Hydraulic Head Gradients Research Articles

Probabilistic Forecast of Single‐Phase Flow in Porous Media With Uncertain Properties

AbstractUncertainty about geologic makeup and properties of the subsurface renders inadequate a unique quantitative prediction of flow and transport. Instead, multiple alternative scenarios have to be explored within the probabilistic framework, typically by means of Monte Carlo simulations (MCS). These can be computationally expensive, and often prohibitively so, especially when the goal is to compute the tails of a distribution, that is, probabilities of rare events, which are necessary for risk assessment and decision making under uncertainty. We deploy the method of distributions to derive a deterministic equation for the cumulative distribution function (CDF) of hydraulic head in an aquifer with uncertain (random) hydraulic conductivity. The CDF equation relies on a self‐consistent closure approximation, which ensures that the resulting CDF of hydraulic head has the same mean and variance as those computed with statistical moment equations. We conduct a series of numerical experiments dealing with steady‐state two‐dimensional flow driven by either a natural hydraulic head gradient or a pumping well. These experiments reveal that the CDF method remains accurate and robust for highly heterogeneous formations with the variance of log conductivity as large as five. For the same accuracy, it is also up to four orders of magnitude faster than MCS in computing hydraulic head with a required degree of confidence (probability).

Error in hydraulic head and gradient time-series measurements: a quantitative appraisal

Abstract. Hydraulic head and gradient measurements underpin practically all investigations in hydrogeology. There is sufficient information in the literature to suggest that head measurement errors can impede the reliable detection of flow directions and significantly increase the uncertainty of groundwater flow rate calculations. Yet educational textbooks contain limited content regarding measurement techniques, and studies rarely report on measurement errors. The objective of our study is to review currently accepted standard operating procedures in hydrological research and to determine the smallest head gradients that can be resolved. To this aim, we first systematically investigate the systematic and random measurement errors involved in collecting time-series information on hydraulic head at a given location: (1) geospatial position, (2) point of head, (3) depth to water, and (4) water level time series. Then, by propagating the random errors, we find that with current standard practice, horizontal head gradients <10-4 are resolvable at distances ⪆170 m. Further, it takes extraordinary effort to measure hydraulic head gradients <10-3 over distances <10 m. In reality, accuracy will be worse than our theoretical estimates because of the many possible systematic errors. Regional flow on a scale of kilometres or more can be inferred with current best-practice methods, but processes such as vertical flow within an aquifer cannot be determined until more accurate and precise measurement methods are developed. Finally, we offer a concise set of recommendations for water level, hydraulic head and gradient time-series measurements. We anticipate that our work contributes to progressing the quality of head time-series data in the hydrogeological sciences and provides a starting point for the development of universal measurement protocols for water level data collection.

Vertically Integrated Hydraulic Conductivity: A New Parameter for Groundwater-Surface Water Analysis.

Numerical modeling of groundwater-surface water interactions provides vital information necessary for determining the extent of nutrient transport, quantifying water budgets, and delineating zones of ecological support. The hydrologic data that drive these models are often collected at disparate scales and subsequently incorporated into numerical models through upscaling techniques such as piecewise constancy or geostatistical methods. However, these techniques either use basic interpolation methods, which often simplifies the system of interest, or utilize complex statistical methods that are computationally expensive, time consuming, and generate complex subsurface configurations. We propose a bulk parameter termed "vertically integrated hydraulic conductivity" (KV ), and defined as the depth-integrated resistance to fluid flow sensed at the groundwater-surface water interface, as an alternative to hydraulic conductivity when investigating vertical fluxes across the groundwater-surface water interface. This bulk parameter replaces complex subsurface configurations in situations dominated by vertical fluxes and where heterogeneity is not of primary importance. To demonstrate the utility of KV , we extracted synthetic temperature time series data from a forward numerical model under a variety of scenarios and used those data to quantify vertical fluxes using the amplitude ratio method. These quantified vertical fluxes and the applied hydraulic head gradient were subsequently input into Darcy's Law and used to quantify KV . This KV was then directly compared to the equivalent hydraulic conductivity (KT ) assuming an infinitely extending layer. Vertically integrated hydraulic conductivity allows for more accurate and robust flow modeling across the groundwater-surface water interface in instances where complex heterogeneities are not of primary concern.

An overview of fault zone permeabilities and groundwater level steps in the Roer Valley Rift System

AbstractFaults in the Roer Valley Rift System (RVRS) act as barriers to horizontal groundwater flow. This causes steep cross-fault groundwater level steps (hydraulic head differences). An overview of the size and distribution of fault-related groundwater level steps and associated fault zone permeabilities is thus far lacking. Such an overview would provide useful insights for nature restoration projects in areas with shallow groundwater levels (wijstgronden) on the foot wall of fault zones. In this review study, data on fault zone permeabilities and cross-fault hydraulic head differences were compiled from 39 sources of information, consisting of literature (starting from 1948), internal reports (e.g. from research institutes and drinking water companies), groundwater models, a geological database and new fieldwork. The data are unevenly distributed across the RVRS. Three-quarters of the data sources are related to the Peel Boundary Fault zone (PBFZ). This bias is probably caused by the visibility of fault scarps and fault-adjacent wet areas for the PBFZ, with the characteristic iron-rich groundwater seepage. Most data demonstrate a cross-fault phreatic groundwater level step of 1.0 to 2.5 m. Data for the Feldbiss Fault zone (FFZ) show phreatic cross-fault hydraulic head differences of 1.0 to 2.0 m.In situmeasured hydraulic conductivity data (K) are scarce. Values vary over three orders of magnitude, from 0.013 to 22.1 m d−1, are non-directional and do not take into account heterogeneity caused by fault zones. The hydraulic conductivity (and hydraulic resistance) values used in three different groundwater models are obtained by calibration using field measurements. They also cover a large range, from 0.001 to 32 m d−1and from 5 to 100,000 days. Heterogeneity is also not taken into account in these models. The overview only revealed locations with a clear cross-fault groundwater level step, and at many locations the faults are visible on aerial photographs as cropmarks or as soil moisture contrasts at the surface. Therefore, it seems likely that all faults have a reduced permeability, which determines the size of the groundwater level steps. In addition, our results show that cross-fault hydraulic head gradients also correlate with topographic, fault-induced offsets, for both the Peel Boundary and the Feldbiss fault zone.

Tomographic Reservoir Imaging with DNA-Labeled Silica Nanotracers: The First Field Validation.

This study presents the first field validation of using DNA-labeled silica nanoparticles as tracers to image subsurface reservoirs by travel time based tomography. During a field campaign in Switzerland, we performed short-pulse tracer tests under a forced hydraulic head gradient to conduct a multisource-multireceiver tracer test and tomographic inversion, determining the two-dimensional hydraulic conductivity field between two vertical wells. Together with three traditional solute dye tracers, we injected spherical silica nanotracers, encoded with synthetic DNA molecules, which are protected by a silica layer against damage due to chemicals, microorganisms, and enzymes. Temporal moment analyses of the recorded tracer concentration breakthrough curves (BTCs) indicate higher mass recovery, less mean residence time, and smaller dispersion of the DNA-labeled nanotracers, compared to solute dye tracers. Importantly, travel time based tomography, using nanotracer BTCs, yields a satisfactory hydraulic conductivity tomogram, validated by the dye tracer results and previous field investigations. These advantages of DNA-labeled nanotracers, in comparison to traditional solute dye tracers, make them well-suited for tomographic reservoir characterizations in fields such as hydrogeology, petroleum engineering, and geothermal energy, particularly with respect to resolving preferential flow paths or the heterogeneity of contact surfaces or by enabling source zone characterizations of dense nonaqueous phase liquids.

Density effects on nanoparticle transport in the hyporheic zone

Abstract A carbon solution composed of nanoparticles (d50 = 85.7 nm) was used in experiments designed to explore nanoparticle transport characteristics within the hyporheic zone of a riverbed. Experiments and numerical simulations demonstrated that nanoparticle transport in the hyporheic zone is affected by hydraulic head gradients due to river flow-bedform interactions as well as density gradients associated with the nano-carbon solution. Differences with similar flow/transport situations were examined, and it was found that particulate-enhanced density can change hyporheic transport appreciably. In addition to density, particle settling enhances downward movement of the nano-carbon plume in the riverbed. While nanoparticle transport in the upper hyporheic zone is largely controlled by advection due to flow driven by head gradients at the bed surface, density gradients and particle settling influence the transport process significantly in the lower hyporheic zone. During the transport process, nanoparticles become deposited due to attachment to sand particles and filtration by small pores in the bed. Compared with transport where density variations are minimal, the particulate-induced density gradient induces downward transport of nanoparticles and entrained liquids, leading to deposition/accumulation at the base of the bed.

A multidisciplinary-based conceptual model of a fractured sedimentary bedrock aquitard: improved prediction of aquitard integrity

A hydrogeologic conceptual model that improves understanding of variability in aquitard integrity is presented for a fractured sedimentary bedrock unit in the Cambrian-Ordovician aquifer system of midcontinent North America. The model is derived from multiple studies on the siliciclastic St. Lawrence Formation and adjacent strata across a range of scales and geologic conditions. These studies employed multidisciplinary techniques including borehole flowmeter logging, high-resolution depth-discrete multilevel well monitoring, fracture stratigraphy, fluorescent dye tracing, and three-dimensional (3D) distribution of anthropogenic tracers regionally. The paper documents a bulk aquitard that is highly anisotropic because of poor connectivity of vertical fractures across matrix with low permeability, but with ubiquitous bed parallel partings. The partings provide high bulk horizontal hydraulic conductivity, analogous to aquifers in the system, while multiple preferential termination horizons of vertical fractures serve as discrete low vertical hydraulic conductivity intervals inhibiting vertical flow. The aquitard has substantial variability in its ability to protect underlying groundwater from contamination. Across widespread areas where the aquitard is deeply buried by younger bedrock, preferential termination horizons provide for high aquitard integrity (i.e. protection). Protection is diminished close to incised valleys where stress release and weathering has enhanced secondary pore development, including better connection of fractures across these horizons. These conditions, along with higher hydraulic head gradients in the same areas and more complex 3D flow where the aquitard is variably incised, allow for more substantial transport to deeper aquifers. The conceptual model likely applies to other fractured sedimentary bedrock aquitards within and outside of this region.

Modeling the Impact of Stream Discharge Events on Riparian Solute Dynamics.

The biogeochemical composition of stream water and the surrounding riparian water is mainly defined by the exchange of water and solutes between the stream and the riparian zone. Short-term fluctuations in near stream hydraulic head gradients (e.g., during stream flow events) can significantly influence the extent and rate of exchange processes. In this study, we simulate exchanges between streams and their riparian zone driven by stream stage fluctuations during single stream discharge events of varying peak height and duration. Simulated results show that strong stream flow events can trigger solute mobilization in riparian soils and subsequent export to the stream. The timing and amount of solute export is linked to the shape of the discharge event. Higher peaks and increased durations significantly enhance solute export, however, peak height is found to be the dominant control for overall mass export. Mobilized solutes are transported to the stream in two stages (1) by return flow of stream water that was stored in the riparian zone during the event and (2) by vertical movement to the groundwater under gravity drainage from the unsaturated parts of the riparian zone, which lasts for significantly longer time (> 400 days) resulting in long tailing of bank outflows and solute mass outfluxes. We conclude that strong stream discharge events can mobilize and transport solutes from near stream riparian soils into the stream. The impact of short-term stream discharge variations on solute exchange may last for long times after the flow event.

Application of new point measurement device to quantify groundwater-surface water interactions

The streambed point velocity probe (SBPVP) measures in situ groundwater velocities at the groundwater-surface water interface without reliance on hydraulic conductivity, porosity, or hydraulic gradient information. The tool operates on the basis of a mini-tracer test that occurs on the probe surface. The SBPVP was used in a meander of the Grindsted Å (stream), Denmark, to determine the distribution of flow through the streambed. These data were used to calculate the contaminant mass discharge of chlorinated ethenes into the stream. SBPVP data were compared with velocities estimated from hydraulic head and temperature gradient data collected at similar scales. Spatial relationships of water flow through the streambed were found to be similar by all three methods, and indicated a heterogeneous pattern of groundwater-surface water exchange. The magnitudes of estimated flow varied to a greater degree. It was found that pollutants enter the stream in localized regions of high flow which do not always correspond to the locations of highest pollutant concentration. The results show the combined influence of flow and concentration on contaminant discharge and illustrate the advantages of adopting a flux-based approach to risk assessment at the groundwater-surface water interface. Chlorinated ethene mass discharges, expressed in PCE equivalents, were determined to be up to 444 kg/yr (with SBPVP data) which compared well with independent estimates of mass discharge up to 438 kg/yr (with mini-piezometer data from the streambed) and up to 372 kg/yr crossing a control plane on the streambank (as determined in a previous, independent study).

Effect of Heterogeneous Sediment Distributions on Hyporheic Flow in Physical and Numerical Models.

Variations in permeability have been found to significantly affect the flow of water though hyporheic systems, especially in regions with discontinuous transitions between distinct streambed lithologies. In this study, we probabilistically arranged two sediments (sand and sandy gravel) in a grid framework and imposed a single hyporheic flow cell across the grid to investigate how discontinuous permeability fields influence volumetric flow and residence time distributions. We used both a physical system and computer simulations to model flow through this sediment grid. A solution of blue dye and salt was pumped into the system and used to detect flow. We recorded the dye location using time-lapse photography and measured the electrolytic conductivity levels as the water exited the system as a proxy for salt concentration. We also used a computer simulation to calculate dye-fronts, residence times, and exiting salt concentrations for the modeled system. Comparison between simulations and physical measurements yielded strong agreement. In further simulations with 300 different grids, we found a strong correlation between volumetric flow rate and the placement of high permeability grid cells in regions of high hydraulic head gradients. One implication is that small anomalies in streambed permeability have a disproportionately large influence on hyporheic flows when located near steep head gradients such as steps. We also used moving averages with varying window sizes to investigate the effect of the abruptness of transitions between sediment types. We found that smoother permeability fields increased the volumetric flow rate and decreased the median residence times.

Design of Remediation Actions for Nutrient Mitigation in the Hyporheic Zone

AbstractAlthough hyporheic exchange has been shown to be of great importance for the overall water quality of streams, it is rarely considered quantitatively in stream remediation projects. A main driver of hyporheic exchange is the hydraulic head fluctuation along the streambed, which can be enhanced by modifications of the streambed topography. Here we present an analytical 2‐D spectral subsurface flow model to estimate the hyporheic exchange associated with streambed topographies over a wide range of spatial scales; a model that was validated using tracer‐test‐results and measurements of hydraulic conductivity. Specifically, engineered steps in the stream were shown to induce a larger hyporheic exchange velocity and shorter hyporheic residence times compared to the observed topography in Tullstorps Brook, Sweden. Hyporheic properties were used to parameterize a longitudinal transport model that accounted for reactions in terms of first‐order decay and instantaneous adsorption. Theoretical analyses of the mitigation effect for nitrate due to denitrification in the hyporheic zone show that there is a Damköhler number of the hyporheic zone, associated with several different stream geomorphologies, that optimizes nitrate mass removal on stream reach scale. This optimum can be limited by the available hydraulic head gradient given by the slope of the stream and the geological constraints of the streambed. The model illustrates the complex interactions between design strategies for nutrient mitigation, hyporheic flow patterns, and stream biogeochemistry and highlights the importance to diagnose a stream prior remediation, specifically to evaluate if remediation targets are transport or reaction controlled.

LOMOS-mini: A coupled system quantifying transient water and heat exchanges in streambeds

The LOMOS-mini is a new experimental system monitoring transient water and heat fluxes in the hyporheic zone at a sub-daily resolution. Given the coupling of high-frequency hydraulic head gradient and temperature measurements, its innovation when compared to currently available monitoring systems is to operate under transient conditions and to allow the quantification of coupled water and heat exchanges with an unprecented fine temporal resolution, such as the rainfall event time scale. The LOMOS-mini is low-cost, easy to construct from individual microelectronic components and has an autonomy of several months in the field. Its robust implementation protocol makes it suitable for a wide variety of geological environments, from soft loamy, sandy, clayey streambeds to compact colluvial environments. Hydraulic head gradients are measured using the pressure sensor technology presented in Greswell et al. (2009). The sensibility of pressure estimates to ambient temperature is demonstrated; the measurement error is evaluated to decrease by up to one order of magnitude when accounting for temperature in the calibration relationship. Coupled with a physically-based hydrothermal model, the LOMOS-mini provides transient estimates of vertically distributed Darcy fluxes, conductive and advective heat fluxes. The methodology is illustrated on a colluvial streambed case study.

Bidirectional stream–groundwater flow in response to ephemeral and intermittent streamflow and groundwater seasonality

AbstractIt is often assumed that the net groundwater flow direction is towards the channel in headwater streams in humid climates, with magnitudes dependent on flow state. However, studies that characterize stream–groundwater interactions in ephemeral and intermittent streams in humid landscapes remain sparse. Here, we examined seasonally driven stream–groundwater interactions in response to temporary streamflow on the basis of field observations of streamflow and groundwater on an adjacent hillslope. The direction of hydraulic head gradients between the stream and groundwater shifted seasonally. The stream gained water (head gradients were towards the stream) when storage state was high. During this period, streamflow was persistent. The stream lost water to the groundwater system (head gradients were away from the stream) when storage state was low. During this period, streamflow only occurred in response to precipitation events, and head gradients remained predominantly away from the stream during events. This suggested that mechanisms other than deep groundwater contributions produced run‐off when storage was low, such as surface and perched subsurface flowpaths above the water table. Analysis of the annual water balance for the study period showed that the residual between precipitation inputs and streamflow and evapotranspiration outputs, which were attributed to the loss of water to the deeper, regional groundwater system, was similar in magnitude to streamflow. This, coupled with results that showed bidirectionality in stream–groundwater head gradients, indicated that headwaters composed of temporary (e.g., ephemeral and intermittent) streams can be important focal areas for regional groundwater recharge, and both contribute to and receive water, solutes, and materials from the groundwater system.

One-dimensional analytical solution for hydraulic head and numerical solution for solute transport through a horizontal fracture for submarine groundwater discharge

Submarine groundwater discharge (SGD) has been recognized as a major pathway of groundwater flow to coastal oceanic environments. It could affect water quality and marine ecosystems due to pollutants and trace elements transported through groundwater. Relations between different characteristics of aquifers and SGD have been investigated extensively before, but the role of fractures in SGD still remains unknown. In order to better understand the mechanism of groundwater flow and solute transport through fractures in SGD, one-dimensional analytical solutions of groundwater hydraulic head and velocity through a synthetic horizontal fracture with periodic boundary conditions were derived using a Laplace transform technique. Then, numerical solutions of solute transport associated with the given groundwater velocity were developed using a finite-difference method. The results indicated that SGD associated with groundwater flow and solute transport was mainly controlled by sea level periodic fluctuations, which altered the hydraulic head and the hydraulic head gradient in the fracture. As a result, the velocity of groundwater flow associated with SGD also fluctuated periodically. We found that the pollutant concentration associated with SGD oscillated around a constant value, and could not reach a steady state. This was particularly true at locations close to the seashore. This finding of the role of fracture in SGD will assist pollution remediation and marine conservation in coastal regions.

Origin and hydrology of a large, intact Early Cambrian paleocave system and its role in overlying fluidisation structures, Arctic Canada

Paleokarst is most commonly expressed as subtle stratigraphic surfaces rather than large void systems penetrating deeply into the paleo-subsurface. In contrast, a regional Precambrian-Cambrian unconformity on Victoria Island (NWT, Canada), is associated with exceptional exposure of large, intact Cambrian paleocaverns (100m diameter; tens of m high). The paleocaves are distributed along a paleo-horizontal plane, and an associated gryke network is present in the 30–60m of Neoproterozoic dolostone between cave rooves and the base of overlying Cambrian sandstone; both are filled by Cambrian sandstone. The formation and preservation of such karst features require aggressive dissolution along a stable paleo-water-table shortly before transgression and deposition of shallow-marine sand over the dolostone. During the transgression, the karst network acted as a conduit for flowing groundwater that was discharged through overlying, unconsolidated Cambrian shallow-marine sand, producing water-escape structures (sand volcanoes and their conduits). The conduits are preserved as cylindrical remnants of the sand volcanoes' feeder pipes. Sediment fluidisation was probably caused by variations in the hydraulic-head gradient in a meteoric lens near the Cambrian coastline under a tropical climate with abundant, probably seasonally variable rainfall that caused pulses in subsurface fluid flow. Spatial distribution of the paleocaves and sand volcanoes suggests their formation on the southeast side of a recently faulted horst of Proterozoic carbonate bedrock that formed a nearshore island during early Cambrian sea-level rise. Fluidisation structures such as those reported here have generally been difficult to interpret owing to a lack of data on the fluid hydraulics of the underlying aquifer. This is the first report linking the hydraulics of a well-characterised paleokarst to development of fluid-escape structures. Such structures are widely known from sandstones overlying the sub-Cambrian unconformity around the circumference of Laurentia.

Water exchange, mixing and transient storage between a saturated karstic conduit and the surrounding aquifer: Groundwater flow modeling and inputs from stable water isotopes

Water exchanges between a karstic conduit and the surrounding aquifer are driven by hydraulic head gradient at the interface between these two domains. The case-study presented in this paper investigates the impact of the geometry and interface conditions around a conduit on the spatial distribution of these exchanges. Isotopic (δ18O and δD), discharge and water head measurements were conducted at the resurgences of a karst system with a strong allogenic recharge component (Val d’Orléans, France), to estimate the amounts of water exchanged and the mixings between a saturated karstic conduit and the surrounding aquifer. The spatio-temporal variability of the observed exchanges was explored using a 2D coupled continuum-conduit flow model under saturated conditions (Feflow®).The inputs from the water heads and stable water isotopes in the groundwater flow model suggest that the amounts of water flowing from the aquifer are significant if the conduit flow discharges are less than the conduit flow capacity. This condition creates a spatial distribution of exchanges from upstream where the aquifer feeds the conduit (recharge area) to downstream where the conduit reaches its maximum discharge capacity and can feed the aquifer (discharge area). In the intermediate transport zone no exchange between the two domains takes place that brings a new criterion to delineate the vulnerable zones to surface water.On average, 4% of the water comes from the local recharge, 80% is recent river water and 16% is old river water. During the November 2008 flood, both isotopic signatures and model suggest that exchanges fluctuate around this steady state, limited when the river water level increases and intensified when the river water level decreases. The existence of old water from the river suggests a transient storage at the aquifer/conduit interface that can be considered as an underground hyporheic zone.

Synoptic estimates of diffuse groundwater seepage to a spring-fed karst river at high spatial resolution using an automated radon measurement technique

Groundwater (GW) seepage can provide a major source of water, solutes, and contaminants to rivers, but identifying magnitudes, directions and locations of seepage is complicated by its diffuse and heterogeneous distributions. However, such information is necessary to develop programs and policies for protecting ecosystems and managing water resources. Here, we assess GW seepage to the Ichetucknee River, a spring-fed, low gradient, gaining stream in north-central Florida, through automated longitudinal surveys of radon (222Rn) activities at three different flow conditions. A 222Rn mass balance model, which integrates groundwater and spring water end member 222Rn activities and longitudinal 222Rn distributions in river water, shows that diffuse groundwater seepage represents about 16% of the total river baseflow, consistent with previous results obtained from ion (Ca2+, Cl−, SRP and Fe) mass balances and dye tracer methods. During high river stage, the contribution from seepage increases to 18–23% of the river flow. The spatial distribution of GW seepage is more variable in the upper 2.2-km reach of the river than the lower 2.8-km reach, regardless of river flow conditions. The upper reach has a narrower flood plain than the lower reach, which limits evapotranspiration and increases hydraulic gradients toward the river following storm events. Seepage in the lower reach is also limited by hydrologic damming by the receiving river, which inundates the floodplain during high flow conditions, and reduces the hydraulic head gradient. These results demonstrate the variable nature of seepage to a gaining river in both time and space and indicate that multiple synoptic analyses of GW seepage are required to assess seepage rates, determine time-averaged solute fluxes, and develop optimal management policies for riverine ecosystems.

Controls on deep critical zone architecture: a historical review and four testable hypotheses

AbstractThe base of Earth's critical zone (CZ) is commonly shielded from study by many meters of overlying rock and regolith. Though deep CZ processes may seem far removed from the surface, they are vital in shaping it, preparing rock for infusion into the biosphere and breaking Earth materials down for transport across landscapes. This special issue highlights outstanding challenges and recent advances of deep CZ research in a series of articles that we introduce here in the context of relevant literature dating back to the 1500s. Building on several contributions to the special issue, we highlight four exciting new hypotheses about factors that drive deep CZ weathering and thus influence the evolution of life‐sustaining CZ architecture. These hypotheses have emerged from recently developed process‐based models of subsurface phenomena including: fracturing related to subsurface stress fields; weathering related to drainage of bedrock under hydraulic head gradients; rock damage from frost cracking due to subsurface temperature gradients; and mineral reactions with reactive fluids in subsurface chemical potential gradients. The models predict distinct patterns of subsurface weathering and CZ thickness that can be compared with observations from drilling, sampling and geophysical imaging. We synthesize the four hypotheses into an overarching conceptual model of fracturing and weathering that occurs as Earth materials are exhumed to the surface across subsurface gradients in stress, hydraulic head, temperature, and chemical potential. We conclude with a call for a coordinated measurement campaign designed to comprehensively test the four hypotheses across a range of climatic, tectonic and geologic conditions. Copyright © 2016 John Wiley & Sons, Ltd.

Determinação do gradiente hidráulico médio em aquífero costeiro influenciado pela variação constante de carga hidráulica em ambiente estuarino

Na determinação dos gradientes hidráulicos das águas subterrâneas em aquíferos estuarinos, as ondas de pressão decorrentes das flutuações das marés e corpos de água devem ser consideradas. Para este estudo, monitorou-se por 45 dias as variações de nível de água do Rio Cubatão e carga hidráulica do aquífero raso (em 3 níveis de profundidade) numa área industrial adjacente ao rio, em Cubatão, SP, a fim de se determinar as cargas hidráulicas médias e gradientes médios pelo método conforme proposto por Serfes (1991). Este monitoramento foi contínuo e realizado em dois cenários: com o sistema de remediação (“Air Sparging”) ativado e desativado. Os resultados mostraram a interferência do sistema de remediação e da variação da maré nas cargas hidráulicas dos poços monitorados nos poços próximos ao rio Cubatão, e somente a variação da maré nos poços mais distantes. No entanto, na porção superior do aquífero monitorado mostrou a condição de influência do rio Cubatão nos dois cenários. Condições de efluência e influência na porção intermediária e efluência predominante desse rio na porção mais profunda foram observadas. Nas porções intermediária e profunda, os gradientes hidráulicos calculados apresentam valores baixos (-0,0027 a 0,0030), de modo que a análise sobre o sentido de fluxo deve ser criteriosa.

Near shore groundwater acidification during and after a hydrological drought in the Lower Lakes, South Australia

An extreme hydrological drought in the Lower Lakes of the Murray–Darling Basin (Ramsar listed site) resulted in exposure of large areas of lake bed (25% of pre-drought lake area), containing the reduced iron (Fe) sulfide mineral pyrite. The pyrite oxidised and the resulting acidification (pH<4) posed risks of acid and metals entering shallow groundwater and potentially discharging to the remaining lake water body. Piezometer transects were installed at four locations and monitoring of the groundwater levels and quality was undertaken for six years from 2009 (drought) to 2014 (4years post-reinundation). Acidic (pH3–5) groundwater was recorded at three of the four piezometer locations and included sites close to the lake water. The acidic groundwater (0.5–2m below lake bed) at these sites is likely to have originated from the transport of acid from the upper oxidised sediment layer formed during the drought. High soluble metal (Fe, Al, Mn) levels were also recorded at acidic locations. Acidic shallow groundwater has persisted at many sites for over 4years following reinundation post-drought, and is likely due to slow diffusion and limited sulfate reduction. Increases in dissolved Fe and Mn with decreases in redox potential suggest that reductive dissolution of Fe and Mn hydrous oxides and Fe oxy-hydroxysulfate minerals (e.g. jarosite) occurred post-drought. Groundwater hydraulic head gradients were low, indicating there was limited potential for groundwater to discharge to the lake. The hydraulic gradients at all locations were dynamic with complex relationships along the near-shore environment. The results highlight the long lasting and severe effects on groundwater that can occur following hydrological drought in aquatic environments with sulfidic sediments.