This study deals with the effects of hydrodynamic functioning of hard-rock aquifers on microbial communities. In hard-rock aquifers, the heterogeneous hydrologic circulation strongly constrains groundwater residence time, hydrochemistry, and nutrient supply. Here, residence time and a wide range of environmental factors were used to test the influence of groundwater circulation on active microbial community composition, assessed by high throughput sequencing of 16S rRNA. Groundwater of different ages was sampled along hydrogeologic paths or loops, in three contrasting hard-rock aquifers in Brittany (France). Microbial community composition was driven by groundwater residence time and hydrogeologic loop position. In recent groundwater, in the upper section of the aquifers or in their recharge zone, surface water inputs caused high nitrate concentration and the predominance of putative denitrifiers. Although denitrification does not seem to fully decrease nitrate concentrations due to low dissolved organic carbon concentrations, nitrate input has a major effect on microbial communities. The occurrence of taxa possibly associated with the application of organic fertilizers was also noticed. In ancient isolated groundwater, an ecosystem based on Fe(II)/Fe(III) and S/SO4 redox cycling was observed down to several 100 of meters below the surface. In this depth section, microbial communities were dominated by iron oxidizing bacteria belonging to Gallionellaceae. The latter were associated to old groundwater with high Fe concentrations mixed to a small but not null percentage of recent groundwater inducing oxygen concentrations below 2.5 mg/L. These two types of microbial community were observed in the three sites, independently of site geology and aquifer geometry, indicating hydrogeologic circulation exercises a major control on microbial communities.

International audienceCrystalline-rock aquifers generally yield limited groundwater resources. However, some highly productive aquifers may be encountered, typically near tectonic discontinuities. In this study, we used a multidisciplinary experimental field approach to investigate the hydrogeological behavior of a sub-vertical permeable fault zone identified by lineament mapping. We particularly focused our investigations on the hydrogeological interactions with neighboring reservoirs. The geometry of the permeable domains was identified from geological information and hydraulic test interpretations. The system was characterized under natural conditions and during a 9-week large-scale pumping test. We used a combination of piezometric analysis, flow logs, groundwater dating and tracer tests to describe the interactions between permeable domains and the general hydrodynamical behaviors. A clear vertical compartmentalization and a strong spatial heterogeneity of permeability are highlighted. Under ambient conditions, the vertical permeable fault zone allows discharge of deep groundwater flows within the superficial permeable domain. The estimated flow across the total length of the fault zone ranged from 170 to 200 m3/day. Under pumping conditions, hydrological data and groundwater dating clearly indicated a flow inversion. The fault zone appears to be highly dependent on the surrounding reservoirs which mainly ensure its recharge. Groundwater fluxes were estimated from tracer tests interpretation. This study demonstrates the hydrogeological capacities of a sub-vertical fault aquifer in a crystalline context. By describing the hydrological behavior of a fault zone, this study provides important constrain about groundwater management and protection of such resources

No 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

No abstract

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

No abstract