At the Dixie Valley geothermal field, Nevada, USA, fluid boiling triggered the precipitation of carbonate scale minerals in concentric bands around tubing inserted into production well 28–33. When the tubing was removed, this mineral scale was sampled at 44 depth intervals between the wellhead and 1227 m depth. These samples provide a unique opportunity to evaluate the effects of fluid boiling on the scale mineralogy and geochemistry of the vapor and liquid phase. In this study, the mineralogy of the scale deposits and the composition of the fluid inclusion gases trapped in the mineral scales were analyzed. The scale consists mainly of calcite from 670–1112 m depth and aragonite from 1125 to 1227 m depth, with traces of quartz and Mg-smectite. Mineral textures, including hopper growth, twinning, and fibrous growth in the aragonite and banded deposits of fine grained calcite crystals, are the result of progressive boiling. The fluid inclusion noncondensable gas was dominated by CO₂. However, significant variations in He relative to N₂ and Ar provide evidence that the geothermal reservoir consists of mixed source deeply circulating reservoir water and shallow, air saturated meteoric water. Gas analyses for many inclusions also showed higher CH₄ and H₂ relative to CO₂ than measured in gas sampled from this well, other production wells, and fumaroles. These inclusions are interpreted to have trapped CH₄- and H₂-enriched gas resulting from early stages of boiling.

High-sulfidation vein gold deposits such as El Indio, Chile, formed in fracture arrays <1000 m beneath paleo-solfatara in volcanic terranes. Stable isotope data have confirmed a predominance of magmatic vapor during the deposition of arsenic-rich sulfide–sulfosalt assemblages in this deposit. These provide a unique opportunity to analyze the processes and products of high-temperature volcanic gas expansion in fractures that form the otherwise inaccessible infrastructure deep inside equivalent present-day fumaroles. We provide field emission scanning electron microscope and LA-ICP-MS micro-analytical data for the wide range of heavy, semi-metals and metalloids (arsenic, antimony, bismuth, tin, silver, gold, tellurium and selenium) in the complex pyrite-enargite-Fe-tennantite assemblages from Copper Stage mineralization in the El Indio deposit. These data document the progressive fractionation of antimony and other heavy metals, such as bismuth, during crystallization from a sulfosalt melt that condensed from expanding vapor at about 15 MPa (150 bars) and >650°C following higher temperature vapor deposition of crystalline pyrite and enargite. The sulfosalt melt aggressively corroded the earlier enargite and pyrite and hosts clusters of distinctive euhedral quartz crystals. The crystallizing sulfosalt melt also trapped an abundance of vugs within which heavy metal sulfide and sulfosalt crystals grew together with K-Al silicates and fluorapatite. These data and their geologic context suggest that, in high-temperature fumaroles on modern active volcanoes, over 90% of the arsenic content of the primary magmatic vapor (perhaps 2000 mg kg⁻¹) was precipitated subsurface as sulfosalt. Subsurface fractionation may also account for the range of exotic Pb-Sn-Bi-Se sulfosalts observed in fumarole sublimates on active volcanoes such as Vulcano, Italy, as well as on extra-terrestrial volcanoes such as Maxwell Montes, Venus.

The elemental fluxes and heat flow associated with large aquifer systems can be significant both at local and at regional scales. In fact, large amounts of heat transported by regional groundwater flow can affect the subsurface thermal regime, and the amount of matter discharged towards the surface by large spring systems can be significant relative to the elemental fluxes of surface waters. The Narni-Amelia regional aquifer system (Central Italy) discharges more than 13 m³ sec⁻¹ of groundwater characterised by a slight thermal anomaly, high salinity and high pCO₂. During circulation in the regional aquifer, groundwater reacts with the host rocks (dolostones, limestones and evaporites) and mixes with deep CO₂-rich fluids of mantle origin. These processes transfer large amounts of dissolved substances, in particular carbon dioxide, and a considerable amount of heat towards the surface. Because practically all the water circulating in the Narni-Amelia system is discharged by few large springs (Stifone-Montoro), the mass and energy balance of these springs can give a good estimation of the mass and heat transported from the entire system towards the surface. By means of a detailed mass and balance of the aquifer and considering the soil CO₂ fluxes measured from the main gas emission of the region, we computed a total CO₂ discharge of about 7.8 × 10⁹ mol a⁻¹ for the whole Narni-Amelia system. Finally, considering the enthalpy difference between infiltrating water and water discharged by the springs, we computed an advective heat transfer related to groundwater flow of 410 ± 50 MW.

Element ratios and water stable isotopes reveal the presence of only two independent deep brines in the Kinnarot Basin, Israel: the evaporite dissolution brine of Zemah-1 and the inferred Ha’on mother brine (HMB) with low and high Br/Cl ratios, respectively. HMB is considered to be a representative of the Late Pliocene evaporated Sedom Sea. The freshwater-diluted evaporation brine emerges as Ha’on brine on the eastern shore of Lake Tiberias and is also identified in the pore water of lake sediments. HMB is converted into Tiberias mother brine (TMB) by dolomitization of limestones and alteration of abundant volcanic rocks occurring along the western side of the lake. The Ha’on and Tiberias brines, both characterized by high δD and δ¹⁸O values, are similar in Na/Cl and Br/Cl ratios but are dissimilar in Br/K ratios because these brines were subjected to different degrees of interactions with rocks and sediments. Excepting the brine from KIN 8, all brines from the Tabigha area including the nearby off-shore Barbutim brine are related to the TMB. The brine KIN 8 and all brines from the Fuliya and Hammat Gader areas are related to the HMB. The brine encountered in wildcat borehole Zemah-1 is generated by halite-anhydrite/gypsum dissolution and is independent from the HMB system.

Although characterized by low seismicity, the Monferrato area of north-western Italy was affected by earthquakes, of magnitude M5.1 and M4.8, in 2000 and 2001. At the same time, marked changes were recorded in water temperature and chemistry in several wells within the epicentral area. In May 2004, an automatic network for the continuous monitoring of groundwater was installed in selected wells to study the phenomenon. Here, we report on data collected during a 3-year period of groundwater monitoring. During the first year, episodes of water heating (by up to 20°C) were observed in one monitored well. The temporal analysis of the seismic activity recorded in the area revealed as almost all seismic events occurred during the period of elevated water temperatures. The similar timing of earthquakes and groundwater-temperature anomalies suggests that both may be triggered by the same processes acting in the crust.

All available information relevant to in situ stress orientations and magnitudes in the Western Canadian Sedimentary Basin (WCSB) were examined to provide a better understanding of how regional stress fields may affect geothermal development. The smallest principal stress is horizontal over most of the Western Canadian Sedimentary Basin, and it varies in magnitude across the region. Horizontal stress trajectories show that S_Hmax axes are generally aligned SW–NE. A total of 1643 measurements of microfracture and minifracture closure pressures, leak-off pressures and fracture breakdown pressures have been harnessed to map S_Hmin gradients across the basin at depths of 156–500, 500–1000, 1000–4185 and 2000–4185 m. Vertical stress magnitudes, calculated in 91 wells, showed that at constant depth, S_V increases towards the Canadian Rocky Mountains. Resultant regional stress maps show consistent trends in orientation of stress axes. As a result, predictions can be made that propagation axes of subsurface hydraulic fractures will be dominantly SW–NE, except over the Peace River Arch area, where they will trend more towards SSW–NNE. Engineered geothermal systems in the WCSB can be optimised by drilling horizontal wells parallel to S_Hmin.

The Central Apennines are affected by frequent earthquakes of moderate magnitude that occur mainly within the upper part of the crust at depths of <15 km. A large number of cold gas emissions that are rich in CO₂ are also found in the region. One particular vent with a high rate of degassing was equipped with a sensor to measure flow rates, which were recorded for a number of different periods between 2005 and 2010. Factors that could affect potentially CO₂ flow rates include barometric pressure, atmospheric temperature, precipitation and local seismicity. Our analysis indicates that the periods of anomalous flow rate were related not to the environmental factors but probably to the deformative processes of the crust associated with the local seismicity. Local seismic events as expression of geodynamic processes occurred always before and during these anomalous gas flow periods. This correlation exists only for events that occurred eastwards of the gas emission site close to the Martana fault zone. We herein consider this correlation as indication for a continuous interaction between the field of static strain and the deep fluid pressure. An approximation of the fluid pressure transmission towards the gas emission site gives reasonable values of 1–10 m² sec⁻¹. To make comparisons with the long-term effects of the static strain, we also recorded the short-term effects of the dynamic release of strain induced by the series of strong earthquakes that took place in L’Aquila in 2009. We detected a significant anomalous flow rate that occurred at the same time as this seismic sequence, during which widespread degassing was induced around the focal zone.

Numerical modelling by finite element methods provides two significant insights into the formation of the giant amethyst geodes of the Paraná volcanic province: the conditions needed to open the cavities and the conditions that control their size and shape. Giant amethyst geodes were formed in the Cretaceous (135 Ma) in altered volcanic rocks by water vapour pressure (Δp) at about 0.5 MPa under an altered basalt cover of 5–20 m. Only rocks with Young’s modulus values (E) in the range 1–2 GPa can sustain ballooning, which is the growth of a cavity in a ductile medium by the pressure of water and its vapour. The size of the proto-geode is dependent on the water vapour pressure, which is directly related to thickness of the overlying basalt. Varying the yield points causes the formation of either prolate or oblate cavities. A low transition point (smaller than 0.18 MPa) generates a prolate-shaped cavity, whereas a high transition point (larger than 0.18 MPa) generates oblate proto-geodes. Proto-geodes are smaller when Young’s modulus is higher (rock is less altered) or when water vapour pressure is lower (because of thinner overburden of basalt). The calculations are an indication that the processes operative in the altered basalts led to the opening of giant cavities by ballooning.

This study reviews and synthesizes the results from geochemical and isotopic case studies across Europe, North America, Antarctica, and Greenland to evaluate the effects of Pleistocene glaciation on continental-scale groundwater circulation in sedimentary basins. The most effective studies, in terms of delineating high-resolution records of paleorecharge to aquifers, combine solute chemistry, stable isotopes of water, age tracers, and dissolved noble gases. Some of the lowest δ¹⁸O values (−22‰), and noble gas temperatures (0°C), and high excess air concentrations were found in confined groundwaters in northern Estonia, likely derived from Scandinavian Ice Sheet subglacial recharge. These results are consistent with groundwater systems in North America that were recharged beneath the Laurentide Ice Sheet. Late Pleistocene precipitation may have also been an important source of recharge, as indicated by low-temperature, isotopically enriched groundwaters in several basins. Detectable age gaps have been observed in several aquifer systems in North America and Europe, likely caused by a hiatus of groundwater recharge in areas covered by permafrost during the Last Glacial Maximum (10–21 ka). Aquifers that were not covered by ice sheets or permafrost contain continuous records of Pleistocene to Holocene recharge with variable δ¹⁸O values and low paleotemperatures (4–9°C lower than today). The maximum depth of glacial meltwater penetration into sedimentary basins is approximately 50–1000 m. Infiltration of dilute meltwaters dissolved large quantities of halite in evaporite-bearing basins. The presence of clay-rich glacial deposits and bedrock confining units enhanced the storage of meltwaters within low-permeability sediments and limited flushing of paleowaters in underlying aquifers. These results demonstrate the importance of continental glaciation as a driver for basinal-scale fluid and solute transport and have implications for long-term storage of radioactive waste and carbon dioxide at depth in high-latitude sedimentary basins.

Hydromechanical effects of continental ice sheets may involve considerably more than the widely recognized direct compression of overridden terrains by ice load. Lithospheric flexure, which lags ice advance and retreat, appears capable of causing comparable or greater stress changes. Together, direct and flexural loading may increase fluid pressures by tens of MPa in geologic units unable to drain. If so, fluid pressures in low-permeability formations subject to glaciation may have increased and decreased repeatedly during cycles of Pleistocene glaciation and can again in the future. Being asynchronous and normally oriented, direct and flexural loading presumably cause normal and shear stresses to evolve in a complex fashion through much or all of a glacial cycle. Simulations of fractured rock predict permeability might vary by two to three orders of magnitude under similar stress changes as fractures at different orientations are subjected to changing normal and shear stresses and some become critically stressed. Uncertainties surrounding these processes and their interactions, and the confounding influences of surface hydrologic changes, make it challenging to delineate their effects on groundwater flow and pressure regimes with any specificity. To date, evidence for hydromechanical changes caused by the last glaciation is sparse and inconclusive, comprising a few pressure anomalies attributed to the removal of direct ice load. This may change as more data are gathered, and understanding of relevant processes is refined.

Characterizing glaciotectonic deformation, glacial erosion and sedimentation, and basal hydrologic conditions of ice sheets is vital for selecting sites for nuclear waste repositories at high latitudes. Glaciotectonic deformation is enhanced by excess pore pressures that commonly persist near ice sheet margins. Depths of such deformation can extend locally to a few tens of meters, with depths up to approximately 300 m in exceptional cases. Rates of glacial erosion are highly variable (0.05–15 mm a⁻¹), but rates <1 mm a⁻¹ are expected in tectonically quiescent regions. Total erosion probably not exceeding several tens of meters is expected during a glacial cycle, although locally erosion could be greater. Consolidation of glacial sediments that is less than expected from independent estimates of glacier thickness indicates that heads at the bases of past ice sheets were usually within 30% of the floatation value. This conclusion is reinforced by direct measurements of water pressure beneath portions of the West Antarctic ice sheet, which indicate average heads <7 m below floatation. Landforms of the Laurentide and Scandinavian ice sheets and recent observations in Greenland indicate that high seasonal discharges of surface water are conducted to the bed, despite thick ice at subfreezing temperatures. Therefore, in models of subglacial groundwater flow used to assess sites for nuclear waste repositories, a flux upper boundary condition based on water input from only basal melting will be far more uncertain than applying a hydraulic head at the upper boundary set equal to a large fraction of the floatation value.

This study reviews the state-of-the-art and promising pathways to advance hydrologic models of groundwater flow systems and related transport processes in response to transient glacial loading. We also discuss the utility of hydrologic and geochemical data sets as a means of providing ground truth for these models. The paleohydrologic models presented herein should be used as analogues to assess high-level nuclear waste repository stability in response to future episodes of glaciations in countries such as Canada, Sweden, and Switzerland. The next generation of fully coupled ice-sheet-aquifer models may also be of use in assessing rates of ice sheet denudation on Greenland and Antarctica in response to global warming. However, significant uncertainty exists in paleoclimatic forcing, paleohydrologic boundary conditions, and effective basin-scale petrophysical parameters. Thus, model results must be viewed with some caution. Model results from studies reviewed herein suggest that during the last glacial maximum, recharge rates across glaciated basin margins increased by as much as 2–6 times modern levels. Paleohydrologic models predict that as ice sheets overran sedimentary basin margins, glacial melt water penetrated to depths of up to hundreds of meters. Recent ice-sheet models that incorporated the effects of groundwater flow suggest that the presence of a 1–10 mm film of water at the glacial bed can increase basal ice sliding rates by up to 4 orders of magnitude. No firm theoretical basis exists for coupling ice sheet and subsurface hydrogeologic models nor the effects of permafrost on hydraulic conductivity. These issues could be resolved, to some degree, by additional careful experimental studies. Analysis of fluid pressures and flow rates beneath modern ice sheets using geochemical tracers would help to reduce the uncertainty regarding suitable hydrogeologic boundary conditions, parameterization of poromechanical coupling, and transport processes. Glacial geologists should work closely with modelers to provide better constraints on model boundary conditions.

Regional-scale groundwater flow modeling of the Fennoscandian shield suggests that groundwater flow can be strongly affected by future climate change and glaciation. We considered variable-density groundwater flow in a 1500-km-long and approximately 10-km-deep cross-section through southern Sweden. Groundwater flow and shield brine transport in the cross-sectional model were analyzed under projected surface conditions for the next 140 ka. Simulations suggest that blockage of recharge and discharge by low-permeability permafrost or cold-based ice causes sinking of brine and consequent freshening of near-surface water in areas of natural discharge. Although recharge of basal meltwater is limited by the requirement that water pressure at the base of the ice sheet not exceed the pressure exerted by the weight of the ice, warm-based ice with basal melting creates a potential for groundwater recharge rates much larger than those of present, ice-free conditions. In the simulations, regional-scale redistribution of recharged water by subsurface flow is minor over the duration of a glacial advance (approximately 10 ka). During glacial retreat, significant upward flow of groundwater may occur below the ice sheet owing to pressure release. If the mechanical loading efficiency of the rocks is high, both subsurface penetration of meltwater during glacial advance and up-flow during glacial retreat are reduced because of loading-induced pressure changes. The maximum rate of groundwater discharge in the simulations occurs at the receding ice margin, and some discharge occurs below incursive postglacial seas. Recharge of basal meltwater could decrease the concentration of dissolved solids significantly below present-day levels at depths of up to several kilometers and may bring oxygenated conditions to an otherwise reducing chemical environment for periods exceeding 10 ka.

A deep geologic repository (DGR) for low- and intermediate-level waste has been proposed by Ontario Power Generation for the Bruce nuclear site in the Municipality of Kincardine, Ontario, Canada. As envisioned, the proposed DGR would be constructed at a depth of about 680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. Within the geologic setting of southern Ontario, the Bruce nuclear site is positioned along the eastern flank of the Michigan Basin. The regional-scale domain for the modeling undertaken in support of the DGR program encompasses an area of approximately 18 000 km² and extends to the deepest points in Lake Huron and Georgian Bay. The conceptual model for the ground water system was developed using data from the DGR site characterization program. Hydraulic parameters for the model hydrostratigraphic units were defined using data from the DGR site boreholes and from lab analyses of cores. Borehole data included hydraulic conductivities from straddle-packer hydraulic tests and pressure measurements from the Westbay MP38 and MP55 multilevel groundwater monitoring system. Data from this system indicate that units of the Salina and the Ordovician sediments are under-pressured relative to hydrostatic levels associated with ground surface at the DGR site. The Silurian Niagaran Group is slightly over-pressured while the Cambrian Formation sandstone is significantly over-pressured. The pressure distribution in the sedimentary rock of the Bruce site was analyzed using a hydromechanical model that assumed homogeneous loads and no lateral strain. Layer dependent specific storage coefficients and one-dimensional loading efficiencies were calculated based on testing of core samples. The impact of glaciation and deglaciation on the groundwater system was investigated in paleohydrogeologic scenarios. The model results indicated that basal meltwater does not penetrate vertically below the units of the Salina at the DGR site. A suite of paleohydrogeologic scenarios were investigated in this study. Based on these analyses, glaciation and deglaciation were unable to yield the abnormal pressure patterns observed in the DGR boreholes for the Ordovician formations. Models that included the presence of a gas phase were found to produce under-pressures that are similar to field observations.