In the coastal multilayer aquifer system of a highly urbanized southern city (Recife, Brazil), where groundwaters are affected by salinization, a multi-isotope approach (Sr, B, O, H) was used to ...investigate the sources and processes of salinization. The high diversity of the geological bodies, built since the Atlantic opening during the Cretaceous, highly constrains the heterogeneity of the groundwater chemistry, e.g. Sr isotope ratios, and needs to be integrated to explain the salinization processes and groundwater pathways. A paleoseawater intrusion, most probably the 120kyB.P. Pleistocene marine transgression, and cationic exchange are clearly evidenced in the most salinized parts of the Cabo and Beberibe aquifers. All 87Sr/86Sr values are above the past and present-day seawater signatures, meaning that the Sr isotopic signature is altered due to additional Sr inputs from dilution with different freshwaters, and water–rock interactions. Only the Cabo aquifer presents a well-delimitated area of Na-HCO3 water typical of a freshening process. The two deep aquifers also display a broad range of B concentrations and B isotope ratios with values among the highest known to date (63–68.5‰). This suggests multiple sources and processes affecting B behavior, among which mixing with saline water, B sorption on clays and mixing with wastewater. The highly fractionated B isotopic values were explained by infiltration of relatively salty water with B interacting with clays, pointing out the major role played by (palaeo)-channels for the deep Beberibe aquifer recharge. Based on an increase of salinity at the end of the dry season, a present-day seawater intrusion is identified in the surficial Boa Viagem aquifer. Our conceptual model presents a comprehensive understanding of the major groundwater salinization pathways and processes, and should be of benefit for other southern Atlantic coastal aquifers to better address groundwater management issues.
•A multi-isotope approach was used to assess the salinization sources and processes.•The geological unit diversity constrains the groundwater chemistry heterogeneity.•A global Pleistocene seawater intrusion and related cationic exchange are evidenced.•The δ11B in groundwater presents highly fractionated values (63–68.5‰).•A present-day seawater intrusion is identified locally in the surficial aquifer.
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.
Here, we report on Sr isotopes, from shallow and deep groundwater from catchments located on granite and schist around the world. This extensive approach of Sr isotope tracing, initiated in France on ...areas impacted by intensive agricultural activities, was enlarged to Africa (granite-gneiss and schists 2200-700 Ma of the Congo Basin; Archean granitoid/gneisses and sedimentary greenstone 3700-2500 Ma of the Orange River); India and Nepal (Archean granites 2500 Ma and Palaeoproterozoic granodiorite and schists 3100-1600 Ma for India; Himalaya metamorphic, silicate metasediments and gneisses 630 to 490 Ma); North America (3500–2500 Ma silicate rocks in the Mackenzie basin; 1000 to 70 Ma silicate rocks in the Fraser Basin in Canada and 1760 to 1430 Ma anorthosite and granite in the Laramie aquifer in Wyoming); South America in French Guiana (Archean gneiss 3400 to 2700 Ma and granite-gneiss rocks 2300 to 1900 Ma); Australia (65 Ma arenaceous and argillaceous rocks; and considering both surface and groundwater). In this extensive approach, the Sr and Mg contents are well correlated and both are partly related to agricultural and weathering inputs. The relationship between Sr-isotope and Mg/Sr ratios allows definition of the relative impact on surface and groundwater of processes occurring in the Critical Zone, mainly rain, agricultural practices and water-rock interactions.
•Sr isotope tracing on areas impacted by intensive agricultural activities initiated in France.•Extensive approach enlarged to Africa; India and Nepal; North and South America and Australia.•Sr and Mg contents are well correlated and related to agricultural and weathering inputs.•The Sr-isotope allows defining processes occurring in the Critical Zone.
► Simulation of F accumulation in Southern India groundwater under paddy field irrigation. ► A solute recycling model was constructed on measured field data. ► The model describes the F progressive ...enrichment in the irrigation return flow. ► F build-up is caused by evaporation and mineral dissolution processes.
Overexploitation of crystalline aquifers in a semi-arid climate leads to a degradation of water quality, with the main processes responsible for the observed salt loads probably being irrigation return flow (IRF) and a high evaporation rate. The present study has focused on modelling the F− accumulation caused by IRF below rice paddy fields in the small endorheic Maheshwaram watershed (Andhra Pradesh, Southern India). The transient simulation was performed with a 1D reactive transport PHREEQC column and took into account IRF evaporation, kinetically controlled mineral dissolution/precipitation, ion adsorption on Fe hydroxides, and mixing with fresh groundwater. The results revealed the role of cationic exchange capacity (CEC) in Ca/Na exchange and calcite precipitation, both favouring a decrease of the Ca2+ activity that prevents fluorite precipitation. Iron hydroxide precipitation offers a not inconsiderable adsorption capacity for F− immobilization. The principal sources of F− are fluorapatite dissolution and, to a lesser extent, allanite and biotite dissolution. Anthropogenic sources of F−, such as fertilizers, are probably very limited. After simulating an entire dry-season irrigation cycle (120days), the results are in good agreement with the observed overall increase of Cl− in the Maheshwaram groundwater. The model enables one to decipher the processes responsible for water-resource degradation through progressive salinization. It shows that F− enrichment of the groundwater is likely to continue in the future if groundwater overexploitation is not controlled.
•Hydrochemical profiles, including fluoride, were run in a crystalline aquifer.•Conductivity profiles enabled to deduce F concentration from I.S.E. measurements.•Variations of F are restricted to the ...weathered/fractured layer of the aquifer.•Fluoride gradient depends on the fracture/fissure connectivity.•Profiles reveal an F-accumulation by water–rock interactions and fertilizers inputs.
Hydrochemical borehole-loggings with a submersible Idronaut Ocean Seven 302 multiparameter probe equipped of F- and NO3-ion-selective electrodes in combination with EC, pH and dissolved oxygen, were applied for characterizing fluoride (F) contamination in a crystalline (hard-rock) aquifer of a small Indian agricultural watershed where groundwater is intensively abstracted for rice irrigation. A high accuracy of F concentrations determined in-situ—shown by comparing with laboratory analyses—was obtained through using conductivity logs for ionic strength consideration. Large variations in chemical composition and particularly of F-concentration were observed inside boreholes, though restricted to the weathered/fractured layer down to 30–35m depth. This conforms to the hydrogeological model of a crystalline aquifer where most groundwater flow occurs in the shallow part of the fractured zone. The general trend of increasing F content with depth results from F accumulation through water–rock interaction, but the shape of the F profile depends on the connectivity of the fracture network close to the borehole. The concentrations seen within the water-table fluctuation zone locally suggest F input from fertilizers in groundwater, in addition to the earlier-demonstrated role of evaporation from irrigation return flow. It is also likely that, locally, the deepening of boreholes has contributed to increasing the population’s vulnerability by tapping F-enriched groundwater at depth.
We investigate denitrification mechanisms through batch experiments using crushed rock and groundwater from a granitic aquifer subject to long term pumping (Ploemeur, France). Except for sterilized ...experiments, extensive denitrification reaction induces NO3 decreases ranging from 0.3 to 0.6mmol/L. Carbon concentrations, either organic or inorganic, remain relatively stable and do not document potential heterotrophic denitrification. Batch experiments show a clear effect of mineral dissolution which is documented through cation (K, Na, Ca) and Fluoride production. These productions are tightly related to denitrification progress during the experiment. Conversely, limited amounts of SO4, systematically lower than autotrophic denitrification coupled to sulfur oxidation stoichiometry, are produced during the experiments which indicates that sulfur oxidation is not likely even when pyrite is added to the experiments.
Analysis of cation ratios, both in isolated minerals of the granite and within water of the batch, allow the mineral dissolution during the experiments to be quantified. Using cation ratios, we show that batch experiments are characterized mainly by biotite dissolution. As biotite contains 21 to 30% of Fe and 0.3 to 1.7% of F, it constitutes a potential source for these two elements. Denitrification could be attributed to the oxidation of Fe(II) contained in biotite. We computed the amount of K and F produced through biotite dissolution when entirely attributing denitrification to biotite dissolution. Computed amounts show that this process may account for the observed K and F produced.
We interpret these results as the development of microbial activity which induces mineral dissolution in order to uptake Fe(II) which is used for denitrification. Although pyrite is probably available, SO4 and cation measurements favor a large biotite dissolution reaction which could account for all the observed Fe production. Chemical composition of groundwater produced from the Ploemeur site indicates similar denitrification processes although original composition shows mainly plagioclase dissolution.
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•We investigate autotrophic denitrification mechanisms through batch experiments.•We show that denitrification is mainly attributed to the oxidation of Fe(II) in biotite.•Nitrate reduction coupled to the oxidation of iron from silicates plays a key role in the Nitrogen cycle.
•We determine groundwater sources in a faulted aquifer system in crystalline media.•We quantify the flow contribution of groundwater reservoir during water abstraction.•We discuss the impact of ...mixing processes on geochemical reactions.
The 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 a deep aquifer system constituted by a graben structure where a permeable fault zone was identified. Analyses of the major chemical elements, trace elements, dissolved gases and stable water isotopes reveal the origin of dissolved components for each permeable domain and provide information on various water sources involved during different seasonal regimes. The geochemical response induced by performing a pumping test in the fault-zone is examined, in order to quantify mixing processes and contribution of different permeable domains to the flow. Reactive processes enhanced by the pumped fluxes are also identified and discussed.
The fault zone presents different geochemical responses related to changes in hydraulic regime. They are 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 older water of inter-glacial origin (recharge temperature around 7°C), suggesting the involvement of water trapped 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 permeable fault zone and its surrounding domains, modern water predominantly flows down to the deep bedrock and ensures recharge at a local scale within the graben.
Pumping in a permeable fault zone induces hydraulic connections with storage-reservoirs. The overlaid regolith 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 major fault zone are dominant. Storage in the connected fracture network within the graben structure mainly ensures the main part of the flow rate (80% in the present case). Reactive processes are induced by mixing of water from different sources and transfer conditions. A specific approach is applied to quantify the reaction rate involved along the pumping time. Autotrophic denitrification coupled with iron minerals oxidation is highlighted and water rock interaction is clearly enhanced by the flux changes induced by pumping.
Denitrification is expected to occur at and near the interface with clayey formations dedicated to radioactive waste disposal, due to the nitrate content in some waste canisters and the high ...probability of introducing denitrifiers during the operational phase. Nitrate reduction to nitrous-oxide gas by pure-strain Pseudomonas mandelii, in the presence or absence of sterilized Callovian–Oxfordian (COx) clay rock, was studied over a period of ~41.5months (1267days) by means of batch experiments. A culture medium with a similar porewater chemistry to that of COx rocks was used, supplemented with acetate and nitrate. Bacterial growth was monitored by genomic-DNA and narG-gene quantification. Nitrite accumulated in solution concomitantly with a decrease in nitrate content and the weak generation of nitrous oxide, but denitrification rates drastically decreased over the study period. Acetate was both oxidized to inorganic carbon and incorporated into biomass.
The presence of solid COx particles significantly affected the geochemical reactions and particularly caused an enhanced nitrate reduction, a higher bacterial growth and the precipitation of calcium carbonate. Moreover, in the presence of COx a delay of several weeks was observed before the accumulation of nitrite in solution, leading to an imbalance between nitrate consumption and the production of nitrite, nitrous oxide and ammonium. Chemical oxidation of clay along with nitrite reduction to dinitrogen is expected to occur, explaining both the delay in nitrite accumulation and the apparent imbalance in nitrogen species. Although the electron donor of the COx oxidation was not identified, several hypotheses may be advanced, and we provide new insight into the biogeochemical and geochemical processes that may occur concomitantly at the excavation damaged interface of the clayey host rock after closure, resaturation and release of waste components in a nuclear waste repository. Because of denitrification, the oxidative impact of nitrate released from waste will diminish. However, nitrites produced through denitrification and diffusing into the rock will likely play in redox reaction via their abiotic reactivity which may occur within the compact clay formation despite a lack of space for bacterial activity.
•The presence of clayey rock enhances nitrifier bacterial growth and activity.•Denitrification occurs in clay medium but its rate decreases with time.•The presence of solid COx particles affects the (bio)geochemical reactions.•Nitrite accumulates temporarily in solution and reacts with solid COx particles.•Accumulated nitrite can oxidize solid COx particles, even where bacteria are excluded.
•Pumping induces a thorough reorganization of fluxes and mixing at long-term time scales.•Mixing of groundwaters due to pumping enhances denitrification processes.•Biotite plays a key role of ...electron donor in the reduction of nitrate.•Crystalline aquifers act as sustainable bioreactors for reducing nitrate over long-term operations.
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We investigated the mixing and dynamic of denitrification processes induced by long-term pumping in the crystalline aquifer of Ploemeur (Brittany, France). Hydrological and geochemical parameters have been continuously recorded over 15 boreholes in 5km2 on a 25-year period. This extensive spatial and temporal monitoring of conservative as well as reactive compounds is a key opportunity to identify aquifer-scale transport and reactive processes in crystalline aquifers. Time series analysis of the conservative elements recorded at the pumped well were used to determine mixing fractions from different compartments of the aquifer on the basis of a Principal Component Analysis approach coupled with an end-member mixing analysis. We could reveal that pumping thus induces a thorough reorganization of fluxes known as capture, favoring infiltration and vertical fluxes in the recharge zone, and upwelling of deep and distant water at long-term time scales. These mixing fractions were then used to quantify the extent of denitrification linked to pumping. Based on the results from batch experiments described in a companion paper, our computations revealed that i) autotrophic denitrification processes are dominant in this context where carbon sources are limited, that ii) nitrate reduction does not only come from the oxidation of pyrite as classically described in previous studies analyzing denitrification processes in similar contexts, and that iii) biotite plays a critical role in sustaining the nitrate reduction process. Both nitrate reduction, sulfate production as well as fluor release ratios support the hypothesis that biotite plays a key role of electron donor in this context. The batch-to-site similarities support biotite availability and the role by bacterial communities as key controls of nitrate removal in such crystalline aquifers. However, the long term data monitoring also indicates that mixing and reactive processes evolve extremely slowly at the scale of the decade.
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•Transport experiments of TiO2 NP suspension through a fractured hard-rock were done in laboratory.•NP deposition and aggregation occurred when NP were transferred through the ...rock.•Ca favored NP mobility by reducing the negative charge of the rock as attested by DLVO.•Exchange of solutes between the mobile and immobile waters controlled the NP behavior.
Successive transport experiments of TiO2 nanoparticles (NP) suspension through fractured hard-rock column were done in laboratory. A low ionic strength (IS) water (0.8–1.3 10−3 M) at pH ∼4.5 was used, corresponding to the chemical composition of groundwater where the rock was collected (Naizin, France). The surface charge of TiO2 NP was positive while that of rock was negative favoring NP deposition. SEM/EDX reveals that NP were retained on a broad distribution of mineral collectors along the preferential flow pathways (i.e., fractures). However, a non-negligible amount of NP (∼10%) was transferred through the rock. Divalent cation (Ca2+) was responsible for the reduction of the negative charge of the rock and thus contributed to limit the NP deposition as attested by DLVO model. Blocking of rock surfaces by NP favored NP transfer while the ripening process and the size exclusion of aggregates decreased NP mobility. Decrease of water flow favored the exchange of solutes from the immobile to the mobile water in the porous medium, which in turn favored the aggregation of the NP and led to their natural attenuation. The result evidences how slight modifications of the environmental conditions can strongly influence the fate of NP in groundwater.
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