Permeable Units Research Articles

Correlation between the parameters of material transfer and granulation indexes in mixtures with varying ratios of concentrate to iron ore fines

The production of high-grade fine material or concentrate continues to increase due to the depletion of high-grade ore deposits. In iron ore granulation, fine materials are involved in producing granules through the coalescence process and auto-layering around coarse particles. The effectiveness of granulation depends on the role of each size fraction of the ore mixture and the level of material transfer between size fractions, and it is measured by different granulation indexes, including maximum permeability, granulation effectiveness (x0.5) and mass fraction of fine material or concentrate. This study focused on the correlation between the level of material transfer between size fractions in granulated mixtures and different granulation indexes. Regular iron ore fines were replaced with 10%, 20%, 30% and 40% concentrate, respectively, to produce granules with an optimum moisture content, determined by the maximum permeability from a pilot-scale JPU (Japanese Permeability Unit) pot. The results indicated that the level of material transfer between size fractions can be used as an alternative index to evaluate the granulation effectiveness of ore mixtures. Adding more concentrate increased the formation of mini-granules that grow slowly, which is associated with a sluggish level of material transfer between size fractions. The growth of large granules was also delayed, leading to a small mean size of granules, low bed permeability and low granulation effectiveness or partition coefficient, x0.5. The coefficients of variance for the material transfer level and maximum permeability were comparable, providing an opportunity to replace the commonly maximum permeability with the level of material transfer in the measurement of the granulation effectiveness.

Bio-Inspired Pressure-Dependent Programmable Mechanical Metamaterial with Self-Sealing Ability.

Self-sealing is one of the fascinating functions in nature that enables living material systems to respond immediately to damage. A prime plant model is Delosperma cooperi, which can rapidly self-seal damaged succulent leaves by systematically deforming until the wound closes. Inspired by this self-sealing principle, a novel programmable mechanical metamaterial has been developed to mimic the underlying damage management concept. This material is able to react autonomously to changes in its physical condition caused by an induced damage. To design this ability into the programmable metamaterial, a permeable unit cell design has been developed that can change size depending on the internal pressure. The parameter space and associated mechanical functionality of the unit cell design is simulated and analyzed under periodic boundary conditions and various pressures. The principles of self-sealing behavior in designed metamaterials are investigated, crack closure efficiency is identified for different crack lengths, the limitations of the proposed approach are discussed, and successful crack closure is experimentally demonstrated in the fabricated metamaterial. Although this study facilitates the first step on the way of integrating new bio-inspired principles in the metamaterials, the results show how programmable mechanical metamaterials might extend materials design space from pure properties to life-likeabilities.

On the Influence of Grain Size Compared with Other Internal Factors Affecting the Permeability of Granular Porous Media: Redefining the Permeability Units

Abstract This study first reviews the influence of grain size on the permeability of porous granular media in comparison to other factors, especially the sorting of grain size distribution, in order to improve the physical knowledge of permeability. The aim of this research is to counter the widespread misconception that the characteristics of water flow in granular porous media can be associated exclusively with an area regarding grain size. This review involves two different aspects. First, the dependence of the intrinsic permeability on the particle size distribution is highlighted, independently of the other internal factors such as porosity and average grain size, by simply reviewing the main existing formulas. Second, the historical literature on the influence of the average grain size in porosity is analyzed, and it is compared with the influence of the granulometric sorting. The most recognized data show that the influence of each of these two factors is of the same order, but it was not expressed in mathematical form, so a relationship of porosity versus average grain size and sorting is established. The two aforementioned steps conclude that the factors influencing permeability do not advise the use of area dimensions because it leads to only link permeability with the average grain size, especially when nonspecialists come into contact with earth sciences. Finally, after a review of the historical evolution of the permeability units, they are redefined to avoid the common misconception that occurs when the established unit leads to only a partial understanding of the key parameters influencing permeability.

Assessing the potential of composite confining systems for secure and long-term CO2 retention in geosequestration

A potential geologic target for CO2 storage should ensure secure containment of injected CO2. Traditionally, this objective has been achieved by targeting reservoirs with overlying seals-regionally extensive, low permeability units that have been proven capable of retaining buoyant fluid accumulations over geologic time. However, considering that the amount of CO2 is limited by a decadal injection period, vertical migration of CO2 can be effectively halted by a composite system of discontinuous shale/silt/mudstone barriers in bedded sedimentary rocks. Here, we studied the impact of depositional architectures in a composite confining system on CO2 migration and confinement at reservoir scale. We stochastically generated lithologically heterogeneous reservoir models containing discontinuous barriers consistent with statistical distributions of net-sand-to-gross-shale ratio (NTG) and horizontal correlation lengths derived from well log data and observations of producing hydrocarbon fields in Southern Louisiana. We then performed an extensive suite of reservoir simulations of CO2 injection and post-injection to evaluate the sensitivity of CO2 plume migration and pressure response of the composite system to a series of geologic and fluid parameters including the lateral continuity of barriers, NTG, permeability anisotropy within the sand body, and capillary pressure contrast between the sand and shale facies. The results indicate that lateral continuity of barriers and NTG are the dominant controls on CO2 plume geometry and pressure build-up in the reservoir, while the impact of NTG is particularly pronounced. The significance of intraformational barriers becomes apparent as they facilitate the local capillary trapping of CO2. Those barriers improve the pore space occupancy by promoting a more dispersed shape of the plume and ultimately retard the buoyancy-driven upward migration of the plume post injection.

Reactive Transport Modelling of Elevated Dissolved Sulphide Concentrations in Sedimentary Basin Rocks

Groundwater with total dissolved sulphide concentrations in excess of 1.0 × 10 − 4 mol L − 1 3 mg L − 1 is relatively common at intermediate depths in sedimentary basins. However, the mechanisms responsible for the formation and spatial distribution of these sulphidic waters in sedimentary basins, which have been affected by periods of glaciation and deglaciation, are not fully understood. Sulphate reduction rates depend on many factors including redox conditions, salinity, temperature, and the presence and abundance of sulphate, organic matter, and sulphate-reducing bacteria. Two-dimensional reactive transport modelling was undertaken to provide potential explanations for the presence and distribution of sulphidic waters in sedimentary basins, partially constrained by field data from the Michigan Basin underlying Southern Ontario, Canada. Simulations were able to generally reproduce the observed depth-dependent distribution of sulphide. Sulphate reduction was most significant at intermediate depths due to anoxic conditions and elevated sulphate concentrations in the presence of organic matter in waters with relatively low salinity. The simulations indicate that glaciation-deglaciation periods increase mixing of waters at this interfacial zone, thereby enhancing rates of sulphate reduction and the formation of sulphide. In addition, the simulations indicate that glaciation-deglaciation cycles do not significantly affect sulphide concentrations in low permeability units, even at shallow depths (e.g., 25 m), while concentrations in permeable units remain stable below depths of 500 m.

Dolomitisation of carbonate platform margins by fault‐controlled geothermal convection: Insights from coupling stratigraphic and reactive transport models

AbstractReactive transport modelling is increasingly deployed to quantitatively evaluate conceptual models of diagenetic processes. However, construction of models of complex systems involves trade‐offs between accuracy and simplification. This tension is explored for models of fault‐associated dolomitisation by sea water convection in a syn‐rift carbonate platform, evaluating the contribution of incorporating stratigraphic growth and fault propagation. Simulations of the high heat flux southern margin of the Derbyshire Platform (Northern England), with heterogeneous matrix permeability that reflects the evolving stratal architecture and burial compaction focusses dolomitisation in more permeable units at all depths. A permeable platform margin fault zone enhances dolomitisation in a broad area on the upper slope and margin, and to a lesser but significant extent, across the interior as platform top waters are entrained and discharge via the fault. Stepwise simulation of flow and reactions during stratigraphic growth suggests that static models over‐predict dolomite abundance in younger sediments and show how regions optimally supplied with reactants and heat to drive dolomite formation migrate vertically and laterally during platform growth. Dolomitisation intensity increases with depth due to greater time for reactions and kinetically favourable temperatures. Adding the fault zone to this model focusses and accelerates flow, giving a more spatially restricted dolostone body and reducing dolomitisation temperature. Changes in fault connectivity with the surface of the evolving platform shift fluid flow pathways and change the rate and temperature of dolomite formation. Results concur with petrographic, isotopic and geochemical observations of the early dolomite on the Derbyshire Platform. This work demonstrates the importance of understanding diagenesis as the product of an evolving set of processes that respond to geological and palaeoenvironmental changes rather than as a sequence of individual diagenetic events. This is particularly critical for reactions, such as dolomitisation by geothermal convection of sea water, which occur over timescales synchronous with platform development.

Assessment of CO2 storage prospect based on physical properties of Rio Bonito Formation rock units

The physical properties of the rock units associated with the Rio Bonito Formation are presented in this study with the focus on modelling reservoir quality based on petrophysics-derived parameters to evaluate CO2 storage potentials. It involves the modelling of the reservoir depths, thicknesses, flow zone indicators (FZI), and effective permeability (Keff) and presenting the CO2 storage efficiency factors peculiar to the rock units of the study location. Research results presented by this study for the stated objectives are not quite common in the region. Keff values range from 200 mD to higher than 2000 mD, and FZI values are generally above 1.0 μm and up to 13.0 μm within the portions covered by the drilled wells. The sandstone units recorded are up to 20 m thick in some cases. The Keff and FZI models indicate the sandstone reservoirs as permeable units to support the injection and circulation of CO2 within the potential reservoir units of the Rio Bonio Formation across São Paulo State. Apart from some points in the southeastern part of the study location, where the Rio Bonito are delineated at depths less than 800 m (minimum CO2 storage depth based on best practices), other portions are deeper, ranging from 950 m to 3500 m. Thin-bedded layers will affect the integrity of the rocks as CO2 storage tanks or reservoir seals/traps/overburden within the region. Sandstone bed thicknesses are up to 20 m in some cases. However, hybrid CO2 reservoir units are feasible, especially in portions where thin siltstone layers are sandwiched between sandstone units to provide considerable thicknesses based on CO2 storage standards. The current study shows that useable areas considering reservoir thickness, depth, and other physical qualities will significantly control the CO2 storage efficiency of the study location. Further studies featuring a detailed geophysical exploration of the site to confirm the availability and saturations of pre-existing fluid (hydrocarbon and water) are encouraged to boost CO2 storage in the region. The related research-based results, as mentioned above, may be combined with the results of this research to determine the area's potentials for CO2 storage or hydrocarbon production with CO2 storage options.

Past and future evolution of the onshore-offshore groundwater system of a carbonate archipelago: The case of the Maltese Islands, central Mediterranean Sea

Offshore freshened groundwater (OFG) is groundwater with a salinity below that of seawater that is stored in sub-seafloor sediments and rocks. OFG has been proposed as an alternative solution to relieve water scarcity in coastal regions and to enhance oil recovery. Although OFG has been documented in most continental margins, we still have a poor understanding of the extent and flow characteristics of OFG systems, and their evolution through time. In view of the general absence of appropriate field data, paleohydrogeological models have been used. The majority of these models are based on 2D approaches, and they rarely consider the future evolution of OFG systems, especially in response to predicted climate change. Here we utilize recently acquired geological, geophysical and hydrogeological data from onshore and offshore the Maltese Islands, and employ 2D and 3D numerical models, to: (i) reconstruct the evolution of the onshore-offshore groundwater system during the last 188 ka, (ii) predict the evolution of the OFG system in response to climate-related changes. We show that the mechanisms emplacing OFG include a combination of active meteoric recharge at present as well as at sea-level lowstands. The Maltese onshore-offshore groundwater system is relatively dynamic, with 23% of groundwater being preserved in the last 18 ka. The control of geology is expressed by the more prevalent distribution of OFG north of the Great Fault, which is associated to the occurrence of low permeability units, and the asymmetry of the groundwater lens during the 18 ka lowstand. A 30% decrease in recharge predicted in the coming 100 years will diminish OFG extent by 38%, whereas sea-level rise will play a negligible role. At present the estimated volume of OFG is 1 km3, which could potentially provide an alternative supply of potable water to the Maltese Islands for 75 years. Exploitation of OFG with minimal salinization of onshore groundwater bodies would require locating pumping wells close to the coast.

The Impact of Ice Sheet Geometry on Meltwater Ingress and Reactive Solute Transport in Sedimentary Basins

AbstractWe investigate the effect of ice sheet geometry on groundwater flow patterns and meltwater ingress in a hypothetical sedimentary basin. The simulation results indicate that meltwater ingress is much greater in 3D domains with a relatively narrow ice sheet extent compared to a wide ice sheet, or a simplified 2D model. In high permeability units (HPUs), the simulated meltwater penetration depth can reach up to 750 m in 3D domains compared to 400 m in a comparable 2D domain. In low permeability units (LPUs), very limited meltwater penetration occurs, indicating that ice sheet geometry and model dimensionality do not substantially affect water flow in these units. A conservative tracer, with a source located at a depth of 500 m in both HPUs and LPUs, illustrates that solutes can be transported to greater depths in HPUs for a narrow ice lobe scenario, in comparison to a wide ice sheet or the 2D approach. Tracer transport in LPUs is unaffected by ice sheet geometry. Similarly, simulation results indicate that the ingress of dissolved oxygen (O2) into HPUs is most substantial below a narrow ice lobe, while O2 ingress into LPUs is not affected by ice sheet geometry. The numerical experiments indicate that 3D analysis will give more comprehensive results for flow patterns and reactive solute transport subjected to glaciation/deglaciation cycles in the case of a narrow ice lobe, but also suggest that a 2D approach might provide an adequate representation for the case of a relatively wide ice sheet.

Determining effective diffusion coefficients of chlorohydrocarbons in natural clays: Unique results from highly resolved controlled release field experiments

This study aims to precisely determine the effective diffusion coefficients of chlorohydrocarbons in low permeable units under in-situ field conditions. To this end, two controlled release field experiments using TCE and PCE as dense non-aqueous phase liquids (DNAPLs) were conducted in two natural clayey deposits. Several months to years after the controlled DNAPL release, highly resolved concentration profiles were determined for the chlorohydrocarbons that had diffused into the clayey deposits. Effective diffusion coefficients for TCE and PCE were then determined by calibrating a 3D numerical and 1D analytical model, respectively, to the measured high-resolution concentration profiles. The simulations revealed that the effective diffusion coefficients vary by as much as a factor of four within the same low permeability unit being consistent with observed small-scale heterogeneities. The determined chlorohydrocarbon effective diffusion coefficients were further used to determine the equivalent thickness of DNAPL that would completely dissolve in an idealized, parallel-plate fracture by diffusion transport into clayey deposits for the time periods of the controlled release field experiments. The equivalent TCE and PCE DNAPL film thicknesses ranged between 36 and 581 μm, respectively, comparable and exceeding fracture apertures measured in naturally fractured clay rich deposits. Hence, films of DNAPL initially contained within fractures in clay-rich deposits can completely dissolve away within a few months to a few years due to diffusion. This stored contaminant mass poses a risk to adjacent aquifers if it is re-released due to diffusion out of the matrix after source depletion or remediation.

The influence of alluvial stratigraphic architecture on liquefaction phenomena: A case study from the Terre del Reno subsoil (southern Po plain, Italy)

Earthquake-induced liquefaction phenomena are commonly observed at shallow depths or at the ground surface above Holocene alluvial deposits. Nonetheless, a high-resolution examination of the relationship between surface manifestations and alluvial stratigraphy has been rarely attempted. In this study, we provide a detailed subsurface model from a selected sector of the southern Po Plain affected by widespread liquefaction processes triggered by the 2012 Emilia-Romagna seismic crisis. We report a comprehensive sedimentological and CPT-based characterization of the Terre del Reno subsoil and investigate how the stratigraphy controls earthquake-induced liquefaction phenomena. The main facies associations making up the alluvial subsoil constitute the basic elements of the liquefaction system, herein named host, source and sedimentary cap. Their subsurface distribution documents the presence of fluvial bodies prone to liquefaction beneath the earthen levees and across an area dominated by flat muddy floodplain deposits with no physical evidence of paleochannels at the surface. In addition, our observations suggest that vertical and lateral changes in stratigraphy control the mode and location of liquefaction evidence at the surface in both settings. In particular, the relative thickness of fluvial channel bodies vs. channel levee deposits (source–seal couplet) appears to control the severity of sand ejection at the surface. Our findings also suggest that vertical and lateral discontinuities within source layers prevent the occurrence of widespread liquefaction features. Conversely, lateral spreading effects and differential underground hydrostatic pressure at the interface between saturated and less permeable units are accountable for repeated sand ejection along surface fractures. This case study constitutes a unique natural laboratory thanks to the quality and density of available data. However, we emphasize that a similar approach can be useful to promote source-layer identification in areas characterized by poor or sparse data and to mitigate liquefaction hazards in urban areas built on alluvial plains.

Subsurface permeability contrasts control shallow groundwater flow dynamics in the critical zone of a glaciated, headwater catchment

AbstractGroundwater flow direction within the critical zone of headwater catchments is often assumed to mimic land surface topographic gradients. However, groundwater hydraulic gradients are also influenced by subsurface permeability contrasts, which can result in variability in flow direction and magnitude. In this study, we investigated the relationship between shallow groundwater flow direction, surface topography, and the subsurface topography of low permeability units in a headwater catchment at the Hubbard Brook Experimental Forest (HBEF), NH. We continuously monitored shallow groundwater levels in the solum throughout several seasons in a well network (20 wells of 0.18–1.1 m depth) within the upper hillslopes of Watershed 3 of the HBEF. Water levels were also monitored in four deeper wells, screened from 2.4 to 6.9 m depth within glacial drift of the C horizon. We conducted slug tests across the well network to determine the saturated hydraulic conductivity (Ksat) of the materials surrounding each well. Results showed that under higher water table regimes, groundwater flow direction mimics surface topography, but under lower water table regimes, flow direction can deviate as much as 56 degrees from surface topography. Under these lower water table conditions, groundwater flow direction instead followed the topography of the top of the C horizon. The interquartile range of Ksat within the C horizon was two orders of magnitude lower than within the solum. Overall, our results suggest that the land surface topography and the top of the C horizon acted as end members defining the upper and lower bounds of flow direction variability. This suggests that temporal dynamics of groundwater flow direction should be considered when calculating hydrologic fluxes in critical zone and runoff generation studies of headwater catchments that are underlain by glacial drift.

Urban geology from a GIS-based geotechnical system: a case study in a medium-sized city (Oviedo, NW Spain)

This paper describes the development of a GIS-based geotechnical system designed to face challenges on urban geology in a Spanish mid-sized city. Its multipurpose nature is based on a relational database that holds a wide variety of georeferenced ground data, mostly extracted from geotechnical reports or acquired during fieldwork. At present it includes, among other unpublished information, more than 2000 site investigations and the results from 5000 tests carried out on rock, soil and groundwater samples. This desk tool provides a better understanding of the bedrock and superficial geology through the spatial analysis of the collected subsurface data. The main achievements include the classification and mapping of man-made grounds, fluvial sediments and residual soils; the identification of unreported faults; the review and detailed study of geotechnical parameters and properties of the rocks and soils; and the hydrogeological characterization of the permeable units. It also provided the surface geological mapping of the urban area; the development of a geo-engineering map based on lithological, geotechnical and construction criteria; and the creation of a preliminary 3D layer-based ground model of the city centre, where the subsurface contains stone used for heritage buildings listed by UNESCO.

Meteorite impact craters as hotspots for mineral resources and energy fuels: A global review

The ever-increasing recovery rate of natural resources from terrestrial impact craters over the last few decades across the globe offers new avenues for further exploration of mineral and hydrocarbon resources in such settings. As of today, 60 of the 208 terrestrial craters have been identified to host diverse resources such as hydrocarbons, metals and construction materials. The potential of craters as plausible resource contributors to the energy sector is therefore, worthy of consideration, as 42 (70%) of the 60 craters host energy resources such as oil, gas, coal, uranium, mercury, critical and major minerals as well as hydropower resources. Among others, 19 craters are of well-developed hydrocarbon reserves. Mineral deposits associated with craters are also classified similar to other mineral resources such as progenetic, syngenetic and epigenetic sources. Of these, the progenetic and syngenetic mineralization are confined to the early and late excavation stage of impact crater evolution, respectively, whereas epigenetic deposits are formed during and after the modification stage of crater formation. Thus, progenetic and syngenetic mineral deposits (like Fe, Ni, Pb, Zn and Cu) associated with craters are formed as a direct result of the impact event, whereas epigenetic deposits (e.g. hydrocarbon) are hosted by the impact structure and result from post-impact processes. In the progenetic and syngenetic deposits, the shock-wave induced fracturing and melting aid the formation of deposits, whereas in the epigenetic deposits, the highly fractured lithostratigraphic units of higher porosity and permeability, like the central elevated area (CEA) or the rim, act as traps. In this review, we provide a holistic view of the mineral and energy resources associated with impact craters, and use some of the remote sensing techniques to identify the mineral deposits as supplemented by a schematic model of the types of deposits formed during cratering process.

Automated approach to reservoir zonation: A case study from the Upper Permian Dalan (Khuff) carbonate ramp, Persian Gulf

Reservoir zonation is one of the fundamental processes conducted on permeability-porosity core data to classify a reservoir thickness based on its productivity potential. In all the methods currently used, human decision plays a prominent role at every stage, from basic reservoir rock clustering to final flow zone determination. This study aims to reduce the often biased human decision-making role at each stage of the reservoir zonation by utilizing simple mathematical algorithms, and enhance the reservoir rock clustering approach. To do so, we propose the application of the elbow algorithm as a straightforward yet effective method to optimize the cluster numbers, and the Pruned Exact Linear Time (PELT) algorithm to effectively determine the zones with identical fluid flow potential (in this paper regarded as Hydraulic Flow Unit – HFU). Both the elbow and PELT algorithms are employed to effectively classify reservoir thickness based on the commonly used flow zone indicator (FZI), reservoir quality index (RQI), and flow unit speed (FUS). The efficiency of the suggested mathematical algorithms was tested against the results of the sedimentological and petrophysical study of the Upper Dalan (Khuff) gas reservoir, the Persian Gulf. Three lithofacies associations (sabkha and tidal flat, lagoon, and shoal) with 11 distinctive lithofacies plus crystalline dolomite have been identified and the major diagenetic processes influencing their petrophysical characteristics have been assessed. Based on the automated approach, the studied Upper Dalan succession can be subdivided into five HFUs with consistent poroperm values and distinct lithofacies and diagenetic modifications: barrier unit (HFU 1; permeability (k) < 0.1 mD), baffle unit (HFU2-3; mean k < 0.1 mD), normal unit (HFU4; k = 10–100 mD, mean k 17 mD), permeable unit (HFU5-6; k = 10–100 mD), and highly-permeable unit (HFU7; k >100 mD). The study suggests that the automated approach may be an effective alternative to a conventional reservoir zonation of similar mixed carbonate-evaporite reservoirs.

Characterization of flow units, rock and pore types for Mishrif Reservoir in West Qurna oilfield, Southern Iraq by using lithofacies data

This study has been accomplished by testing three different models to determine rocks type, pore throat radius, and flow units for Mishrif Formation in West Qurna oilfield in Southern Iraq based on Mishrif full diameter cores from 20 wells. The three models that were used in this study were Lucia rocks type classification, Winland plot was utilized to determine the pore throat radius depending on the mercury injection test (r35), and (FZI) concepts to identify flow units which enabled us to recognize the differences between Mishrif units in these three categories. The study of pore characteristics is very significant in reservoir evaluation. It controls the storage mechanism and reservoir fluid properties of the permeable units while pore structure is a critical controlling factor for the petrophysical properties and multiphase-flow characteristics in reservoir rocks. Flow zone indicator (FZI) has been used to identify the hydraulic flow units approach (HFUs). Each (HFU) was reproduced by certain FZI and was supposed to have similar geological and petrophysical properties. The samples were from four lithofacies, mA, CRII, mB1, and mB2. Because of the wide range of cored-wells samples (20 wells), this paper is updated the previous studies and indicated some differences in the resulting categories. It was noticed as results of this study that the rocks types of the lower Mishrif were mostly ranged from wackestone to packstone in the upper part of mB2 which reflected mid-ramp facies while the upper part of mB2 referred to shoal facies and for the mB1 unit the rocks types mostly range from packstone to grainstone with some points as wackestone marked as shoal and rudist bioherm facies. Grainstone relatively decreases with the increasing of depth from upper to lower Mishrif while wackestone and packstone indicated increasing in the same direction. The unit mA is marked as mesopores and macropores, while megapores and macropores feature increased in mB1 which has been noticed in the northern part of West Qurna oilfield due to increasing shoal and rudist bioherm facies, the mB2 unit revealed increasing in mesoporous and decreasing in megaporous. The upper Mishrif (mA) has three flow units, while the lower Mishrif (mB1, mB2) has eight flow units four for each reservoir unit.

Including Vertical Fault Structures in Layered Groundwater Flow Models.

Accurate representation of groundwater flow and solute transport requires a sound representation of the underlying geometry of aquifers. Faults can have a significant influence on the structure and connectivity of aquifers, which may allow permeable units to connect, and aquifers to seal when juxtaposed against lower permeability units. Robust representation of groundwater flow around faults remains challenging despite the significance of faults for flow and transport. We present a methodology for the inclusion of faults utilizing the unstructured grid features of MODFLOW-USG and MODFLOW 6. The method focuses on the representation of fault geometries using non-neighbor connections between juxtaposed layers. We present an illustration of the method for a synthetic fluvial aquifer. The combined impact of the heterogeneous aquifer and fault offset is clearly visible where channel features at different depths in the aquifer were connected at the fault. These results highlight the importance of representing fault features in groundwater flow models.

Electrical Resistivity Anomalies Offshore a Carbonate Coastline: Evidence for Freshened Groundwater?

AbstractCarbonate lithologies host considerable quantities of the Earth's freshwater resources and partially supply a quarter of the global population with drinkable water. In addition, carbonates constitute substantial amounts of the global coastlines, yet it is not known if and how they can sustain freshened groundwater offshore. Here, we use controlled‐source electromagnetic, seismic reflection, and core sample data to derive a lithological model for the eastern margin of the Maltese Islands and identify four distinct resistivity anomalies within the Upper Coralline and Globigerina Limestone formations. The anomalies hosted in the former are likely associated with low porosities, whereas the anomaly within the latter is indicative of pore fluid freshening. Hydrogeological modeling suggests that freshened pore fluids, emplaced during sea‐level lowstands and preserved in low permeability units, are potentially still found within carbonate shelves. However, resource potential is low due to its relict nature and low permeability host environment.

Matangkaka manganese deposit, Ambitle Island, Feni Island Group, Papua New Guinea: a Quaternary epithermal stratabound manganese oxide deposit

The Matangkaka manganese deposit is an unusual Quaternary epithermal stratabound manganese oxide deposit from Ambitle Island in the Feni Island Group, New Ireland Province, Papua New Guinea. Ambitle is a small volcanic island that has undergone Quaternary explosive and effusive volcanic activity. Oldest volcanic rocks on the island consist of mafic–intermediate lavas and epiclastic rocks of the Pleistocene Ambitle Volcano (Waramung Volcanics). These rocks rest unconformably on Oligocene calcilutites of the New Ireland Basin. The summit of Ambitle Volcano is now modified as a semi-circular topographic rim around Nanum Valley, known as the Nanum Valley Crater. This crater formed during gravity induced failure (or sector collapse) of the southwest flanks of the summit of Ambitle Volcano. Ambitle Crater is a resurgent Pleistocene–Holocene crater located within the summit crater of Ambitle Volcano. Deposits of resurgent volcanism include block and ash deposits, mafic–intermediate lavas, trachyte lavas, tephra and lacustrine sediments (Babamis Volcanics). Two new members are described from the Babamis Volcanics (Nanmikit Trachyte Member and Danmagal Tephra Member). The tephra member is the youngest deposit in the Plio-Pleistocene Tabar–Lihir–Tanga–Feni alkalic volcanic arc. Three new intrusive units are described (Kabang Syenite, Matangkaka Monzonite and Saddle Monzonite). The Saddle hydrothermal system (new name) is an active geothermal system, elongated along the arc-parallel northwest-trending Niffin structural zone. The Matangkaka occurrence is classified as a Mn–Fe–Cu–Mo deposit. Active northwest-trending structures and northeast-trending faults allowed sea water to circulate to depth, leach metals from basement and overlying volcaniclastic rocks and ascend to shallow levels. Manganese mineralisation was deposited in shallow marine conditions, focussed in porous and permeable volcaniclastic units and forming 3–4 m-thick bedded deposits. Circulating neutral chloride waters of the Saddle hydrothermal system played a key role in the deposition of manganese in porous and permeable volcaniclastic units. Mineralising fluids were probably 120 °C. KEY POINTS The Matangkaka manganese deposit is an unusual Quaternary epithermal stratabound manganese oxide deposit on Ambitle Island in the Feni Island Group, Tabar–Lihir–Tanga–Feni alkalic volcanic arc, New Ireland Province, Papua New Guinea. Ambitle Island is a small volcanic island that has undergone Pleistocene–Holocene explosive and effusive volcanic activity. Active fractures have allowed sea water to circulate to depth, leach metals from basement and overlying volcaniclastic rocks and convect to shallow levels. Manganese mineralisation was deposited by neutral chloride waters, probably 120°C. Y/Ho ratios indicate there was a sea water component to mineralising fluids. The manganese occurrence is classified as a Mn–Fe–Cu–Mo deposit. Manganese mineralisation was deposited in shallow marine conditions, focussed in porous and permeable volcaniclastic units, and forming 3–4 m-thick bedded deposits.

Assessing Stream-Aquifer Connectivity in a Coastal California Watershed

We report the results of field and laboratory investigations of stream-aquifer interactions in a watershed along the California coast to assess the impact of groundwater pumping for irrigation on stream flows. The methods used include subsurface sediment sampling using direct-push drilling, laboratory permeability and particle size analyses of sediment, piezometer installation and instrumentation, stream discharge and stage monitoring, pumping tests for aquifer characterization, resistivity surveys, and long-term passive monitoring of stream stage and groundwater levels. Spectral analysis of long-term water level data was used to assess correlation between stream and groundwater level time series data. The investigations revealed the presence of a thin low permeability silt-clay aquitard unit between the main aquifer and the stream. This suggested a three layer conceptual model of the subsurface comprising unconfined and confined aquifers separated by an aquitard layer. This was broadly confirmed by resistivity surveys and pumping tests, the latter of which indicated the occurrence of leakage across the aquitard. The aquitard was determined to be 2–3 orders of magnitude less permeable than the aquifer, which is indicative of weak stream-aquifer connectivity and was confirmed by spectral analysis of stream-aquifer water level time series. The results illustrate the importance of site-specific investigations and suggest that even in systems where the stream is not in direct hydraulic contact with the producing aquifer, long-term stream depletion can occur due to leakage across low permeability units. This has implications for management of stream flows, groundwater abstraction, and water resources management during prolonged periods of drought.