Groundwater sources and geochemical processes in a crystalline fault aquifer

Abstract

International audienceThe origin of water flowing in faults and fractures at great depth is poorly known in crystalline media.This paper describes a field study designed to characterize the geochemical compartmentalization of adeep aquifer system constituted by a graben structure where a permeable fault zone was identified. Analysesof the major chemical elements, trace elements, dissolved gases and stable water isotopes reveal theorigin of dissolved components for each permeable domain and provide information on various watersources involved during different seasonal regimes. The geochemical response induced by performinga pumping test in the fault-zone is examined, in order to quantify mixing processes and contributionof different permeable domains to the flow. Reactive processes enhanced by the pumped fluxes are alsoidentified and discussed.The fault zone presents different geochemical responses related to changes in hydraulic regime. Theyare interpreted as different water sources related to various permeable structures within the aquifer.During the low water regime, results suggest mixing of recent water with a clear contribution of olderwater of inter-glacial origin (recharge temperature around 7 C), suggesting the involvement of watertrapped in a local low-permeability matrix domain or the contribution of large scale circulation loops.During the high water level period, due to inversion of the hydraulic gradient between the major permeablefault zone and its surrounding domains, modern water predominantly flows down to the deep bedrockand ensures recharge at a local scale within the graben.Pumping in a permeable fault zone induces hydraulic connections with storage-reservoirs. The overlaidregolith domain ensures part of the flow rate for long term pumping (around 20% in the present case).During late-time pumping, orthogonal fluxes coming from the fractured domains surrounding the majorfault zone are dominant. Storage in the connected fracture network within the graben structure mainlyensures the main part of the flow rate (80% in the present case). Reactive processes are induced by mixingof water from different sources and transfer conditions. A specific approach is applied to quantify thereaction rate involved along the pumping time. Autotrophic denitrification coupled with iron mineralsoxidation is highlighted and water rock interaction is clearly enhanced by the flux changes induced bypumping

A methodology for regionalizing 3‐D effective porosity at watershed scale in crystalline aquifers

“…They help in mapping the total weathering profile thickness. The maps are, however, valid at large scale (here 500 × 500 m) and do not consider local variations, for instance, deepening of the weathering front because of local geological heterogeneity (faults and veins; Dewandel, Lachassagne, Zaidi, & Chandra, ; Roques, Aquilina, et al, ; Roques, Bour, et al, ). In Anantapur, additional field data should be collected to determine if the relationship varies significantly according to the geology, which is more complex.…”

“…Over the past three decades, the geological and hydrogeological characterization of crystalline aquifers has seen significant improvements (Ayraud et al, ; Chilton & Foster, ; Dewandel, Lachassagne, Wyns, Maréchal, & Krishnamurthy, ; Guihéneuf et al, ; Lachassagne, Wyns, & Dewandel, , ; Maréchal et al, ; Roques, Bour, et al, ; Taylor & Howard, ; Wyns et al, ). These works show that, where such hard rocks are exposed to deep weathering processes, the geometry and hydrodynamical properties of aquifers are closely related to the weathering grade of the parent rock.…”

Assessment of Hydraulic Connectivity of Fractures in Mudstones by Single‐Borehole Investigations

“…In the case of a derivative curve similar to those obtained at t 2 in Figures c and d, even if an upward trend is observed during the middle to late period, the derivative may still be an open type if small‐scale fracture networks are developed in the host rock, as shown in Figure c. Such derivatives are indicated by pore pressure and water chemistry data (Dewandel et al, ; Maréchal et al, ; Roques et al, ). Thus, the identification of the type of derivative may entail uncertainty, depending on the scale and duration of the hydraulic packer test.…”

“…Linear flow can be interpreted with various models; however, if flow is due to channel structures (e.g., a one‐dimensional open channel within fractures, one‐dimensionally connected fractures, or the intersection of two fractures or faults; Enachescu et al, ; Ferroud et al, ; Tsang & Neretnieks, ), the extent of such flow paths is limited (Ji et al, ; Yabuuchi et al, ). Although linear flow may be caused by one‐dimensional flow toward large faults from surrounding small‐joint systems (i.e., a dual‐porosity model for multiscale fracture networks; Figure c; Jia et al, ; Roques et al, ), this case is limited to crystalline rocks or mechanically layered rocks where joint systems tend to develop, and is unlikely in massive mudstones such as those of the Koetoi, Wakkanai, and Palfris Formations (Eichhubl & Boles, ; Le Garzic et al, ; Odling et al, ). This view is also supported by the observation that joint systems are not common in the Koetoi, Wakkanai, and Palfris Formations, and that flow anomalies (flow paths) are detected almost solely in single shear fractures in the Koetoi Formation and in fault zones of the Wakkanai and Palfris Formations (Ishii, ; Mazurek et al, ).…”

Geochemical Alteration and Mineralogy of Coals under the Influence of Fault Motion: A Case Study of Qi’nan Colliery, China

“…Geochemical and geophysical characteristics of fault rocks are primarily controlled by fault zones or fault systems [1][2][3][4][5]. Fault zones in rocks are generally divided into fault core (comprised of ultracataclasite), damage zone and undeformed zone (comprised of protolith) [6,7].…”