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

抽水井通常安装在识别良好的含水层中，含水层要么是粒状(由砂砾组成)，要么是裂隙岩石。假设从井中抽取的地下水来自安装井的含水层。然而，有一个特殊的情况尚未在文献中得到很好的记录，也没有得到彻底的调查：泵井安装在沉积物或裂隙岩石中，但在这两种情况下，都靠近这两种含水层之间的界面。在颗粒层通常覆盖在破碎的基岩上的区域地质框架内，抽水井可能恰好安装在矿床的下部恰好在基岩上方，或在基岩的上部，恰好在矿床的下方。在这两种配置中，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：一种来自沉积物，另一种来自基岩。如果这两类含水层之间存在水力连接，就会出现这种情况。这样的连接可能是自然预先存在的，在那里破碎岩石的不连续面自然会与矿床相交。或者，如果通过岩石的钻探作业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)，则可能是人为诱导的。在这两种配置中，抽水井可以被称为“混合井”，因为它们收集来自颗粒含水层和裂隙岩石含水层的水。这项拟议研究的目的是建立一种方法，一方面能够探测混合井，另一方面能够表征这两种含水层的水力连接和优先流量。这种探测和定性将使我们能够评估混合井的后果，量化靠近井的两种含水层之间以及在地方和区域范围内的地下水交换，并有助于了解井供应的地下水的来源。在适当管理地下水资源并能够更好地量化地下水流量时，这些方面至关重要，不仅是在含水层内，而且是在不同含水层之间。此外，更好地了解地下水的来源将有助于更好地了解地下水的化学成分及其在混合井抽水时可能受到的污染(含水层之间的交叉污染)，这取决于地下水在不同含水层中的流动路径。将获得更多关于可能污染的来源的知识。这项拟议的研究还将导致建立新的工具，以评估含水层的脆弱性并确保更好地保护资源。拟议的进行这项研究的方法本质上是全面的，并结合了以下几个方面：建立所调查问题的理论基础、进行现场试验、实验室试验、模拟和数值试验。将更具体地制定研究的两个方面，以解决方法论问题。首先，在混合井中进行的水力试验中，裂隙岩石和颗粒含水层之间的联系的严格物理后果(水力学)应该使我们能够检测到这些联系。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由两个不同的化学特征组合而成，一个来自裂隙岩石含水层，另一个来自沉积含水层。

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.