Modeling approaches and strategies for data-scarce aquifers: example of the Dar es Salaam aquifer in Tanzania

Management of groundwater resources can be improved by using groundwater models to perform risk analyses and to improve development strategies, but a lack of extensive basic data often limits the implementation of sophisticated models. Dar es Salaam in Tanzania is an example of a city where increasing groundwater use in a Pleistocene aquifer is causing groundwater-related problems such as saline intrusion along the coastline, lowering of water-table levels, and contamination of pumping wells. The lack of a water-level monitoring network introduces a problem for basic data collection and model calibration and validation. As a replacement, local water-supply wells were used for measuring groundwater depth, and well-top heights were estimated from a regional digital elevation model to recalculate water depths to hydraulic heads. These were used to draw a regional piezometric map. Hydraulic parameters were estimated from short-time pumping tests in the local wells, but variation in hydraulic conductivity was attributed to uncertainty in well characteristics (information often unavailable) and not to aquifer heterogeneity. A MODFLOW model was calibrated with a homogeneous hydraulic conductivity field and a sensitivity analysis between the conductivity and aquifer recharge showed that average annual recharge will likely be in the range 80–100 mm/year.     

Stormwater harvesting in ephemeral streams: how to bypass clogging and unsaturated layers

To cope with water scarcity in drylands, stormwater is often collected in surface basins and subsequently stored in shallow aquifers via infiltration. These stormwater harvesting systems are often accompanied by high evaporation rates and hygiene problems. This is commonly a consequence of low infiltration rates, which are caused by clogging layers that form on top of the soil profile and the presence of a thick vadose zone. The present study aims to develop a conceptual solution to increase groundwater recharge rates in stormwater harvesting systems. The efficiency of vadose-zone wells and infiltration trenches is tested using analytical equations, numerical models, and sensitivity analyses. Dams built in the channel of ephemeral streams (wadis) are selected as a study case to construct the numerical simulations. The modelling demonstrated that vadose-zone wells and infiltration trenches contribute to effective bypassing of the clogging layer. By implementing these solutions, recharge begins 2250–8100% faster than via infiltration from the bed surface of the wadi reservoir. The sensitivity analysis showed that the recharge rates are especially responsive to well length and trench depth. In terms of recharge quantity, the well had the best performance; it can infiltrate up to 1642% more water than the reservoir, and between 336 and 825% more than the trench. Moreover, the well can yield the highest cumulative recharge per dollar and high recharge rates when there are limitations to the available area. The methods investigated here significantly increased recharge rates, providing practical solutions to enhance aquifer water storage in drylands.

     

Statistical forecast of the marine surge

Natural Hazards - This paper studies different machine learning methods for solving the regression problem of estimating the marine surge value given meteorological data. The marine surge is...     

Comparison between natural and artificial maturation series of humic coals from the Mahakam delta,Indonesia

Type III (humic) organic matter from the Mahakam delta (Indonesia) was chosen to compare artificial and natural coal series. Powdered and concentrated immature organic matter was heated in sealed gold tubes for 24 hr at temperatures ranging from 250 to 550°C and under pressures ranging from 0.5 to 4 kb, with and without water. Both elemental and Rock-Eval analyses were used to characterize the products. A comparison between our results, published data and the natural model shows that, quantitatively, natural maturation is simulated better when pyrolysis is performed under confined conditions (no free volume, no diluting inert gas). Thus, pyrolysis in a medium swept by an inert gas, vacuum pyrolysis and some pyrolysis in sealed glass tubes must be considered to be poor simulation tools. The presence of water does not seem to have an essential effect. Allowing the hydrocarbons formed to reach a certain value of partial pressure seems to be important. Results are unchanged when external pressure varies from 0.5 to 4 kb.     

Low melting temperature for calcite at 1000 bars on the join CaCO3‐H2O – some geological implications

Melting experiments of calcite were performed on the join CaCO₃‐H₂O at a pressure of 1000 bars. The system evolves to the ternary CaO‐H₂O‐CO₂ system during melting experiments. Our experiments show that partial melting of calcite begins at a low temperature, below 650 °C. Such a low partial melting temperature for carbonates revives the debate about the presence of carbonate melts in the upper crust. More specifically, the conditions for carbonate partial melting are present in carbonate host rocks undergoing contact metamorphism at high temperatures in the presence of water‐rich fluid. The presence of carbonate melts influences physical parameters such as viscosity and permeability in contact aureoles, and, furthermore, decarbonation reactions release massive amounts of CO₂.     

Lateral extrusion along the Altyn Tagh Fault,Qilian Shan (NE Tibet): insight from a 3D crustal budget

The lithospheric strike‐slip Altyn Tagh Fault has accommodated hundreds of kilometres of displacement between the Qaidam and Tarim blocks since its Eocene reactivation. However, the way the deformation is accommodated in the Qilian Shan and further east remains uncertain. Based on 360 km of north‐eastward migration of the relatively rigid Qaidam block along the Altyn Tagh Fault and 3D isovolumetric balancing of the crustal deformation within the Altyn Tagh Fault–Qilian Shan system, we demonstrate that 250 ± 28 km (43.8–49.4%) of N20E directed crustal shortening and an additional ~250–370 km of eastward motion of the Qilian Shan crust must be accounted for by strike‐slip faulting in the Qilian Shan and crustal thickening in the Qinling area, as well as by extension in the adjoining North China block graben systems.     

The pattern of Ni and Co abundances in lunar olivines

Near liquidus experiments on peridotite and other olivine normative compositions from 1.7 to 6 GPa confirm the applicability of exchange-based empirical models of Ni and Co partitioning between olivine and silicate liquids with compositions close to the liquidus of peridotite. Given that most estimates of lunar bulk composition are peridotitic, the partitioning models thus lend themselves to calculation of olivine compositions produced during the early stages of magma ocean crystallization. Calculation of olivine compositions produced by fractional crystallization of a model lunar magma ocean, initially 700 km deep, reveals a prominent maximum in Ni concentration versus fraction crystallized or Mg’ (molar MgO/(MgO + FeO)), but a pattern of monotonically increasing Co concentration. These patterns qualitatively match the puzzling patterns of Ni and Co concentrations observed in lunar rocks in which forsteritic olivines in magnesian suite cumulates have lower Ni and Co abundances than do less magnesian olivines from low-Ti mare basalts, and olivines from the ferroan anorthosite suite (FAS) have lower Ni, but similar Co to mare basalt olivines.The Ni and Co abundances in olivines from the magnesian suite cumulates can be reconciled in terms of fractional crystallization of a deep magma ocean which initially produces a basal dunite comprised of the hottest and most magnesian olivine overlain by an olivine-orthopyroxene (harzburgite) layer that is in turn overlain by an upper zone of plagioclase-bearing cumulates. The ultramafic portion of the cumulate pile overturns sending the denser harzburgite layer, which later becomes a portion of the green glass source region, to the bottom of the cumulate pile with Ni- and Co-rich olivine. Meanwhile, the less dense, but hottest, most magnesian olivines with much lower Ni and Co abundances are transported upward to the base of the plagioclase-bearing cumulates where subsequent heat transfer leads to melting of mixtures of primary dunite, norite, and gabbronorite with KREEP (a K-REE-P enriched component widely believed to be derived from the very latest stage magma ocean liquid). These hybrid melts have Al₂O₃, Ni, and Co abundances and Mg’ appropriate for parent magmas of the magnesian suite. Ni and Co abundances in the FAS are consistent with either direct crystallization from the magma ocean or crystallization of melts of primary dunite-norite mixtures without KREEP.     

Seasonal control of the Saint-Lawrence maximum turbidity zone by tidal-flat sedimentation

The core of the turbidity maximum zone in the Saint-Lawrence Estuary is located in the North Channel and oscillates in front of the large (3×10⁶ m²) intertidal flats and marshes of Cap Tourmente. It is shown that seasonal fluctuations in the intensity and the position of this core are mainly determined by suspended sediment exchanges between the channel and the marshes. Fine sediments, most of them found 20 km downstream in the channel off Cap Maillard in late winter and early spring, are advected upstream over the flats during the summer months by the tide. The deposition, favored by marsh plant growth, reaches 5×10⁵ metric tons in three months. A period of intense erosion, at a mean rate of 4,500 metric tons per tide, coincides with the destruction of the plant cover by migratory geese. The material removed fills up the Chenal de l’Île d’Orléans upstream and is flushed back into the water column during the next spring freshette. This rotating seasonal sediment circulation, although very localized, exerts a major influence on the distribution and storage time of suspended particles in the upper estuary.     

Die Entstehung einiger tektonischer Grundbegriffe. Ein Beitrag zur Geschichte der Geologie

Zusammenfassung Die ersten tektonischen Vorstellungen legte 1669Nic. Steno in seinem Prodromus nieder. Seine Beobachtungen an horizontalen und geneigten Schichten führten später zu dem Begriffe der orogenetischen Diskordanzen. L.Élie de Beaumont ist sein direkter Nachfolger geworden, während J.Hutton ganz unabhängig von beiden den Diskordanzbegriff entwickelte. Die neptunistische Lehre von A. G.Werner ging von einem primären, von Wasserfluten geschaffenen Relief der Erdoberfläche aus, das überschichtet wurde. Doch auchWerner dachte daneben ebenfalls an große Höhlen, deren Decken einbrechen würden. Die neptunistischen Schwierigkeiten ergaben sich aus der Unmöglichkeit einer vertikalen Sedimentation von primär senkrecht gestellten Schichten. Die Anfänge der Erforschung der Tektonik des Rheingrabens werden dargelegt, und es wird besonders gezeigt, daßÉlie de Beaumont, der 1836 von einem eingebrochenen Gewölbe sprach, schon vorher Kenntnis eines eingebrochenen Gewölbes von Island und eines Schichtgewölbes von Württemberg über Schwarz wald — Vogesen nach Lothringen hatte. Die Zeit des Einbruches legteÉ. de Beaumont irrtümlich in die Zeit des Buntsandsteins.Theorien der Gebirgsbildung wurden mit L. v.Buch's Erhebungstheorie und geognostischen Gebirgssystemen um zwei weitere Ideen 1824–1835 erweitert.Élie de Beaumont baute diese Theorie sehr aus und sprach von Revolutionen der Erdkruste (1829). Die Faltengebirge fanden in J.Thurmann (1832) ihren ersten gründlichen Bearbeiter, der am Beispiel des Faltenjuras nachwies, daß doch nur Horizontalkräfte am Werke gewesen sein können und die Erhebungstheorie L. v.Buch's für diese Gebirgsarten keine Erklärung bringe. Zum Schlusse wird die Geschichte des Geologenkompasses gebracht.

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Nic. Steno was the first who developed tectonic ideas in his Prodromus in 1669. His observations in the surroundings of Firenze (Italy) led him to the notion what nowadays is called a discordance by orogeny. He was followed by the French L.Élie de Beaumont, while the Scotchman J.Hutton developed this term of tectonic discordance quite independently of both.The neptunic theory of A. G.Werner in Freiberg (Saxony) started from the idea that the primary relief of the surface of the earth was formed by floods and covered with sediments. Furtheron he assumed large caves, the tops of them having broken down, explaining in this manner very inclined strata. The difficulties in this neptunic theory resulted from the fact that it was impossible to account for a sedimentation in a vertical line as seen in vertical layers.The author gives a survey of the first researches in the tectonics of the Rhinegraben and shows thatÉlie de Beaumont, who in 1836 mentioned the breakdown of the central ridge of a vault, so forming the rift-valley, had already some information of such a pressure arch in Iceland and of a second one stretching from Würtemberg via Black Forest—Vosges to Lorraine.Élie de Beaumont misdated the collapse of this large vault, stating erroneously the time of Buntsandstein (=New Red Sandstone).Between 1824 and 1835 L. v.Buch developed two further theories on orogenesis, that of the elevation-crater hypothesis and that of the geognostic systems of mountains. É.de Beaumont completedBuch's theory and in 1829 he spoke of revolutions of the earth's crust.The first who thoroughly researched the Swiss Jura was J.Thurmann. Quoting the folded strata of this region as an example he proved that only horizontal stresses could have been responsible for the formation of these mountains and that the elevation-crater hypothesis of L. v.Buch acting only with vertical forces could not give any explanation for it.Finally the author recapitulates the history of the geologist's compass.

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Résumé Les premières conceptions tectoniques ont été exposées en 1669 par N.Steno dans son « Prodrome ». Ses observations sur des strates horizontales ou inclinées ont conduit ultérieurement à la notion de discordances orogéniques. L.Élie de Beaumont fut son successeur direct, cependant que J.Hutton, tout à fait indépendamment des deux premiers développa la notion de discordance. La théorie neptuniste de A. G.Werner se basait sur l'existence préalable d'un relief terrestre créé par les flots et qui aurait été noyé sous les strates des dépôts ultérieurs. Pourtant, dans un autre ordre d'idées,Werner pensait aussi à l'existence d'immenses grottes dont les toits s'effondreraient. Les neptunistes éprouvèrent des difficultés résultant de l'impossibilité d'expliquer des couches redressées par une sédimentation verticale. L'auteur expose les débuts des recherches sur la tectonique du fossé rhénan et montre en particulier qu'Élie de Beaumont, qui parla en 1836 d'une voûte effondrée avait déjà auparavant eu connaissance d'une voûte effondrée en Islande et d'une voûter structurale allant du Wurttemberg en Lorraine par-dessus la Forêt-Noire et les Vosges. Par erreur,É de Beaumont plaça le moment de cet effondrement à l'époque du grès bigarré.Avec la théorie des soulèvements et des orogènes géognostiques L. v.Buch enrichit de 1824–1835 les théories orogéniques de deux concepts supplémentaires.Élie de Beaumont les développa et parla de révolution de la croûte terrestre (1829). Les premières recherches approfondies sur les chaînes plissées sont dues à J.Thurmann (1832) qui, d'après l'exemple du Jura plissé, démontra que seules des forces horizontales ont pu avoir été à l'oeuvre et que la théorie des soulèvements de L.von Buch n'apportait pas d'explication pour les chaînes de ce type.Pour terminer, l'auteur expose l'histoire de la boussole du géologue.

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South Equatorial Current (SEC) driven changes at DSDP Site 237, Central Indian Ocean, during the Plio-Pleistocene: Evidence from Benthic Foraminifera and Stable Isotopes

This study attempts to analyse paleoceanographic changes in the Central Indian Ocean (Deep Sea Drilling Project Site 237), linked to monsoon variability as well as deep-sea circulation during the Plio-Pleistocene. We used factor and cluster analyses of census data of the 34 most dominant species of benthic foraminifera that enabled us to identify five biofacies: Astrononion umbilicatulum–Uvigerina proboscidea (Au–Up), Pullenia bulloides–Bulimina striata (Pb–Bs), Globocassidulina tumida–Nuttallides umbonifera (Gt–Nu), Gyroidinoides nitidula–Cibicides wuellerstorfi (Gn–Cw) and Cassidulina carinata–Cassidulina laevigata (Cc–Cl) biofacies. Knowledge of the environmental preferences of modern deep-sea benthic foraminifera helped to interpret the results of factor and cluster analyses in combination with oxygen and carbon isotope values. The biofacies indicative of high surface productivity, resulting from a stronger South Equatorial Current (Au–Up and Pb–Bs biofacies), dominate the early Pliocene interval (5.6–4.5 Ma) of global warmth. An intense Indo-Pacific ‘biogenic bloom’ and strong Oxygen Minimum Zone extended to intermediate depths (1000–2000 m) over large parts of the Indian Ocean in the early Pliocene. Since 4.5 Ma, the food supply in the Central Indian Ocean dropped and fluctuated while deep waters were corrosive (biofacies Gt–Nu, Gn–Cw). The Pleistocene interval is characterized by an intermediate flux of organic matter (Cc–Cl biofacies).