International audienceUsing groundwater age determination done through CFC analysis and geochemical data obtained from seven sites in Brittany (France), a hydrogeochemical model for hard-rock aquifers is presented. According to the geological structure, three zones can be defined: the weathered layer, about 30 m thick; the weathered-fissured layer (fractured rock with a high density of fissures induced by weathering), which represents a transition zone between the weathered zone and the lower fractured zone; and the unweathered part of the aquifer. (1) The weathered layer (alterites) is often considered as a porous medium and is the only part frequently used in hard-rock aquifers. Recent apparent ages (0­10 a) are observed in the groundwater fluctuation zone in a thin layer, which is from 1­2 m-thick in the lower parts and 10­15 m-thick in the upper parts of the catchments. Below this thin layer, the groundwater apparent age is high (between 10 and 25 a) and is unexpectedly homogeneous at the regional scale. This groundwater apparent age contrast, which also corresponds to a Cl- concentration contrast, is attributed to rapid lateral transfers in the fluctuation zone which limit water transfer to the underlying weathered zone. Groundwater chemistry is characterized by and Cl- concentrations related to land uses (high in agricultural areas, low in preserved ones). (2) At the interface between the weathered and the weathered-fissured layers a strong biogeochemical reactivity is observed. Autotrophic denitrification is enhanced by a higher availability of sulfides. (3) Under this interface, in the weathered-fissured layer and the underlying fractured deep part of the aquifer, groundwater apparent age is clearly correlated to depth. The vertical groundwater velocity is estimated to be 3 m/a, whatever be the site, which seems to indicate a regional topographic control on groundwater circulation in the deep part of the aquifer. In this deep part, groundwater chemistry is modified by water­rock interaction processes as indicated by Ca and Na concentrations, and a slight sea-water contribution (from 0.1% to 0.65%) in the sites close to the seacoast. One site inland shows a saline and old end-member. The global hydrogeochemical scheme is modified when the aquifer is pumped at a high rate in the fissured-weathered layer and/or the fractured layer. The increase in water velocity leads to a homogeneous groundwater apparent age, whatever be the depth in the weathered-fissured and fractured layers

SignificanceAlthough groundwater is a critical source of drinking water and irrigation, it has been polluted worldwide by agriculture, industry, and domestic activity. Because assessing groundwater quality and recovery rates is challenging, we developed a method for determining where and how quickly nitrate is removed in aquifers using just a few point measurements of groundwater chemistry. This methodology opens new avenues for characterizing catchment-scale nutrient dynamics, including nitrogen, carbon, and silica, with existing datasets for ecosystems around the globe. Understanding the subsurface structure of reactivity would also improve estimates of recovery time frames for polluted ecosystems and inform sustainable limits for anthropogenic activity.

International audienceNitrogen pollution of freshwater and estuarine environments is one of the most urgent environmental crises. Shallow aquifers with predominantly local flow circulation are particularly vulnerable to agricultural contaminants. Water transit time and flow path are key controls on catchment nitrogen retention and removal capacity, but the relative importance of hydrogeological and topographical factors in determining these parameters is still uncertain. We used groundwater dating and numerical modeling techniques to assess transit time and flow path in an unconfined aquifer in Brittany, France. The 35.5 km2 study catchment has a crystalline basement underneath a ∼60 m thick weathered and fractured layer, and is separated into a distinct upland and lowland area by an 80 m-high butte. We used groundwater discharge and groundwater ages derived from chlorofluorocarbon (CFC) concentration to calibrate a free-surface flow model simulating groundwater flow circulation. We found that groundwater flow was highly local (mean travel distance = 350 m), substantially smaller than the typical distance between neighboring streams (∼1 km), while CFC-based ages were quite old (mean = 40 years). Sensitivity analysis revealed that groundwater travel distances were not sensitive to geological parameters (i.e. arrangement of geological layers and permeability profile) within the constraints of the CFC age data. However, circulation was sensitive to topography in the lowland area where the water table was near the land surface, and to recharge rate in the upland area where water input modulated the free surface of the aquifer. We quantified these differences with a local groundwater ratio (rGW-LOCAL), defined as the mean groundwater travel distance divided by the mean of the reference surface distances (the distance water would have to travel across the surface of the digital elevation model). Lowland, rGW-LOCAL was near 1, indicating primarily topographical controls. Upland, rGW-LOCAL was 1.6, meaning the groundwater recharge area is almost twice as large as the topographically-defined catchment for any given point. The ratio rGW-LOCAL is sensitive to recharge conditions as well as topography and it could be used to compare controls on groundwater circulation within or between catchments

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.

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Quantifying nutrient attenuation at watershed scales requires long-term water chemistry data, water discharge, and detailed nutrient input chronicles. Consequently, nutrient attenuation estimates are largely limited to long-term research areas or modeling studies, constraining understanding of the ecological characteristics controlling nutrient attenuation and complicating efforts to protect or restore water quality in developed and developing regions. Here, we combined long-term data and a broad suite of biogeochemical parameters from 49 watersheds in northwestern France to test how well instantaneous measurements can predict nitrogen (N) and phosphorus (P) attenuation at watershed scales. We evaluated 13 biogeochemical and 12 hydrological proxies of hydrological flowpaths, residence time, and biogeochemical transformation. Across the 49 watersheds, nutrient attenuation ranged from 88 to −2% for N and 99-96% for P. The strongest biogeochemical proxies of N attenuation were NO − 3 isotopes, rare earth elements (REEs), radon, and turbidity, together explaining 75% of observed variation. For P attenuation, REEs, NO − 3 isotopes, molecular weight of dissolved organic matter, and radon were the strongest proxies, but only explained 27% of observed variation. However, a single hydrological parameter-annual runoff-explained 91% of N attenuation and the relative abundance of schist bedrock explained 56% of P attenuation. We discuss how runoff both controls and reflects watershed hydrology, biogeochemistry, and nutrient attenuation. For example, runoff was correlated with long-term decreases in nutrient concentration, demonstrating how leakier watersheds recover more quickly from nutrient saturation. Given the immense fertilization capacity of modern society, we propose that eutrophication can only be solved by reducing nutrient inputs, though hydrochemical proxies can provide valuable information on where to carry out essential food production activities.

The spectrophotometric iodine measurement for oxygen determination by the Winkler method is reexamined for theoretical and operational aspects. It is shown that the selection of an isobestic point for measuring the mixture of iodine and tri-iodide in the solution enhances reliability. The wavelength value of 466 nm was selected after a spectrum study. Then, performances are assessed by mean of statistical approach based on repeated standardization curves at the isobestic point compared with other wavelengths. At 466 nm, the method is linear up to about 1000 μmol kg−1 of O2. Optimization and validation of the spectrophotometric method is realized through a robustness study based on factors that may alter the results (temperature, reagent volumes, storage time). Temperature is the only factor that exerts a significant influence on the absorbance (0.5% per °C), hence samples and standards should be kept at the same temperature within ±1 °C. Precision was estimated over a 10-day period. The standard deviation was 0.45% for inter-sample repeatability and 0.73% for reproducibility near 250 μmol kg−1. The repeatability of 12 samples taken at the same immersion was around 0.12%. The detection limit, determined from standardization curves, is <2 μmol kg−1. Accuracy was verified with O2 saturated seawater and found to be within the precision confidence interval. The method can be applied to the determination of dissolved oxygen in fresh or seawater samples, suboxic, or waters supersaturated in dissolved oxygen. It is suggested as being a suitable alternative to titration in most applications of the Winkler determination of dissolved oxygen.

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