Electrical imaging and fluid modeling of convective fingering in a shallow water-table aquifer

Unstable density-driven flow can lead to enhanced solute transport in groundwater. Only recently has the complex fingering pattern associated with free convection been documented in field settings. Electrical resistivity (ER) tomography has been used to capture a snapshot of convective instabilities at a single point in time, but a thorough transient analysis is still lacking in the literature. We present the results of a 2 year experimental study at a shallow aquifer in the United Arab Emirates that was designed to specifically explore the transient nature of free convection. ER tomography data documented the presence of convective fingers following a significant rainfall event. We demonstrate that the complex fingering pattern had completely disappeared a year after the rainfall event. The observation is supported by an analysis of the aquifer halite budget and hydrodynamic modeling of the transient character of the fingering instabilities. Modeling results show that the transient dynamics of the gravitational instabilities (their initial development, infiltration into the underlying lower-density groundwater, and subsequent decay) are in agreement with the timing observed in the time-lapse ER measurements. All experimental observations and modeling results are consistent with the hypothesis that a dense brine that infiltrated into the aquifer from a surficial source was the cause of free convection at this site, and that the finite nature of the dense brine source and dispersive mixing led to the decay of instabilities with time. This study highlights the importance of the transience of free convection phenomena and suggests that these processes are more rapid than was previously understood.

Map of water table depths of theKenigezkiǐ fond

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Map of water table depths of the Semi-Kolodez︠ia︡nskiǐ fond

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Analysis of an Aquifer-water table aquitard system

The problem of pumping an aquifer in an aquifer-water table aquitard system is considered, accounting for the elastic properties of both the aquifer and the aquitard, the gravity drainage in the aquitard and treating the water table as an unknown boundary. The coupled partial differential equations are nondimensionalised, yielding three principal parameters governing the problem. The numerical solution of these equations is obtained for a wide range of parameter values. Type curves are generated and their use is illustrated through a field application.

Dendrochronological evidence for a lower water-table on peatland around 3200-3000 BC from subfossil pine in northern Scotland

Tree-ring analysis of subfossil Pinus sylvestris L., from nine new peatland sites located beyond the species’ current northern limit in Scotland, established a regional chronology called WRATH-9. The chronology has been provisionally dated against Irish pine chronologies and provides the first annual resolution picture of Scots pine expansion from c. 3200 bc and subsequent demise from c. 3000 bc. Pine germination and growth is suggested to be associated with a widespread fall in bog water-tables that indicates a regional climatic control. Bog pines progressively declined in number, rather than died out in a single event, reflecting their growth in a marginal habitat, close to a critical ecological threshold. The use of tree-ring sequences from in situ bog pine macrofossils provides a higher resolution insight into past conditions than possible with existing radiocarbon and pollen-based chronologies.

Examining the uncertainties in a 'tuned and stacked' peatland water table reconstruction

Tuning and stacking approaches have been used to compile non-annually resolved peatland palaeo-water table records in several studies. This approach has been proposed as a potential way forward to overcome the chronological problems that beset the correlation of records and may help in the upscaling of palaeoclimate records for climate model-data comparisons. This paper investigates the uncertainties in this approach using a published water table compilation from Northern Ireland. Firstly, three plausible combinations of chronological match points are used to assess the variability of the reconstructions. It is apparent that even with markedly different match point combinations, the compilations are highly similar, especially when a 100-year running mean line is used for interpretation. Secondly, sample-specific reconstruction errors are scaled in relation to the standardised water table units and illustrated on the compiled reconstruction. Thirdly, the total chronological errors for each reconstruction are calculated using Bayesian age-modelling software. Although tuning and stacking approaches may be suitable for compiling peat-based palaeoclimate records, it is important that the reconstruction and chronological errors are acknowledged and clearly illustrated in future studies. The tuning of peat-based proxy climate records is based on a potentially flawed assumption that events are synchronous between sites. © 2011 Elsevier Ltd and INQUA.

Increased temperature sensitivity of net DOC production from ombrotrophic peat due to water table draw-down.

The production and release of dissolved organic carbon (DOC) from peat soils is thought to be sensitive to changes in climate, specifically changes in temperature and rainfall. However, little is known about the actual rates of net DOC production in response to temperature and water table draw-down, particularly in comparison to carbon dioxide (CO2) fluxes. To explore these relationships, we carried out a laboratory experiment on intact peat soil cores under controlled temperature and water table conditions to determine the impact and interaction of each of these climatic factors on net DOC production. We found a significant interaction (P < 0.001) between temperature, water table draw-down and net DOC production across the whole soil core (0 to −55 cm depth). This corresponded to an increase in the Q10 (i.e. rise in the rate of net DOC production over a 10 °C range) from 1.84 under high water tables and anaerobic conditions to 3.53 under water table draw-down and aerobic conditions between −10 and − 40 cm depth. However, increases in net DOC production were only seen after water tables recovered to the surface as secondary changes in soil water chemistry driven by sulphur redox reactions decreased DOC solubility, and therefore DOC concentrations, during periods of water table draw-down. Furthermore, net microbial consumption of DOC was also apparent at − 1 cm depth and was an additional cause of declining DOC concentrations during dry periods. Therefore, although increased temperature and decreased rainfall could have a significant effect on net DOC release from peatlands, these climatic effects could be masked by other factors controlling the biological consumption of DOC in addition to soil water chemistry and DOC solubility. These findings highlight both the sensitivity of DOC release from ombrotrophic peat to episodic changes in water table draw-down, and the need to disentangle complex and interacting controls on DOC dynamics to fully understand the impact of environmental change on this system.

Artworks : Water Table and Rain Table : presented as part of the Periscope Exhibition, Castlemaine State Festival

Rain Table and Water Table: Delicate splashes and droplets of water act like primitive lenses bringing transparency to the diffused images of celestial bodies. These two installation pieces are inspired by the beauty of the night sky and invite the viewer to consider the cosmos in relation to ones self and to contemplate the discoveries which have changed our understanding of the universe. Water Table and Rain Table are the two works being presented as part of Periscope. Through the form of the science bench or museum cabinet, luminous and projected images play against glass and water invoking the sublime sense of wonder that we have when we look to the starry night sky. Water Table - In 1912 the astronomer, Vesto Slipher made the discovery that “Nebula” were moving at incredible velocities due to the expansion of space itself. This discovery revealed these “Nebula” to be vastly remote and independent galaxies. Water Table speculates on the understanding that when we look into deep space, we also look into deep time. Rain Table is a new work produced for the festival and makes reference to the first telescopic observations of the Moon made by the mathematician, philosopher and astronomer, Galileo Galilei in 1610. The implication of Galileo’s observations gave rise to a radical new understanding of the heavens and our place in it and the final acceptance that the Earth was not the centre of the Universe.

Castlemaine State Festival 2013: Periscope Exhibition: Artwork No 1: "Water Table"

Rian Table and Water Table: Delicate splashes and droplets of water act like primitive lenses bringing transparency to the diffused images of celestial bodies. These two installation pieces are inspired by the beauty of the night sky and invite the viewer to consider the cosmos in relation to ones self and to contemplate the discoveries which have changed our understanding of the universe. Water Table and Rain Table are the two works being presented as part of Periscope. Through the form of the science bench or museum cabinet, luminous and projected images play against glass and water invoking the sublime sense of wonder that we have when we look to the starry night sky. Water Table In 1912 the astronomer, Vesto Slipher made the discovery that “Nebula” were moving at incredible velocities due to the expansion of space itself. This discovery revealed these “Nebula” to be vastly remote and independent galaxies. Water Table speculates on the understanding that when we look into deep space, we also look into deep time. Rain-Table is a new work produced for the festival and makes reference to the first telescopic observations of the Moon made by the mathematician, philosopher and astronomer, Galileo Galilei in 1610. The implication of Galileo’s observations gave rise to a radical new understanding of the heavens and our place in it and the final acceptance that the Earth was not the center of the Universe.

Stochastic simulation of time-series models combined with geostatistics to predict water-table scenarios in a Guarani Aquifer System outcrop area, Brazil

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)