Fractured Basalt Research Articles

Stimulation of tight basalt reservoirs using supercritical carbon dioxide: Implications for large-scale carbon sequestration

Although supercritical carbon dioxide (SC-CO2) fracturing shows tremendous potential for maximizing injection efficiency and enhancing storage volumes, few investigations have been reported on the SC-CO2 fracturing characteristics of tight basalts and the reactions between fractured basalt and SC-CO2. In this study, hydraulic fracturing experiments were conducted on cylindrical basalt specimens using water and SC-CO2 as fracturing fluids. Geometric parameters were proposed to characterize the fracture morphologies based on the three-dimensional (3D) reconstructions of fracture networks. The rock slices with induced fractures after SC-CO2 fracturing were then processed for fluid (deionized water/SC-CO2)-basalt reaction tests. The experimental results demonstrate that SC-CO2 fracturing can induce complex and tortuous fractures with spatially dispersed morphologies. Other fracturing behaviors accompanying the acoustic emission (AE) signals and pump pressure changes show that the AE activity responds almost simultaneously to variation in the pump pressure. The fractured basalt blocks exposed to both SC-CO2 and water exhibit rough and uneven surfaces, along with decreased intensities in the element peaks, indicating that solubility trapping predominantly occurs during the early injection stage. The above findings provide a laboratory research basis for understanding the fracturing and sequestration issues related to effective CO2 utilization.

Carbon evolution and mixing effects on groundwater age calculations in fractured basalt, southwestern Idaho, U.S.A.

Using hydrochemical and isotopic compositions of springs and wells, we trace carbon from critical zone carbon dioxide (CO2) into groundwater of the semi-arid Reynolds Creek Experimental Watershed - Critical Zone Observatory, southwestern Idaho, USA. Dissolved inorganic carbon (DIC) concentrations, pH and stable isotope tracers of carbon for DIC (δ13CDIC), are used to show that most groundwater evolves under open system conditions, moving carbon into the groundwater and acting as a carbon sink. However, one sample (−10.94‰ δ13CDIC, 6,350 14C years before present (yrs. BP)) may have evolved under closed system conditions with a higher partial pressure of critical zone CO2 than present-day soils. By characterizing the carbon cycle, we show that (1) carbon evolution is primarily under open-system conditions, (2) shallow groundwater samples are generally less mixed and more recent (10 to 70 3H yrs. BP) than deeper groundwater samples (1,469 to 6,350 14C yrs. BP), and (3) the older portion of the groundwater may be even older than the calculated 14C ages, as indicated by the mixing of age tracers in intermediate wells. Our global conception of the deep critical zone should include carbon cycling of critical zone CO2 in old groundwater. Characterizing the deep critical zone in a semi-arid weathered silicate watershed improves our global understanding of carbon, nutrient and water cycling.

Megaflood Erosion on Mars—How Lava‐Filled Craters Became Mesas (With Insights From Lava Physics, Stream Power, and Rock Mechanics)

AbstractRound mesas up to 500 m high occur in Martian outflow channels. Mesas in Ravi and Elaver Valles occur in deepest parts of the channels where the most intense megaflood erosion occurred. I theorize that Noachian basalts poured into craters which acted as lava traps, similar to Kilauean lava lakes. Subsequent flood basalts buried the infilled craters. Hesperian megafloods stripped away hundreds of meters of basalts, exhuming erosion‐resistant strata. Lava solidification theory is explored to assess the role of cooling in governing the structure and strength of the Martian basalts. The postulated lava lakes that formed the Ravi Vallis mesas would have needed millennia to cool. The larger Elaver Vallis mesa may have needed up to 20,000 years to solidify. Long cooling times led to coarse, doleritic mineral textures and reduced numbers of cooling joints, greatly increasing bulk rock strength and resistance to hydrodynamic erosion. Using rock mechanics theory, cores of exhumed lava‐filled craters would have bulk rock strengths at least 5 to 30 times greater than more highly fractured basalts. Discharge hydrographs for the Morella Crater breach flood that carved Elaver Vallis peak at ∼2.1 × 107 m3 s−1. Stream power per unit streambed area ranged up to >150,000 W m−2 as the breach grew in size. The calculations provide an envelope of power sufficient to erode hundreds of meters of basalts, but not great enough to remove the large mesa. Thus the legacy of an ancient, lava‐filled crater is a high mesa on the floor of Elaver Vallis.

Occurrence of Carbonated Groundwater and Hydrocarbons at the Uranium Deposits of the Khiagda Ore Field (Republic of Buryatia)

A generalization of unpublished and published data since 1985 on biogeochemical, hydrogeochemical, geochemical, mineralogical studies and soil geochemistry at uranium deposits of the Khiagda ore field in the Vitim uranium ore district made it possible to reveal a genetic relation of deep cold carbonated hydrocarbonate–magnesium groundwater containing dissolved hydrocarbons (HCs) to uranium mineralization and ore preservation. The through penetration of epigenetic HCs was traced from the disintegration zone of basement granitoids through overlying sedimentary ore-bearing and volcanosedimentary rocks up to overlying fractured basalts. The assemblage of clarified rocks–HCs–siderites–uranium phosphates U(IV) was commonly found. Carbonated hydrocarbonate–magnesium groundwater and anomalous HC contents in soils can be additional criteria for identifying the Vitim-type uranium deposits in the Trans-Baikal region.

A Concept of Fuzzy Dual Permeability of Fractured Porous Media

The interpretation of the results of hydrogeological field observations and the modeling of fractured porous subsurface media is often conducted using dual-porosity and/or dual-permeability concepts. These concepts, however, do not consider the effects of spatial and temporal variations and uncertainties, or fuzziness, in the evaluation of the subsurface flow characteristics of fractured porous media. The goal of the paper is to introduce a concept of fuzzy dual permeability of fractured porous media based on the fuzzy system analysis of the results of ponded infiltration tests in fractured basalt. The author revisited the results of the tests conducted in areas close to the Idaho National Laboratory (INL), Idaho, USA: small-scale (approximately 0.5 m2) ponded tests at the Hell’s Half Acre site, mesoscale (56 m2) ponded tests at the Box Canyon site, and a large-scale infiltration test (31,416 m2) at the Radioactive Waste Management Complex at INL. Methods of fuzzy clustering and fuzzy regression were applied to describe the time-depth waterfront penetration and to characterize the phenomena of rapid flow through a predominantly fractured component and slow flow through a predominantly porous matrix component. The concept of fuzzy dual permeability is presented using a series of fuzzy membership functions of the waterfront propagation with depth and time. To describe the time variation of the flux, a fuzzy Horton’s model is presented. The developed concept can be used for the uncertainty quantification in flow and transport in geologic media.

Subsurface investigation for groundwater potential of biu plateau basalt north eastern Nigeria, using vertical electrical sounding

Groundwater is a very important component of water resources in nature. Since the demand of groundwater increases with population growth, it is necessary to explore groundwater more intensively. The importance of groundwater cannot be overemphasized. For this reason, the exploration for water is therefore a vital aspect of Geophysics. The resistivity method of surveying was carried out for the study of Groundwater potential in Biu town, Borno State, Nigeria. Data were acquired using the ABEM Terrameter 300C using the Schlumberger array. The data obtained were subjected to interpretation by partial curve matching and then by computer iteration and the results correlated with records from existing wells. A total of four geologic layers namely; top soil, weathered basalt, fractured basalt and fresh basement was delineated in this study. Results showed that the aquifer is located within the second and third layers comprising mainly of weathered and fracture materials. The first aquifer is the weathered basalt with resistivity ranging from 11.5 to 106.8 Ωm and thickness ranges from 2.73 to 31.32 m. while the second aquifer is the fractured basalt with resistivity ranging from 16.5 to 372.2 Ωm with thickness from 2.9 to 11.17 m. Appropriate depths to which potable water can be obtained from the various locations varies from 21.89 m to 38.33 m are recommended in this study. While depths in VES 6 and VES 7 are 10.45 m and 14.97 m are not good for groundwater exploration.

Understanding the spatio-temporal evolution of fractures in pillow basalt

We investigated the origin and spatio-temporal evolution of cooling fractures in pillow basalt which undergo thermal contraction after their eruption in an aqueous environment. Through a computer-based simulation using Fourier transformation, the thermo elastic stress displacement profiles within individual pillow units are determined. The scaled model (pillow diameter - 1 meter) generated radial, linear fractures perpendicular to pillow margin and irregular discrete flaws in the pillow interior like the ones observed in natural examples. Radial linear fractures of 3–5 centimetre in length have been measured in pillows of average one-metre diameter from the Maradihalli region, in the Chitradurga Schist Belt, India. An estimated time of 94–118 minutes was required to get radial fractures of similar length in the simulation. Our model efficiently replicated the generation and distribution of thermal fractures and allowed an estimation of cooling time for the peripheral glassy zone but has limitations in deciphering the formation of fracture networks in progressively crystalline inner zone of pillows.

Estimating Groundwater Flow Velocity in Shallow Volcanic Aquifers of the Ethiopian Highlands Using a Geospatial Technique

The shallow volcanic aquifer is the major rural water supply source in the Ethiopian highlands. A significant number of hand pump wells in these aquifers experience a rapid decline in yield and poor performance within a short period of time after construction. Hence, reliable estimation of groundwater flow velocity is important to understand groundwater flow dynamics, aquifer responses to stresses and to optimize the sustainable management of groundwater resources. Here, we propose the geospatial technique using four essential input raster maps (groundwater elevation head, transmissivity, effective porosity and saturated thickness) to investigate groundwater flow velocity magnitude and direction in the shallow volcanic aquifers of the Ethiopian highlands. The results indicated that the high groundwater flow velocity in the Mecha site, ranging up to 47 m/day, was observed in the fractured scoraceous basalts. The Ejere site showed groundwater flow velocity not exceeding 7 m/day in the fractured basaltic aquifer and alluvial deposits. In the Sodo site, the groundwater flow velocity was observed to exceed 22 m/day in the fractured basaltic and rhyolitic aquifers affected by geological structures. The Abeshege site has a higher groundwater flow velocity of up to 195 m/day in the highly weathered and fractured basaltic aquifer. In all study sites, aquifers with less fractured basalt, trachyte, rhyolite, welded pyroclastic, and lacustrine deposits exhibited lower groundwater flow velocity values. The groundwater flow velocity directions in all study sites are similar to the groundwater elevation head, which signifies the local and regional groundwater flow directions. This work can be helpful in shallow groundwater resource development and management for rural water supply.

Characterizing groundwater and surface water interaction using geological, environmental tracers (222Rn, EC, δ18O, and δ2H) and baseflow index methods for part of the Upper Awash and the adjacent Blue Nile Basin, Ethiopia

Characterization of groundwater-surface water interaction using a combined approach of geological, environmental tracers (222Rn, EC, δ18O, and δ2H) and baseflow index in a complex geological environment was carried out for part of the Upper Awash and adjacent Blue Nile Basins. In this work, total measurements of 59 water samples were used (52 from different reaches of main Rivers and tributaries, 4 from springs, and 3 from boreholes) for 222Rn analysis in dry and wet seasons, along with recent 8–20 years of daily stream flow records from 11-gauge stations. For sub-surface inference, regional geological structures and lineament density zones were determined using Kriging interpolation techniques. 222Rn values for the river and streams show 2.7–468 Bq/m3 during the rainy season and 17.1–1467 Bq/m3 for the non-rainy season. The credible justification related to moderate to high seasonal 222Rn values (100–400 Bq/m3) in the upstream parts of the two Basins, including areas falling along the surface water divide between the two adjacent Basins, shows a link between groundwater and surface water systems. In the same area, a very high mean annual baseflow index (BFI >0.5) is dominantly associated with high lineament density (0.9 < LD < 3.0), regional faults, and geologic units of highly permeable scoriaceous basalt, rift basalt, and fractured ignimbrite units. Relatively, low 222Rn (<100 Bq/m3) in both seasons and low baseflow index (BFI <0.35) values mainly in the central part of the Upper Awash basin are attributed to existing impervious superficial deposits (alluvial, tuff and ash). This, in turn, shows the poor connection between the groundwater and surface water systems in the central plain part of the Upper Awash Basin and, as also evidenced by the storage of floodwater for an extended time after the end of the rainy season, prove poor connectivity between groundwater and surface water systems. The very high 222Rn (400–1467 Bq/m3) with varied mean annual baseflow index (0.35 < BFI <0.5) at different stations in the extreme southern and southeastern parts of the Upper Awash Basin show Awash River is gaining from the aquifer system through highly fractured basalt and fractured ignimbrite geologic units. Evidence from analysis of measured electrical conductivity and stable isotopes (δ18O, δ2H) generally agree with the observed 222Rn value and baseflow analysis.

Experimental investigation on frictional properties of stressed basalt fractures

Experimental investigation on frictional properties of stressed basalt fractures

Lithology, Pore‐Filling Media, and Pore Closure Depth Beneath InSight on Mars Inferred From Shear Wave Velocities

AbstractWe quantify the volume and distribution of water, cement, sediments, and fractured rocks within the Martian crust beneath NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport mission) lander by using rock physics models to interpret shear wave velocities (Vs) measured from InSight data. The models assume that Mars' crust comprises sediments and fractured rocks whose pores and fractures host variable combinations of gas, liquid water, and mineral cements. Measured Vs in the upper crust (0–8 km) can be explained by layers of minimally (&lt;2%) cemented sediments and gas‐filled fractured basalts. Measured Vs in the deeper crust (8–20 km) can be explained by fractured basalts or more felsic igneous rocks (modeled here as 100% plagioclase feldspar) that is unfractured or has up to 23% porosity. Open pores in the deeper crust could host gas, liquid water, and up to 2% cement. Modeled Vs are too low for a seismically detectable ice‐saturated cryosphere in the upper crust and temperatures are too high to freeze liquid water in the deeper crust. Notably, with Vs alone, we are unable to distinguish between liquid water and gas within the pores.

Pathways and Estimate of Aquifer Recharge in a Flood Basalt Terrain; A Review from the South Fork Palouse River Basin (Columbia River Plateau, USA)

Aquifer recharge is one of the most important hydrologic parameters for understanding available groundwater volumes and making sustainable the use of natural water by minimizing groundwater mining. In this framework, we reviewed and evaluated the efficacy of multiple methods to determine recharge in a flood basalt terrain that is restrictive to infiltration and percolation. In the South Fork of the Columbia River Plateau, recent research involving hydrologic tracers and groundwater modeling has revealed a snowmelt-dominated system. Here, recharge is occurring along the intersection of mountain-front alluvial systems and the extensive Miocene flood basalt layers that form a fractured basalt and interbedded sediment aquifer system. The most recent groundwater flow model of the basin was based on a large physio-chemical dataset acquired in laterally and vertically distinctive locations that refined the understanding of the intersection of the margin alluvium and the spatially variable basalt flows that filled the basin. Modelled effective recharge of 25 and 105 mm/year appears appropriate for the basin’s plain and the mountain front, respectively. These values refine previous efforts on quantifying aquifer recharge based on Darcy’s law, one-dimensional infiltration, zero-flux plane, chloride, storage, and mass-balance methods. Overall, the combination of isotopic hydrochemical data acquired in three dimensions and flow modelling efforts were needed to simultaneously determine groundwater dynamics, recharge pathways, and appropriate model parameter values in a primarily basalt terrain. This holistic approach to understanding recharge has assisted in conceptualizing the aquifer for resource managers that have struggled to understand aquifer dynamics and sustainable withdrawals.

Changes in Pore Geometry and Connectivity in the Basalt Pore Network Adjacent to Fractures in Response to CO2‐Saturated Fluid

AbstractPores connected to fractures provide an increased surface area for fluid‐rock interactions when reactive fluids, such as CO2‐saturated water, flow within fractures. Diffusion‐controlled transport of ions including dissolved CO2, Ca2+, Mg2+, Fe2+, and Si within the connected pore network can lead to local mineral undersaturation or supersaturation and respective local dissolution or precipitation of secondary minerals. In this study, a diffusion‐controlled experiment was conducted on a fractured basalt under subsurface conditions (60°C and 80 bars) to investigate the changes in the pore volume, the connectivity within the pore network, the pore and throat size distribution and to quantify the volume of dissolved and precipitated mineral phases. CO2‐saturated water with a supply of ions from the dissolution of basalt powder was reacted with an artificially fractured basalt sample over 12 weeks. The net pore volume of the sample decreased by 158 mm3, equivalent to 7.7% of the initial pore volume (2,041 mm3). The number of pores decreased considerably (15%) while the decline in the number of throats was small (4%) suggesting precipitation primarily occurred in pores. The number of isolated pores declined, which is attributed to mineral dissolution leading to greater connectivity within the pore network. Overall, the results provide insights into the early phase of reactive fluid flow in fractured basalt eventually leading to self‐sealing of fractures and the adjacent pore network.

Technological innovations in construction of underground caverns in basaltic rocks at Baihetan Hydropower Station on Jinsha River

The two underground powerhouses on the left and right banks of Baihetan Hydropower Station (BHT-HS) are symmetrically arranged inside basaltic rocks in the alpine gorge reach of the lower Jinsha River. Each of them is equipped with 8 sets of 1,000 MW hydraulic turbine generators, with the largest unit capacity, total installed capacity, span of main powerhouse (i.e. 34 m), diameter of dome-shaped tail-regulating chamber (max diameter of 48 m) and scale of underground caverns around the world. Due to complex layout, interaction between caverns and high hollowing rate, the construction process of the underground cavern group is faced many challenges. For example, the cavern excavation is located in the high ground stress area dominated by horizontal tectonic stress; the underlying basalt fractures are well developed, and the rock is hard and brittle, with low crack initiation strength; columnar jointed basalts develop locally; a number of large and weak gently-sloped shear zones develop diagonally cutting the main powerhouse. Following the construction ideology of “understanding, utilization, protection, monitoring, and feedback analysis on the surrounding rock masses during the construction period”, and with the engineering construction procedure of “the excavation, analysis, prediction and inspection of each rock layer”, this paper proposes a set of innovation construction technologies including “advanced pre-control, thin vertical layer, small planar partition, short excavation progress, fine blasting, strong and quick support, short interval monitoring, quick feedback, precise dynamic optimization” to control the internal cracking of the hard and brittle basaltic rocks, discontinuous deformation of staggered zones and spatio-temporal deformation of underground caverns in high geostress. It has promoted the safe and orderly construction of BHT-HS, the technical progress of the construction of hydropower station projects in China, and may also be applicable to the design and construction of similar projects worldwide.

Genesis of calcite vein in basalt and its effect on reservoir quality: A case study of the Carboniferous in the east slope of Mahu sag, Junggar Basin, NW China

The characteristics and genesis of the calcite veins in Carboniferous basalt in the east slope of Mahu Sag, Junggar Basin are investigated based on observation of cores and thin sections; analyses of X-ray fluorescence, in situ major, trace and rare earth elements (REE), carbon, oxygen and strontium isotopes, fluid inclusions, as well as basin modeling. There are three periods of calcite fillings. The Period I calcite is characterized by low Mn content, flat REE pattern, strong negative cerium (Ce) anomaly, weak to moderate negative Eu anomaly, and light carbon isotopic composition, indicating the formation of the calcite was affected by meteoric water. The Period II calcite shows higher Mn and light REE contents, weak positive Ce anomaly and slight positive europium (Eu) anomaly, and a little heavier carbon isotopic composition and slightly lower strontium isotope ratio than the Period I calcite, suggesting that deep diagenetic fluids affected the formation of the Period II calcite to some extent. The Period III calcite is rich in iron and manganese and has REE pattern similar to that of the Period II calcite, but the cerium and europium enomalies vary significantly. The Period I and II calcites were formed in shallow diagenetic environment at approximately 250–260 Ma, corresponding to Late Hercynian orogeny at Late Permian. The Period III calcite was probably formed in the Indo-China movement during Late Triassic. It is believed that the precipitation of calcite in basalt fractures near unconformity was related to leaching and dissolution of carbonates in the overlying Lower Permian Fengcheng Formation by meteoric water, which destructed the Carboniferous weathering crust reservoirs in early stage. Relatively high quality reservoirs could be developed in positions with weak filling and strong late dissolution, such as structural high parts with Fengcheng Formation missing, distant strata from Fengcheng Formation vertically, buried hills inside lake basin, etc.

Simulation of CO2 mineral trapping and permeability alteration in fractured basalt: Implications for geologic carbon sequestration in mafic reservoirs

Basalt formations are potentially attractive targets for carbon capture and sequestration (CCS) on the basis of favorable CO2-water-rock reactions, which result in permanent CO2 isolation through mineral trapping. Recent pilot-scale experiments in Iceland and Washington state, USA, provide promising results that indicate rapid carbon mineralization occurs within basalt reservoirs. Nevertheless, transitioning these pilot-scale results to large-scale industrial CCS operations is fraught with uncertainty because fluid flow in basalt formations is governed by fracture-controlled hydraulic properties that are highly heterogeneous and difficult to map in situ. This uncertainty is exacerbated by feedbacks between multi-phase fluid dynamics (CO2 and water) and fluid-rock reactions, which may result in a reinforcing feedback comprising CO2 mineralization, permeability alteration, and fluid mobility. To begin to understand the feedbacks between multi-phase fluid flow and mineralization in fractured basalt, this study uses reactive transport simulation methods to model CO2 infiltrating a meter-scale, synthetic basalt fracture overlying a storage reservoir while accounting for porosity change due to mineralization and its corresponding effect on permeability and fluid mobility. Results show that (i) carbonate and clay mineralization tends to occur downgradient of a fracture intersection, (ii) mineralization reduces porosity, which leads to permeability reduction and slows free-phase CO2 migration, (iii) stronger porosity-permeability coupling increases the proportion of mineralized carbon while reducing CO2 mass that can enter fracture, which may lead to self-sealing behavior as fluid mobility approaches nil, and (iv) errors caused by unknown porosity-permeability relationships are small in comparison to errors that arise by omitting mineralization-induced permeability reduction when simulating CO2 sequestration scenarios in basalt reservoirs.

Experimental study on the velocity-dependent frictional resistance of a rough rock fracture exposed to normal load vibrations

Changes in shear velocity can strengthen or weaken the frictional resistance of joints/faults in natural systems, but the mechanism remains unclear. We investigated the shear behavior of a rough basalt fracture in well-controlled, repeatable shear tests under constant and dynamic normal load conditions at different shear velocities. Normal load vibrations, simulating a dynamic normal load, were applied to the upper block of a fractured basalt sample. Simultaneously, a shear load was applied to the bottom block, providing a constant shear velocity. The peak shear strength increased with increasing shear velocity under constant normal load conditions. The peak shear strength decreased at a lower shear velocity under normal load vibrations. When the shear velocity exceeded the critical value, vc, the peak shear strength increased. The apparent coefficient of friction reduced under normal load vibrations. The reduction in the dynamic coefficient of friction increased with increasing shear velocity. We identified a phase shift between the peak normal load and peak shear load with peak shear load delay (D1) and a phase shift between peak normal load and the peak coefficient of friction with the peak coefficient of friction delay (D2). D1 and D2 were dependent on the quasi-static coefficient of friction and shear velocity, and both decreased with increasing shear velocity. D1 decreased with the increasing quasi-static coefficient of friction, while D2 was almost constant with changes in the quasi-static coefficient of friction. A new shear strength criterion was proposed for a rough joint under a constant shear velocity and normal load vibrations.

On the transpiration of wild olives under water-limited conditions in a heterogeneous ecosystem with shallow soil over fractured rock

Abstract Mediterranean ecosystems are typically heterogeneous and savanna-like, with trees and grass competing for water use. By measuring sap flow, we estimated high transpiration of wild olive, a common Mediterranean tree, in Sardinia despite dry conditions. This estimate agrees with independent estimates of tree transpiration based on energy balance, highlighting the wild olive’s strong tolerance of dry conditions. The wild olive can develop an adaptation strategy to tolerate dry conditions. In this Sardinian case study, the wild olive grew in shallow soil, and the tree roots expanded into the underlying fractured basalt. The trees survived in dry periods using water infiltrated during wet seasons into fractured rocks and held in soil pockets. We estimated a high upward vertical flux through the bottom soil layer from the underlying substrate, which reached 97% evapotranspiration in August 2011. The water taken up by tree roots from bedrock hollows is usually neglected in ecohydrological modeling.

Noble gases, dead carbon, and reinterpretation of groundwater ages and travel time in local aquifers of the Columbia River Basalt Group

Groundwater noble gas concentrations are used to model the temperature of aquifer recharge but also may be used to understand geologic influences on water chemistry. Aquifers in the South Fork Palouse River Basin are hosted within the fractured basalts of the Columbia River Basalt Group and the interbedded sediments of the Latah Formation. The travel time of groundwater, and the associated recharge rate, in this multiple aquifer system is of primary interest to water resource managers because of declining water levels. Prior investigations of the deeper groundwater indicated low values of percent modern carbon and elevated alkalinity and δ13C values compared to shallower groundwater. The groundwater travel time suggested by the uncorrected carbon-14 ages (up to 31,000 BP) implies a recharge rate much slower than hypothesized from identified recharge pathways, low storativity values, and relatively large groundwater withdrawals. No sources of dead carbon were previously identified, but the disconnect between old groundwater and likely recharge pathways suggest input from a dead carbon source. Groundwater collected for this study contained elevated alkalinity, δ13C, He, and 3He/4He values deeper in this aquifer system located in this young flood basalt province. Elevated alkalinity and δ13C values correlate with a mantle CO2 source that is reflected in oversaturated He and elevated 3He/4He (R/Ra) ratios. The flood basalts are Miocene expressions of the modern Yellowstone hotspot, which exudes relatively large concentrations of He (with elevated 3He/4He (R/Ra) ratios) and CO2. A deep and 14C-free geologic source of CO2 helps to explain the low percent modern carbon in the deeper groundwater. Correction of groundwater ages through incorporation of this dead carbon source produced ages up to 64% smaller than previous age estimates; although, greater age corrections may be necessary because of differences in sample and analysis methods for alkalinity, δ13C, and 14C that influence detection of the dead carbon input.

Shallow geothermal energy system in fractured basalt: A case study from Kollafjørður, Faroe Islands, NE-Atlantic Ocean

A shallow (≈200 m) geothermal energy system is examined in the Faroe Islands, a 60-million-year-old volcanic archipelago in the Northeast Atlantic. The geothermal water has a heating capacity of approximately 150 individual households and consists of meteoric water approximately 3 years old. Water temperatures as high as 27 °C in artesian wells are explained by a topography-driven vertical convection. The water flows into the boreholes from the north-northwest through fractures and flow tops and bases in the basalt exposed in surrounding high terrains. Of six influx zones, three are water carrying fractures that strike N–S and dip E.