Fractured Crystalline Bedrock Research Articles

HYDROGEOLOGICAL CHARACTERIZATION OF LOCAL GROUNDWATER RESERVOIRS IN THE KŁODZKO LAND (SUDETES - SW POLAND) BASED ON AN ANALYSIS OF THE DISCHARGES OF SPRINGS

The hydrogeological characteristics of the two types of local groundwater reservoirs found in the Kłodzko Land (Sudetes) have been made based on the analysis of the discharge of springs in Szczytna and Różanka. In the first of these spring discharge points, water flows out of Upper Cretaceous sandstones and mudstones. The other spring is associated with fractured and weathered mica schists and gneisses. The Różanka spring characterized by greater discharge variability than the fissured porous outflow in Szczytna. The latter one, due to the slower rate of water filtration in the rock medium, belongs to stable springs in terms of discharge. The local groundwater reservoir located in sedimentary rocks characterized by a much higher value of the groundwater volume stored (264,000 m3) than the reservoir consisting of weathered and fractured crystalline rocks (1,918 m3). In the case of the reservoir draining by Szczytna spring, the rate of groundwater exchange in the active zones is about 66 weeks. A distinctly shorter time (12 weeks) is necessary to exchange water in the fractured crystalline bedrock. The hydraulic conductivity for the aquifers in Szczytna and Różanka, calculated based on the recession coefficient, differ substantially from each other. They are 4.64 and 24.9 m/d, respectively.

Capability for Hydrogeochemical Modelling within Discrete Fracture Networks

A new method for simulating solute transport and geochemical interactions within fractured rock is presented. This will be an important capability for assessing the safety of radioactive waste disposal facilities that are located within fractured crystalline bedrock. Specifically, the discrete fracture network (DFN) module within the ConnectFlow groundwater flow and transport software has been updated to: (i) simulate the advection and diffusion of more than one solute species (with the flow and transport equations coupled by the evolving density and viscosity); (ii) model the diffusion of solutes into the rock matrix between fractures; and (iii) utilise the iPhreeqc library to model chemical reactions involving solutes, minerals on fracture/pore surfaces and rock minerals. The performance of ConnectFlow’s DFN module has also been significantly improved via parallelisation which allows more complex calculations to be attempted. These developments are significant because hydrogeochemistry within fractured rock is more accurately represented in an explicit DFN, rather than using more approximate equivalent continuous porous medium (ECPM) methods. Illustrative calculations have been completed for the disposal facility for spent nuclear fuel at Olkiluoto in Finland, and the former candidate site for spent fuel disposal, Laxemar, in Sweden. These calculations show that DFN simulations provide results that are qualitatively similar to results from ECPM calculations. However, because the ECPM is a less direct approach, notable differences exist when compared to the DFN approach.

Laboratory Measurements of Zeta Potential in Fractured Lewisian Gneiss: Implications for the Characterization of Flow in Fractured Crystalline Bedrock

Despite the broad range of interest and possible applications, the controls on the electric surface charge and the zeta potential of gneiss at conditions relevant to naturally fractured systems remain unreported. There are no published zeta potential measurements conducted in such systems at equilibrium, hence, the effects of composition, concentration and pressure remain unknown. This study reports zeta potential values for the first time measured in a fractured Lewisian gneiss sample saturated with NaCl solutions of various concentrations, artificial seawater and artificial groundwater solutions under equilibrium conditions at confining pressures of 4 MPa and 7 MPa. The constituent minerals of the sample were identified using X-ray diffraction and linked to the concentration and composition dependence of the zeta potential. The results reported in this study demonstrate that the zeta potential remained negative for all tested solutions and concentrations. However, the values of the zeta potential of our Lewisian gneiss sample were found to be unique and dissimilar to pure minerals such as quartz, calcite, mica or feldspar. Moreover, the measured zeta potentials were smaller in magnitude in the experiments with artificial complex solutions compared with those measured with NaCl, thus suggesting that divalent ions (Ca2+, Mg2+ and SO42−) acted as potential determining ions. The zeta potential was also found to be independent of salinity in the NaCl experiments, which is unusual for most reported data. We also investigated the impact of fracture aperture on the electrokinetic response and found that surface electrical conductivity remained negligibly small across the range of the tested confining pressures. Our novel results are an essential first step for interpreting field self-potential (SP) signals and facilitate a way forward for characterization of water flow through fractured basement aquifers.

Review: Saltwater intrusion in fractured crystalline bedrock

During the past few years, the number of regional and national assessments of groundwater quality in regard to saltwater intrusion in coastal aquifers has increased steadily. However, most of the international literature on saltwater intrusion is focused on coastal plains with aquifers in unconsolidated material. Case studies, modelling approaches and parameter studies dealing with saltwater intrusion in those systems are abundant. While the hydrogeology of fractured rock has been intensively studied with both modelling approaches and parameter studies—mainly in relation to deep-laying fractured crystalline bedrock as potential waste repositories—case studies on saltwater intrusion in shallow fractured rocks are still an exception. This review summarizes the actual knowledge on saltwater intrusion in fractured crystalline rock. In combination with short overviews of the processes of saltwater intrusion, flow in fractured systems and the genesis of these systems, the review highlights the importance of the fracture systems and its specific characteristics. Fracture properties are a direct consequence of the geological history as well as the current situation of the coastal area. A holistic assessment of water quality in coastal areas hosting fractured crystalline bedrock therefore requires the combination of different approaches in order to investigate the impact of saltwater intrusion through the fractured system.

Mapping Contaminant Plume at a Landfill in a Crystalline Basement Terrain in Ouagadougou, Burkina Faso, Using Self-Potential Geophysical Technique

The delineation of pollution plumes generated by household waste landfills is not easy, particularly in the case of discontinuous or intricately extending water tables, such as those developed in a fractured crystalline bedrock context. In Ouagadougou (Burkina Faso), there are many uncontrolled landfills throughout the urban area. The water table, generally located between 3 and 10 m deep, is likely to be contaminated by the leachate from these landfills. More than 1000 measurements of spontaneous potential (self-potential), referenced by GPS, have been carried out on a landfill and its immediate surroundings to the south of the urban area. The geostatistical processing by analysis of variograms and correlograms highlights an adapted prospecting technique and reliable cartography. The response seems to be mainly due to the electrochemical component with hot spots within the landfill and a plume heading towards the North-East. The distribution of the spontaneous potential seems to be controlled, not by the topography of the site, but by the fracturing of the mother rock of dominant direction 15° N, and by the mother rock/saprolite contact. Thus, the plume does not flow to the market gardening just below the landfill but rather to a residential area where monitoring of the quality of the borehole water is required.

Atmospheric Precipitation and Anomalous Upper Mantle in Relation to Intraplate Seismicity in Norway

AbstractTwo enigmatic regions of high intraplate seismicity in Norway (Western Norway and the Nordland area) show a temporal correlation between the number of earthquakes within the upper crystalline crust and intensity of rain and snowmelt at the Earth's surface. This correlation is obvious in the Nordland area where detailed and reliable data on seismicity are available, whereas the correlation is weaker in Western Norway where detailed data on seismicity are missing. Moreover, these discrete zones of high seismic activity coincide spatially with prominent, low‐velocity, and, most likely, thermally anomalous zones in the upper mantle. We propose that in addition to other causes, the high seismicity may be controlled by the anomalous upper mantle, along with topographically induced gravitational potential energy and crustal density variations. Precipitation‐induced, seasonal increases in pore‐fluid pressure within the fractured crystalline bedrock and changes in water loads may modulate the tectonically controlled seismicity.

A Combined X-ray Absorption and Mössbauer Spectroscopy Study on Fe Valence and Secondary Mineralogy in Granitoid Fracture Networks: Implications for Geological Disposal of Spent Nuclear Fuels.

Underground repository in crystalline bedrock is a widely accepted solution for long-term disposal of spent nuclear fuels. During future deglaciations, meltwater will intrude via bedrock fractures to the depths of future repositories where O2 left in the meltwater could corrode metal canisters and enhance the migration of redox-sensitive radionuclides. Since glacial meltwater is poor in reduced phases, the quantity and (bio)accessibility of minerogenic Fe(II) in bedrock fractures determine to what extent O2 in future meltwater can be consumed. Here, we determined Fe valence and mineralogy in secondary mineral assemblages sampled throughout the upper kilometer of fractured crystalline bedrock at two sites on the Baltic Shield, using X-ray absorption and Mössbauer spectroscopic techniques that were found to deliver matching results. The data point to extensive O2-consuming capacity of the bedrock fractures, because Fe(II)-rich phyllosilicates were abundant and secondary pyrite was dispersed deep into the bedrock with no overall increase in Fe(II) concentrations and Fe(II)/Fe(III) proportions with depth. The results imply that repeated Pleistocene deglaciations did not cause a measurable decrease in the Fe(II) pool. In surficial fractures, largely opened during glacial unloading, ferrihydrite and illite have formed abundantly via oxidative transformation of Fe(II)-rich phyllosilicates and recently exposed primary biotite/hornblende.

Radium isotopes to trace uranium redox anomalies in anoxic groundwater

223Ra, 224Ra, 226Ra, and 228Ra isotopes have been measured in groundwaters from depths ranging 50–900m in fractured crystalline bedrock (Forsmark, Sweden) to understand the reason for elevated (up to 150μg/L) aqueous uranium (Uaq) at 400–650m depth. Ra isotope data is interpreted alongside previously reported 222Rn, 234U, and 238U data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [223Ra/226Ra]GW-based approach (where brackets and “GW” subscript refer to expression of an activity ratio measured from groundwater) to groundwater residence time estimation shows that elevated [Uaq] is most common in Holocene-age groundwaters of marine origin. Although these groundwaters are geochemically reducing, the [223Ra/228Ra]corr (where “corr” subscript refers to a correction applied to compare [223Ra/228Ra]GW to the more commonly reported [226Ra/228Ra]GW) suggest that they interact with U-rich pegmatites containing Proterozoic- and Palaeozoic-age Ca-U(VI)-silicate minerals, which are undersaturated in the present groundwaters. Local aqueous U(VI) can be stabilized in Ca2UO2CO30 complexes at pe-values as low as −4.5 but is susceptible to reduction after a modest decrease in pe-value, alkalinity, or Ca concentration. The [223Ra/228Ra]corr and [224Ra/228Ra]GW also suggest that U(VI)aq precipitates as UO2+X at the interface between marine and non-marine groundwaters. From these data, local [Uaq] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals.

Micro-scale isotopic variability of low-temperature pyrite in fractured crystalline bedrock ― A large Fe isotope fractionation between Fe(II)aq/pyrite and absence of Fe-S isotope co-variation

This study assessed Fe-isotope ratio (56Fe/54Fe, expressed as δ56Fe relative to the IRMM-014 standard) variability and controls in pyrite that has among the largest reported S-isotope variability (maximum δ34S: 140‰). The pyrite occurs as fine-grained secondary crystals in fractures throughout the upper kilometer of granitoids of the Baltic Shield, and was analyzed here for δ56Fe by in situ secondary ion mass spectrometry (SIMS). Part of these pyrite crystals were picked from borehole instrumentation at depths of >400 m below sea level (m.b.s.l.), and thus are modern (known to have formed within 17 years) and can be compared with the δ56Fe of the source dissolved ferrous iron. The δ56Fe values of the modern pyrite crystals (−1.81‰ to +2.29‰) varied to a much greater extent than those of the groundwaters from which they formed (−0.48‰ to +0.13‰), providing strong field evidence for a large Fe isotope fractionation during the conversion of Fe(II)aq to FeS and ultimately to pyrite. Enrichment of 56Fe in pyrite relative to the groundwater was explained by equilibrium Fe(II)aq-FeS isotope fractionation, whereas depletion of 56Fe in pyrite relative to the groundwater was mainly the result of sulfidization of magnetite and kinetic isotopic fractionation during partial transformation of microsized FeS to pyrite. In many pyrite crystals, there is an increase in δ34S from crystal center to rim reflecting Rayleigh distillation processes (reservoir effects) caused by the development of closed-system conditions in the micro-environment near the growing crystals. A corresponding center-to-rim feature was not observed for the δ56Fe values. It is therefore unlikely that the groundwater near the growing pyrite crystals became progressively enriched in the heavy Fe isotope, in contrast to what has been found for the sulfur in sulfate. Other pyrite crystals formed following bacterial sulfate reduction in the time period of mid-Mesozoicum to Quaternary, had an almost identical Fe-isotope variability (total range: −1.50‰ to +2.76‰), frequency-distribution pattern, and relationship with δ34S as the recent pyrite formed on the borehole instrumentation. These features suggest that fundamental processes are operating and governing the Fe-isotope composition of pyrite crystals formed in fractured crystalline bedrock over large time scales.

Evaluation of the Lund deep geothermal exploration project in the Romele\xe5sen Fault Zone, South Sweden: a case study

The bedrock of Skåne, the southernmost province of Sweden, has been targeted for geothermal feasibility studies since the late 1970s. An exploration project concerning the geothermal potential in the Romeleåsen Fault Zone was launched outside the town of Lund in 2001. Besides geophysical imaging of the thrust fault zone, the investigations included drilling and investigations of a 3701.8-m-deep exploration well, DGE-1, with the aim to find > 100 °C warm and hydraulically conductive fractured crystalline bedrock associated to the fault zone. The well penetrates a heavily thrusted and predominantly strongly inclined sedimentary succession in hanging rock blocks along the main fault before entering the fractured crystalline basement at 1946 m, primarily composed of gneiss, granite, and metabasite. This paper represents the first comprehensive description and evaluation of the geological, physical, and hydrological properties of the bedrock at these depths in the Romeleåsen Fault Zone coupled to a geothermal assessment. In addition, the applicability of the four drilling methods used in the crystalline basement section is discussed. The outcome of the DGE-1 well shows significant fracturing in the crystalline bedrock at target depth. The investigations show an average thermal gradient of 22 °C/km, an average heat flow of 58 mW/m2, and an average heat production of 5.8 µW/m3. The values are relatively high in comparison to thermal conditions noted in other deep wells in the Fennoscandian Shield. However, a bottomhole temperature of around 85 °C and insufficient fluid production rate made a commercial geothermal system unviable. Despite this, the experiences from drilling and investigations of the crystalline bedrock at several kilometers depth constitute important proxies for assessing the geothermal potential in similar geological settings and for engineered geothermal systems in the crystalline bedrock of south Sweden.

Identifying cross-well fracture connections using the dissolved oxygen alteration method

Determining how wells in fractured crystalline bedrock are interconnected is essential in defining contaminant flow paths and in interpreting water quality data from wells. Although various tracers and geophysical techniques have been used with success, the high cost or logistics may restrict their implementation. This study demonstrates a low-cost technique for identifying fracture connections between wells by combining the dissolved oxygen alteration method (DOAM) with cross borehole hydraulic pumping tests. Following the alteration of the dissolved oxygen (DO) profile in the observation well through bubbler aeration, an interconnected well is pumped, which results in changes in borehole flow in the aerated observation well. The DO profile in the observation well is monitored over time to characterize the changes in borehole flow and identify fractures that hydraulically connect to the pumping well. The method was demonstrated at three test sites in Storrs, Connecticut. Each site has two or three interconnecting wells in fractured crystalline bedrock and varying flow conditions. The DOAM successfully identified interconnecting fractures in wells 15–84 m apart.

Arsenic variability and groundwater age in three water supply wells in southeast New Hampshire

Three wells in New Hampshire were sampled bimonthly over three years to evaluate the temporal variability of arsenic concentrations and groundwater age. All samples had measurable concentrations of arsenic throughout the entire sampling period and concentrations in individual wells had a mean variation of more than 7 μg/L. The time series data from this sampling effort showed that arsenic concentrations ranged from a median of 4 μg/L in a glacial aquifer well (SGW-65) to medians of 19 μg/L and 37 μg/L in wells (SGW-93 and KFW-87) screened in the bedrock aquifer, respectively. These high arsenic concentrations were associated with the consistently high pH (median ≥ 8) and low dissolved oxygen (median <0.1 mg/L) in the bedrock aquifer wells, which is typical of fractured crystalline bedrock aquifers in New Hampshire. Groundwater from the glacial aquifer often has high dissolved oxygen, but in this case was consistently low. The pH also is generally acidic in the glacial aquifer but in this case was slightly alkaline (median = 7.5). Also, sorption sites may be more abundant in glacial aquifer deposits than in fractured bedrock which may contribute to lower arsenic concentrations.Mean groundwater ages were less than 50 years old in all three wells and correlated with conservative tracer concentrations, such as chloride; however, mean age was not directly correlated with arsenic concentrations. Arsenic concentrations at KFW-87 did correlate with water levels, in addition, there was a seasonal pattern, which suggests that either the timing of or multiple sampling efforts may be important to define the full range of arsenic concentrations in domestic bedrock wells.Since geochemically reduced conditions and alkaline pHs are common to both bedrock and glacial aquifer wells in this study, groundwater age correlates less strongly with arsenic concentrations than geochemical conditions. There also is evidence of direct hydraulic connection between the glacial and bedrock aquifers, which can influence arsenic concentrations. Correlations between arsenic concentrations and the age of the old fraction of water in SGW-65 and the age of the young fraction of water in SGW-93 suggest that water in the two aquifers may be mixing or at least some of the deeper, older water captured by the glacial aquifer well may be from a similar source as the shallow young groundwater from the bedrock aquifer. The contrast in arsenic concentrations in the two aquifers may be because of increased adsorption capacity of glacio-fluvial sediments, which can limit contaminants more than fractured rock. In addition, this study illustrates that long residence times are not necessary to achieve more geochemically evolved conditions such as high pH and reduced conditions as is typically found with older water in other regions.

Simplified representative models for long-term flow and advective transport in fractured crystalline bedrock

Simplified representative models (SRMs) of the hydrogeological system at radioactive-waste repository sites are presented and demonstrated to give useful predictions of the key hydrogeological factors affecting long-term safety. The SRM is constructed from complex site-descriptive models, which have been developed to be consistent with detailed site information and data from short-term (with duration of days, weeks, up to months) field experiments, by keeping elements that are important for long-term predictions into thousands of years and simplifying features of less importance. The simplified approach relies only on fundamental hydrogeological principles and the mathematics can be kept relatively simple. The purpose of SRM is to provide a means of verifying predictions from complex numerical models, with an approach that is easy to evaluate and allows transparent evaluation of factors influencing long-term results. The approach is applied to evaluations of sites for two repositories in fractured crystalline bedrock in Sweden: one for spent nuclear fuel rod assemblies and one for waste with lower levels of radioactivity. The results indicate that the SRMs are able to yield results similar to those of calculations based on much more complex models. Further, the approach allows an evaluation of additional sources of uncertainty that are difficult or expensive to conduct with the complex models. These capabilities make SRMs a very useful and transparent tool for regulatory review.

Groundwater Potential Zone Identification in the Crystalline Basement Rock Terrain of Liberia Using Integrated Remote Sensing and GIS

This paper presents groundwater favorability mapping of fractured crystalline bedrock terrain throughout Liberia with a focus on 43 selected health centers. The aim of this research is to create groundwater suitability maps to be used by the Liberian Ebola Emergency Response Project. Hydrogeological investigations can often be cost prohibitive, particularly in developing nations. Therefore, this study was performed using only publicly available free geospatial data. The materials acquired included a geologic map obtained from the USGS, a Digital Elevation Map, and images from Landsat 8. The data was analyzed with both remote sensing software and ArcGIS and processed to produce several thematic maps. The maps were combined in a multi-criteria analysis (weighted linear combination) to produce a variety of groundwater favorability maps. The results suggest significant spatial variability within 5 km of all selected health centers. These models will be further refined and may prove useful for the Emergency Response Project and future groundwater development in Liberia.

Hydrogeochemical mechanisms governing the mineralization and elevated fluoride $$\left( {\text{F}^{ - } } \right)$$ F - contents in Precambrian crystalline aquifer groundwater in central Benin, Western Africa

In the central part of Benin (Western Africa), high fluoride $$\left( {{\text{F}}^{ - } } \right)$$ contents have been reported in groundwater from Precambrian crystalline bedrock aquifer which is the main source of drinking water. The hydrogeochemical mechanisms leading to such elevated fluoride concentrations are usually not fully understood. In this context, the objective is to identify the hydrogeochemical processes governing groundwater mineralization and the origin of the high fluoride concentrations. A dataset of 162 groundwater samples was collected from the aquifer consisting of a thin altered bedrock layer (shallow aquifer) and a deep fractured crystalline bedrock (deep aquifer). Geochemical approaches and multivariate statistics have been used to explore the data. Fluoride concentrations vary between 0.00 and 7.19 mg/L in groundwater. Samples collected in the southern part of the investigated area, close to Dassa-Zoume, show the highest concentrations, with more than 75% greater than the guideline value of 1.5 mg/L. The deep fractured aquifer shows higher concentrations of fluoride than the shallow regolith reservoir. Results reveal that groundwater mineralization is derived mainly from the hydrolysis of silicate minerals, but it is also influenced by anthropogenic effects, particularly in the shallow reservoir. However, fluoride has a geogenic origin, primarily from the weathering of silicate minerals, primarily biotite. $${\text{Ca}}^{2 + }/{\text{Na}}^{ + }$$ cation exchanges and $${\text{F}}^{ - }/{\text{OH}}^{ - }$$ anion exchanges between groundwater and the rock matrix also occur as minor processes. Saturation of the water with respect to calcite and the precipitation of this mineral, which reduces calcium activity, also favor the release of fluoride from rocks.

Fracture hydromechanical response measured by fiber optic distributed acoustic sensing at milliHertz frequencies

AbstractA new method of measuring dynamic strain in boreholes was used to record fracture displacement in response to head oscillation. Fiber optic distributed acoustic sensing (DAS) was used to measure strain at mHz frequencies, rather than the Hz to kHz frequencies typical for seismic and acoustic monitoring. Fiber optic cable was mechanically coupled to the wall of a borehole drilled into fractured crystalline bedrock. Oscillating hydraulic signals were applied at a companion borehole 30 m away. The DAS instrument measured fracture displacement at frequencies of less than 1 mHz and amplitudes of less than 1 nm, in response to fluid pressure changes of less 20 Pa (2 mm H2O). Displacement was linearly related to the log of effective stress, a relationship typically explained by the effect of self‐affine fracture roughness on fracture closure. These results imply that fracture roughness affects closure even when displacement is a million times smaller than the fracture aperture.

Analysis of crystalline bedrock aquifer productivity: Case of central region in Cameroon

Hydrogeology in the central region of Cameroon is marked by the presence of two types of aquifers: one of them in the weathered rocks and another in the fractured crystalline bedrock. This study focuses on the fractured bedrock aquifer through the examination of records from 67 boreholes drilled in the area. An analysis of the productivity of this aquifer system has been done to enhance the water supply, in view of the drilling data of the region. Boreholes have been grouped according to the depth, weathering depth, rock type, topography, and proximity to lineaments. The results show transmissivity values ranging between 0.1 and 131m2 / day with an average and a median of 4.7 and 0.7m 2 / day, respectively. No correlation between productivity and borehole depth or weathering depth was found. However, beyond 70m depth, the values of transmissivity tend to fall. In addition, weathering depths between 5 and 35m appear to provide the highest transmissivity. In terms of topography, the boreholes located in the valleys are more productive than those located in the upland. Schist appears to be more productive than gneisses and quartzites. Wells within 150m of lineaments are more productive than those located further. Moreover, wells along NW-SE lineaments (120–150°) seem to be more productive. All these factors play an important role in the productivity of wells in the central region of Cameroon. This study thus provides a basis for rural water-supply development programs.

Isotopic evidence for microbial production and consumption of methane in the upper continental crust throughout the Phanerozoic eon

Microorganisms produce and consume methane in terrestrial surface environments, sea sediments and, as indicated by recent discoveries, in fractured crystalline bedrock. These processes in the crystalline bedrock remain, however, unexplored both in terms of mechanisms and spatiotemporal distribution. Here we have studied these processes via a multi-method approach including microscale analysis of the stable isotope compositions of calcite and pyrite precipitated in bedrock fractures in the upper crust (down to 1.7 km) at three sites on the Baltic Shield. Microbial processes have caused an intriguing variability of the carbon isotopes in the calcites at all sites, with δ13C spanning as much as −93.1‰ (related to anaerobic oxidation of methane) to +36.5‰ (related to methanogenesis). Spatiotemporal coupling between the stable isotope measurements and radiometric age determinations (micro-scale dating using new high-spatial methods: LA-ICP-MS U–Pb for calcite and Rb–Sr for calcite and co-genetic adularia) enabled unprecedented direct timing constraints of the microbial processes to several periods throughout the Phanerozoic eon, dating back to Devonian times. These events have featured variable fluid salinities and temperatures as shown by fluid inclusions in the calcite; dominantly 70–85 °C brines in the Paleozoic and lower temperatures (<50–62 °C) and salinities in the Mesozoic. Preserved organic compounds, including plant signatures, within the calcite crystals mark the influence of organic matter in descending surficial fluids on the microbial processes in the fracture system, thus linking processes in the deep and surficial biosphere. These findings substantially extend the recognized temporal and spatial range for production and consumption of methane within the upper continental crust.

Impacts of Road Salting on Water Quality in Fractured Crystalline Bedrock.

Many rural communities depend on bedrock wells as a primary water source, which raises the issue as to whether increasing amounts of salt application are affecting bedrock water quality and to what degree. Through wellbore profiling, this study investigated changes in specific conductance in two crystalline bedrock wells at the University of Connecticut in Storrs, CT, from 2003 to 2016, with particular emphasis on the impacts of increased salt application with a change in deicing practices at the university after 2009. Hourly specific conductance measurements were collected in 2014 to determine how water source may affect wellbore concentrations seasonally. Chloride was found to be highly persistent in the bedrock, with concentrations consistently increasing from 2003 to 2016 despite year-to-year variations in salt application. A dramatic increase in chloride occurred in the wellbores in response to the change in deicing practices, with an immediate response in fractures having a direct connection to the overburden. In light of the long-term implications of road salting on subsurface chloride contamination, this study argues that consideration be given to deicing management strategies to reduce road salt contamination.

Cerium sequestration and accumulation in fractured crystalline bedrock: The role of Mn-Fe (hydr-)oxides and clay minerals

This study focuses on the mechanisms of Ce sequestration and accumulation in the fracture network of the upper kilometer of the granitoid bedrock of the Baltic Shield in southeast Sweden (Laxemar area, Sweden). The material includes 81 specimens of bulk secondary mineral precipitates (“fracture coatings”) collected on fracture walls identified in 17 drill cores, and 66 groundwater samples collected from 21 deep boreholes with equipment designed for retrieval of representative groundwater at controlled depths. The concentrations of Ce in the fracture coatings, although varying considerably (10–90th percentiles: 67–438mgkg−1), were frequently higher than those of the wall rock (10–90th percentiles: 70–118mgkg−1). Linear combination fitting analysis of Ce LIII-edge X-ray absorption near-edge structure (XANES) spectra, obtained for 19 fracture coatings with relatively high Ce concentrations (⩾145mgkg−1) and a wide range of Ce-anomaly values, revealed that Ce(IV) occurs frequently in the upper 10m of the fracture network (Ce(IV)/Cetotal=0.06–1.00 in 8 out of 11 specimens) and is mainly associated with Mn oxides (modeled as Ce oxidatively scavenged by birnessite). These features are in line with the strong oxidative and sorptive capacities of Mn oxide as demonstrated by previous studies, and abundant todorokite and birnessite-like Mn oxides identified in 3 out of 4 specimens analyzed by Mn K-edge X-ray absorption spectroscopy (XAS) in the upper parts of the fracture network (down to 5m). For a specimen with very high Ce concentration (1430mgkg−1) and NASC-normalized Ce anomaly (3.63), the analysis of Ce XANES and Mn XAS data revealed (i) a predominance of Ce oxide in addition to Ce scavenged by Mn oxide; and (ii) a large fraction of poorly-crystalline hexagonal birnessite and aqueous Mn2+, suggesting a recent or on-going oxidation of Mn2+ in this fracture. In addition, the Ce oxide precipitates on this fracture observed by in situ SEM-EDS contained considerable amounts of Mn. These spectroscopic and microscopic features led us to suggest that the remarkable accumulation of Ce(IV) in this fracture is a result of repeated formation and dissolution of Mn oxides, that is, formation of Mn oxide followed by oxidative scavenging of Ce as Ce oxide nanoparticles, which largely remained during the subsequent reductive dissolution of the Mn oxides. In addition, the XANES data indicate that goethite has the capability to oxidize Ce at near-neutral pH under our experimental conditions (goethite reacted with 0.001M Ce for 48h in a glove box with O2 <1ppm). This previously unrecognized Ce oxidation pathway also seems to contribute to a minor extent to the oxidative scavenging of Ce in the fracture network. Trivalent Ce in the fracture coatings, in particular below 2.5m, is mainly sorbed as inner-sphere complexes on clay minerals. Taking into account the facts that Ce in the present groundwater is scarce and modeled to be largely complexed with humic substance, it is argued that the inner-sphere complexes were mainly formed from past (Paleozoic) hydrothermal fluids.