Marty Bernard, Dewonck Sarah, France-Lanord Christian
Centre de Recherches Pétrographiques et Géochimiques (CRPG), 15 Rue Notre-Dame des Pauvres, France.
Nature. 2003 Sep 4;425(6953):55-8. doi: 10.1038/nature01966.
Aquitards-layers of rock having low permeability-have been suggested as potential long-term reservoirs for toxic materials such as nuclear or chemical waste. But information about the isolation properties of aquitard layers is essential to evaluate whether they can indeed be used safely as reservoirs. Here we investigate the long-term mobility of groundwaters between two aquifers surrounding an aquitard layer in the eastern recharge area of the Paris basin, France, using helium isotopes as a geochemical tracer. The deeper Trias sandstone aquifer, which lies above the crystalline basement, accumulates radiogenic (4)He and primordial (3)He from large regions of the crust and mantle at rates comparable to the degassing of the whole crust and of mid-ocean ridges. We show that the overlying carbonate Dogger aquifer, which is separated from the Trias aquifer by an aquitard layer consisting of a approximately 600 m succession of shales and clays, is stagnant and has been extremely well isolated from the Trias over the past several million years. This finding, together with previous studies at the centre of the Paris basin, shows that diffusive mass transfer across aquitards is negligible and that cross-formational flow in basins takes place preferentially in faulted areas.
弱透水层——渗透率低的岩层——被认为是核废料或化学废料等有毒物质潜在的长期储存库。但是，关于弱透水层隔离特性的信息对于评估它们是否真的能安全用作储存库至关重要。在这里，我们以氦同位素作为地球化学示踪剂，研究了法国巴黎盆地东部补给区一个弱透水层周围两个含水层之间地下水的长期流动性。较深的三叠系砂岩含水层位于结晶基底之上，从地壳和地幔的大片区域积累放射性成因（4）He和原始（3）He的速率与整个地壳和大洋中脊的脱气速率相当。我们发现，上覆的碳酸盐多格含水层与三叠系含水层被一层由约600米厚的页岩和粘土组成的弱透水层隔开，该含水层处于停滞状态，在过去几百万年里与三叠系含水层的隔离程度极高。这一发现，连同巴黎盆地中部以前的研究，表明穿过弱透水层的扩散性质量传递可以忽略不计，并且盆地中的跨地层流动优先发生在断层区域。
