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

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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.