
波西米亚白垩纪盆地北部一直是密集开采铀的地点，对地下水质量造成了有害影响。对地下水流动和年龄分布的了解对于预测未来污染的扩散和影响至关重要。因此，使用了最先进的示踪方法(H-3、He-3、He-4、Kr-85、Ar-39和C-14)，以了解切诺马尼亚和中图罗尼亚两个最重要的含水层中心南北流线上地下水的老化和混合过程。地下水测年在这一地区尤为复杂，因为：(I)塞诺马尼亚含水层中的地下水局部受到地源气体和生物气体(如CO2、CH4、He)的流动以及富含氯和SO4的基岩中的化石卤水的影响；(Ii)图罗尼亚含水层覆盖着厚厚的非饱和带；(Iii)末次冰盛期(LGM)期间盛行的冰缘气候和永久冻土条件；以及(Iv)水井大多是在大深度区间内进行筛选的。Kr-85和H-3/He-3年龄的巨大差异表明，除老化以外的其他过程影响了都灵含水层的示踪数据。Ar-39的活动证实了与较老的水域(>；50a)的混合。采用了一种包括示踪剂在整个非饱和区运输的时间滞后和He-3脱气的反向模拟方法来估计地下水的年龄。对于从现代到几百年的平均停留时间，模型和现场结果之间得到了最好的拟合。这一含水层中现代水的存在与污染加剧(如硝酸盐)的发生有关。反应地球化学指标(如Na)和放射性成因He-4的增加以及沿流动方向的C-14的减少证实了较深承压的塞诺马尼亚含水层中地下水的老化。放射性碳的年龄从几百年到20多年不等。通过Ar-39测量对放射性碳测年的初始C-14活度进行了标定。在末次盛冰期重新充填的样品的C-14年龄进一步被耗尽的稳定同位素特征和接近冰点的惰性气体温度所证实。放射性成因的He-4在地下水中积累，随着C-14年龄的增加而呈线性增加。这使得能够使用He-4来验证C-14的年龄范围，并将其扩展到该含水层的其他部分。在断层附近，Ar-39超过现代浓度和C-14死CO2源、He-3/He-4比值升高和渐新世至第四纪火山活动表明了深层成因气体的影响，阻碍了He-4、Ar-39和C-14在地下水测年中的应用。(C)2012爱思唯尔有限公司。保留所有权利。
The northern section of the Bohemian Cretaceous Basin has been the site of intensive U exploitation with harmful impacts on groundwater quality. The understanding of groundwater flow and age distribution is crucial for the prediction of the future dispersion and impact of the contamination. State of the art tracer methods (H-3, He-3, He-4, Kr-85, Ar-39 and C-14) were, therefore, used to obtain insights to ageing and mixing processes of groundwater along a north-south flow line in the centre of the two most important aquifers of Cenomanian and middle Turonian age. Dating of groundwater is particularly complex in this area as: (i) groundwater in the Cenomanian aquifer is locally affected by fluxes of geogenic and biogenic gases (e.g. CO2, CH4, He) and by fossil brines in basement rocks rich in Cl and SO4; (ii) a thick unsaturated zone overlays the Turonian aquifer; (iii) a periglacial climate and permafrost conditions prevailed during the Last Glacial Maximum (LGM), and iv) the wells are mostly screened over large depth intervals.Large disagreements in Kr-85 and H-3/He-3 ages indicate that processes other than ageing have affected the tracer data in the Turonian aquifer. Mixing with older waters (> 50 a) was confirmed by Ar-39 activities. An inverse modelling approach, which included time lags for tracer transport throughout the unsaturated zone and degassing of He-3, was used to estimate the age of groundwater. Best fits between model and field results were obtained for mean residence times varying from modern up to a few hundred years. The presence of modern water in this aquifer is correlated with the occurrence of elevated pollution (e.g. nitrates).An increase of reactive geochemical indicators (e.g. Na) and radiogenic He-4, and a decrease in C-14 along the flow direction confirmed groundwater ageing in the deeper confined Cenomanian aquifer. Radiocarbon ages varied from a few hundred years to more than 20 ka. Initial C-14 activity for radiocarbon dating was calibrated by means of Ar-39 measurements. The C-14 age of a sample recharged during the LGM was further confirmed by depleted stable isotope signatures and near freezing point noble gas temperature. Radiogenic He-4 accumulated in groundwater with concentrations increasing linearly with C-14 ages. This enabled the use of He-4 to validate the dating range of C-14 and extend it to other parts of this aquifer. In the proximity of faults, Ar-39 in excess of modern concentrations and C-14 dead CO2 sources, elevated He-3/He-4 ratios and volcanic activity in Oligocene to Quaternary demonstrate the influence of gas of deeper origin and impeded the application of He-4, Ar-39 and C-14 for groundwater dating. (c) 2012 Elsevier Ltd. All rights reserved.