Detection and Implications of Hydraulic Relationships between Fractured-Rock and Deposit Aquifers

泵井通常安装在识别良好的含水层中，要么是颗粒状（由砂和砾石组成），要么是破碎的岩石。假定从井中抽取的地下水来自井所在的含水层。然而，有一种特殊的情况尚未在文献中得到很好的记录，到目前为止也没有进行彻底的调查：抽水井要么安装在沉积物中，要么安装在破裂的岩石中，但在这两种情况下，都靠近这两种含水层之间的界面。在一个区域地质格架中，颗粒层通常覆盖在破碎的基岩上，泵井可能恰好安装在基岩上方的沉积物下部，或者位于基岩下方的基岩上部。在这两种构造中，人们可能会怀疑抽水井产生的水可能由两种不同的地下水组成：一种来自矿床，另一种来自基岩。如果两种含水层之间存在水力连接，就会出现这种情况。这种连接可能是自然存在的，在那里，断裂岩石的不连续面会自然地与矿床相交。或者，当钻穿岩石的钻井作业已经在含水层之间建立了水力连接（沿着套管底部的优先通道，穿透破裂岩石的上部）时，它们也可能是人为诱发的。在这两种配置中，抽水井都可以被称为“混合井”，因为它们既收集来自颗粒含水层的水，也收集来自裂缝岩石含水层的水。本研究的目的是建立一种方法，一方面可以检测混合井，另一方面可以表征两种含水层的水力连接和优先流量。这种检测和表征将使我们能够评估混合井的后果，量化水井附近两种含水层之间以及当地和区域尺度上的地下水交换，并有助于了解水井供应的地下水的来源。这些方面在适当管理地下水资源和能够更好地量化含水层内以及不同含水层之间的地下水流量时至关重要。此外，更好地了解地下水的来源将使我们能够更好地了解地下水的化学成分，以及通过混合井抽水时地下水可能受到的污染（含水层之间的交叉污染），这取决于地下水流经不同含水层的流动路径。将获得更多关于可能的污染来源的知识。这项拟议的研究还将导致建立新的工具来评估含水层的脆弱性，并确保更好地保护资源。提出的研究方法是整体性的，包括建立研究问题的理论基础、进行现场试验、实验室试验、建模和数值试验。为了解决方法问题，将更具体地发展研究的两个方面。首先，在混合井中进行的水力测试中，裂缝岩石和颗粒含水层之间的连接的严格物理结果（水力）应该使我们能够检测到这些连接。其次，我们将研究水源的化学特征，确定混合井供应的地下水是由两种不同化学特征的组合组成的，一种来自裂缝岩石含水层，另一种来自沉积含水层。

Pumping wells are usually installed in a well-identified aquifer, either granular (composed of sand and gravel) or fractured rock. It is assumed that the groundwater extracted from the well comes from the aquifer in which the well is installed. However, there is a particular case which has not yet been well documented in the literature, nor thoroughly investigated so far: that of pumping wells which are installed either in deposit or in fractured rock, but in both instances, close to the interface between these two types of aquifers. Within a regional geological framework where granular layers usually overlay the fractured bedrock, pumping wells may happen to be installed either in the lower part of the deposit just above the bedrock, or in the upper part of the bedrock, just below the deposit. In both of these configurations, one may suspect that the water produced by the pumping well could be composed of a mix of two different groundwaters: one coming from the deposit and the other coming from the bedrock. This situation would arise if hydraulic connections exist between the two types of aquifers. Such connections may be naturally preexisting, where discontinuities of the fractured rock would naturally intersect with the deposit. Or they may be artificially induced, in the case where drilling operations through the rock have created a hydraulic connection between the aquifers (preferential pathways along the bottom part of the casing, penetrating the upper part of the fractured rock). In both configurations, the pumping wells could be qualified as “mixt wells” because they collect waters originating from both the granular aquifer and the fractured rock aquifer. The aim of this proposed research is to set up a methodology allowing on the one hand to detect mixt wells, and on the other hand to characterize the hydraulic connections and preferential flow rates of the two types of aquifers. This detection and characterization would allow us to assess the consequences of mixt wells, to quantify the groundwater exchanges between the two types of aquifers close to the wells and at local and regional scales, and to help to understand the origin of groundwater supplied by wells. These aspects are of crucial importance when appropriately managing groundwater resources and being able to better quantify the quantity of groundwater flowing not only within an aquifer but also between different aquifers. Furthermore, a better knowledge of the origin of groundwater will allow to better understand the chemical composition of groundwater and its possible contamination when pumped by a mixt well (cross-contamination between aquifers), depending on its flow paths through different aquifers. More knowledge will be gained about origins of possible contaminations. This proposed research will also lead to the establishment of new tools for assessing the vulnerability of aquifers and ensuring a better protection of the resource. The proposed approach to perform this research is holistic in nature and combines the following: establishing the theoretical bases of the investigated issue, performing in situ field tests, laboratory tests, modeling and numerical tests. Two aspects of the research will be developed more specifically in order to address methodology issues. Firstly, the strictly physical consequences (hydraulics) of the connections between fractured rock and granular aquifers on the hydraulic tests performed in mixt wells should allow us to detect these connections. Secondly, we will study the chemical signature of the water sources, establishing that groundwater supplied by mixt wells is composed of a combination of two different chemical signatures, one from a fractured rock aquifer and the other from a deposit aquifer.