Coastal groundwater reserves often reflect a complex evolution of marine transgressions and regressions, and are only rarely in equilibrium with current boundary conditions. Understanding and ...managing the present-day distribution and future development of these reserves and their hydrochemical characteristics therefore requires insight into their complex evolution history. In this paper, we construct a paleo-hydrogeological model, together with groundwater age and origin calculations, to simulate, study and evaluate the evolution of groundwater salinity in the coastal area of the Netherlands throughout the last 8.5 kyr of the Holocene. While intended as a conceptual tool, confidence in our model results is warranted by a good correspondence with a hydrochemical characterization of groundwater origin. Throughout the modeled period, coastal groundwater distribution never reached equilibrium with contemporaneous boundary conditions. This result highlights the importance of historically changing boundary conditions in shaping the present-day distribution of groundwater and its chemical composition. As such, it acts as a warning against the common use of a steady-state situation given present-day boundary conditions to initialize groundwater transport modeling in complex coastal aquifers or, more general, against explaining existing groundwater composition patterns from the currently existing flow situation. The importance of historical boundary conditions not only holds true for the effects of the large-scale marine transgression around 5 kyr BC that thoroughly reworked groundwater composition, but also for the more local effects of a temporary gaining river system still recognizable today. Model results further attest to the impact of groundwater density differences on coastal groundwater flow on millennial timescales and highlight their importance in shaping today's groundwater salinity distribution. We found free convection to drive large-scale fingered infiltration of seawater to depths of 200 m within decades after a marine transgression, displacing the originally present groundwater upwards. Subsequent infiltration of fresh meteoric water was, in contrast, hampered by the existing density gradient. We observed discontinuous aquitards to exert a significant control on infiltration patterns and the resulting evolution of groundwater salinity. Finally, adding to a long-term scientific debate on the origins of groundwater salinity in Dutch coastal aquifers, our modeling results suggest a more significant role of pre-Holocene groundwater in the present-day groundwater salinity distribution in the Netherlands than previously recognized. Though conceptual, comprehensively modeling the Holocene evolution of groundwater salinity, age and origin offered a unique view on the complex processes shaping groundwater in coastal aquifers over millennial timescales.
Complex conductivity of soils Revil, A.; Coperey, A.; Shao, Z. ...
Water resources research,
August 2017, Volume:
53, Issue:
8
Journal Article
Peer reviewed
Open access
The complex conductivity of soils remains poorly known despite the growing importance of this method in hydrogeophysics. In order to fill this gap of knowledge, we investigate the complex ...conductivity of 71 soils samples (including four peat samples) and one clean sand in the frequency range 0.1 Hz to 45 kHz. The soil samples are saturated with six different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m−1, NaCl, 25°C) in order to determine their intrinsic formation factor and surface conductivity. This data set is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in‐phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This data set confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent.
Plain Language Summary
Geophysical methods are increasingly popular in agriculture. Usually, DC (DIrect Current) resistivity is the preferred method but the interpretation of resistivity data suffers a major flaw: the inability to distinguish between bulk and surface conductivity. This has yield to unrealistc interpretation schemes in hydrogeophysics and an abuse of Archie's law. We propose a way to cure this flaw by extending the DC resistivity method to what is called induced polarization. This paper is the first work entirely focused on the study of induced polarization of soils including a comparison with a mechanistic model and a study of the influence of both temperature and saturation.
Key Points
A large data set of complex conductivity data on soils is presented
Complex conductivity data are explained to the light of the dynamic Stern layer model
Surface conductivity is related to the normalized chargeability or quadrature conductivity, which opens new perspectives in hydrogeophysics
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•We physically separated and measured flow paths to an agricultural ditch.•High frequency measurements revealed dynamic origin of tile drain and ditch outflow.•Tile drains transport the majority of ...groundwater-derived salts to surface water.•Salinity variations explained by interaction of water velocity and wave celerity.•Surface water flushing demand was found to greatly exceed sprinkling demand.
The search for management strategies to cope with projected water scarcity and water quality deterioration calls for a better understanding of the complex interaction between groundwater and surface water in agricultural catchments. We separately measured flow routes to tile drains and an agricultural ditch in a deep polder in the coastal region of the Netherlands, characterized by exfiltration of brackish regional groundwater flow and intake of diverted river water for irrigation and water quality improvement purposes. We simultaneously measured discharge, electrical conductivity and temperature of these separate flow routes at hourly frequencies, disclosing the complex and time-varying patterns and origins of tile drain and ditch exfiltration. Tile drainage could be characterized as a shallow flow system, showing a non-linear response to groundwater level changes. Tile drainage was fed primarily by meteoric water, but still transported the majority (80%) of groundwater-derived salt to surface water. In contrast, deep brackish groundwater exfiltrating directly in the ditch responded linearly to groundwater level variations and is part of a regional groundwater flow system. We could explain the observed salinity of exfiltrating drain and ditch water from the interaction between the fast-responding pressure distribution in the subsurface that determined groundwater flow paths (wave celerity), and the slow-responding groundwater salinity distribution (water velocity). We found water demand for maintaining water levels and diluting salinity through flushing to greatly exceed the actual sprinkling demand. Counterintuitively, flushing demand was found to be largest during precipitation events, suggesting the possibility of water savings by operational flushing control.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Groundwater‐surface water (GW‐SW) interaction in numerical groundwater flow models is generally simulated using a Cauchy boundary condition, which relates the flow between the surface water and the ...groundwater to the product of the head difference between the node and the surface water level, and a coefficient, often referred to as the “conductance.” Previous studies have shown that in models with a low grid resolution, the resistance to GW‐SW interaction below the surface water bed should often be accounted for in the parameterization of the conductance, in addition to the resistance across the surface water bed. Three conductance expressions that take this resistance into account were investigated: two that were presented by Mehl and Hill (2010) and the one that was presented by De Lange (1999). Their accuracy in low‐resolution models regarding salt and water fluxes to a dense drainage network in a confined aquifer system was determined. For a wide range of hydrogeological conditions, the influence of (1) variable groundwater density; (2) vertical grid discretization; and (3) simulation of both ditches and tile drains in a single model cell was investigated. The results indicate that the conductance expression of De Lange (1999) should be used in similar hydrogeological conditions as considered in this paper, as it is better taking into account the resistance to flow below the surface water bed. For the cases that were considered, the influence of variable groundwater density and vertical grid discretization on the accuracy of the conductance expression of De Lange (1999) is small.
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Water management in the Netherlands applies to a dense network of surface waters for discharge, storage and distribution, serving highly valuable land-use. National and regional water authorities ...develop long-term plans for sustainable water use and safety under changing climate conditions. The decisions about investments on adaptive measures are based on analysis supported by the Netherlands Hydrological Instrument NHI based on the best available data and state-of-the-art technology and developed through collaboration between national research institutes. The NHI consists of various physical models at appropriate temporal and spatial scales for all parts of the water system. Intelligent connectors provide transfer between different scales and fast computation, by coupling model codes at a deep level in software. A workflow and version management system guarantees consistency in the data, software, computations and results. The NHI is freely available to hydrologists via an open web interface that enables exchange of all data and tools.
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•Operational model is result of 35yr development by authorities and institutes.•Integrated physical models, different scales (time, space), entire water system.•Intelligent connectors at deep level in software include transfer across scales.•Workflow and version management system for sustainable multi-purpose use.•Focus on national scale: climate change scenarios, water distribution during drought.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
End‐member mixing models have been widely used to separate the different components of a hydrograph, but their effectiveness suffers from uncertainty in both the identification of end‐members and ...spatiotemporal variation in end‐member concentrations. In this paper, we outline a procedure, based on the generalized likelihood uncertainty estimation (GLUE) framework, to more inclusively evaluate uncertainty in mixing models than existing approaches. We apply this procedure, referred to as G‐EMMA, to a yearlong chemical data set from the heavily impacted agricultural Lissertocht catchment, Netherlands, and compare its results to the “traditional” end‐member mixing analysis (EMMA). While the traditional approach appears unable to adequately deal with the large spatial variation in one of the end‐members, the G‐EMMA procedure successfully identified, with varying uncertainty, contributions of five different end‐members to the stream. Our results suggest that the concentration distribution of “effective” end‐members, that is, the flux‐weighted input of an end‐member to the stream, can differ markedly from that inferred from sampling of water stored in the catchment. Results also show that the uncertainty arising from identifying the correct end‐members may alter calculated end‐member contributions by up to 30%, stressing the importance of including the identification of end‐members in the uncertainty assessment.
Key Points
We propose a new method to better quantify uncertainty in mixing models
Both end‐member identification and spatiotemporal variation add to uncertainty
Sampling of stored water may not adequately represent input to the stream
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Microreactor technology creates opportunities for the development of miniature chemical devices, in which several unit operations are integrated. In this paper, we describe the design, model ...simulation, and experimental validation and operation of a microdevice for the preferential oxidation of carbon monoxide in hydrogen-rich reformate gas. The microdevice integrates two heat exchangers and one reactor, all consisting of welded stacks of microstructured plates. We show that the device is able to reduce the carbon monoxide concentration to 10
ppm in combination with a high heat recovery efficiency of 90%. The design of the microstructured plates was based on a combination of three-dimensional computational fluid dynamics simulations of the flow pattern and a two-dimensional heat transfer model, in which the flow pattern was approximated. An integral one-dimensional micro heat exchange model was then used to design the integrated device. This approach may be well suited for designing microstructured reaction systems.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Microreactor technology creates opportunities for the development of miniature chemical devices, in which several unit operations are integrated. We describe in this paper the design, experimental, ...and modelling work concerning a microdevice for the preferential oxidation of carbon monoxide in hydrogen-rich reformate gas. The microdevice consists of two heat exchangers and one reactor, all integrated in a single stack of microstructured plates. Experiments show that the initial carbon monoxide conversion is high. However, the catalyst deactivates rapidly. It takes over one hour to reach the required reactor temperature during startup, which is too long for application in a portable fuel processor. The measured temperature gradients in the heat exchangers are twice as small as predicted by a one-dimensional heat exchange model of the microchannels. A two-dimensional model shows that large differences in temperature exist between channels close to the inlet and channels further from the inlet, causing the one-dimensional model to fail. This paper shows that for an accurate description of heat transfer in a micro heat exchanger, the complete (two-dimensional) plate geometry needs to be considered.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Microreactors generally consist of microstructured plates containing a large number of equal channels. The small diameter of the channels enables high heat and mass transfer rates. To exploit this ...feature and realize a high throughput within a small volume, it is necessary to use high flow rates. However, at these high flow rates it is not straightforward to obtain an even distribution of fluid flow over the individual microchannels. A three-dimensional computational fluid dynamics (CFD) model was used to calculate the flow distribution on a microstructured plate. Calculation time was reduced by introducing an artificial viscosity in the channel region. The calculations show that a transitional velocity exists, below which the flow distribution is independent of velocity and above which inertia effects start to influence the distribution. To optimize the flow distribution, nine different plate geometries were studied at flow rates between 0.1 and 100 m s
–1, or 4 × 10
–4 to 0.4 m
3 h
–1 per plate. By optimizing the plate geometry, the relative standard deviation of the flow distribution was reduced from 19 to 3%. Furthermore, it is shown that the optimal geometry depends on the flow rate, which thus needs to be taken into account in the design of microchannel plates.
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