Using stable water isotopes to identify spatio-temporal controls on groundwater recharge in two contrasting East African aquifer systems

Understanding the spatio-temporal variability in groundwater recharge is a prerequisite to sustainable management of aquifers. Spatial analysis of groundwater stable isotopes uncovered predominant controls on groundwater recharge in the Nairobi aquifer system (NAS) and the South Coast aquifer (SC), two exemplar East African aquifers relied upon by 7 million people. We analysed 368 samples for stable isotopes and basic physico-chemical parameters. The NAS groundwater isotopes are controlled by precipitation orographic effects and enriched recharge from impounded lakes/wetlands; the SC isotopes are correlated with water-table depth influencing evapotranspiration. Global Network of Isotopes in Precipitation (GNIP) data revealed groundwater recharge during months of heavy rains in the NAS, whilst the SC experiences spatio-temporally diffuse recharge. Inferred “isoscapes” show: in the NAS, (1) direct, rapid recharge favoured by faults, well-drained soils and ample rainfall in uplands, (2) delayed recharge from impounded lakes and wetlands in mid-lands, and (3) focused, event-based recharge in floodplains; and in the SC, diffuse recharge complicated by significant water-table evapotranspiration processes.     

Hydrogen and oxygen in brine shrimp chitin reflect environmental water and dietary isotopic composition

Hydrogen and oxygen isotope ratios of the common structural biopolymer chitin are a potential recorder of ecological and environmental information, but our understanding of the mechanisms of incorporation of H and O from environmental substrates into chitin is limited. We report the results of a set of experiments in which the isotopic compositions of environmental water and diet were varied independently in order to assess the contribution of these variables to the H and O isotopic composition of Artemia franciscana chitin. Hydrogen isotope ratios of chitin were strongly linearly correlated with both food and water, with approximately 26% of the hydrogen signal reflecting food and approximately 38% reflecting water. Oxygen isotopes were also strongly correlated with the isotopic composition of water and food, but whereas 69% of oxygen in chitin exchanged with environmental water, only 10% was derived from food. We propose that these observations reflect the position-specific, partial exchange of H and O atoms with brine shrimp body water during the processes of digestion and chitin biosynthesis. Comparison of culture experiments with a set of natural samples collected from the Great Salt Lake, UT in 2006 shows that, with some exceptions, oxygen isotope compositions of chitin track those of water, whereas hydrogen isotopes vary inversely with those of lake water. The different behavior of the two isotopic systems can be explained in terms of a dietary shift from allochthonous particulate matter with relatively higher δ²H values in the early spring to autochthonous particulate matter with significantly lower δ²H values in the late summer to autumn. These results suggest oxygen in chitin may be a valuable proxy for the oxygen isotopic composition of environmental water, whereas hydrogen isotope values from the same molecule may reveal ecological and biogeochemical changes within lakes.     

A new correction model for C ages in aquifers with complex geochemistry – Application to the Neogene Aquifer,Belgium

The objective of this paper is to build a general correction model that takes into account all the different radiocarbon-dilution reactions and resolving the processes that are geochemically “aging” the groundwater in the Neogene Aquifer. For this, δ¹³C and radiocarbon in groundwater are investigated with their relationship to other chemical components in groundwater. The δ¹³C values in the Neogene Aquifer are influenced by various geochemical reactions like calcite dissolution, oxidation of organic matter and methanogenesis. Calcite dissolution and CH₄ production increase δ¹³C while the oxidation of organic matter decreases δ¹³C in the groundwater. The reactions that modify δ¹³C also influence the ¹⁴C activity. Due to the complex geochemical environment, existing correction models are not applicable to this situation. A correction model for initial ¹⁴C activity is formulated in which the different C sources that influence ¹⁴C activity are taken into account. It is observed that recent dissolved organic matter plays an important role in redox reactions. The corrected ¹⁴C ages lie between −0.792 and 6.425 ka representing the maximum age. If a part of the organic matter that oxidises is fossil, the determined age will represent an overestimated age.     

A new groundwater radiocarbon correction approach accounting for palaeoclimate conditions during recharge and hydrochemical evolution: The Ledo-Paniselian Aquifer,Belgium

The particular objective of the present work is the development of a new radiocarbon correction approach accounting for palaeoclimate conditions at recharge and hydrochemical evolution. Relevant climate conditions at recharge are atmospheric pCO₂ and infiltration temperatures, influencing C isotope concentrations in recharge waters. The new method is applied to the Ledo-Paniselian Aquifer in Belgium. This is a typical freshening aquifer where recharge takes place through the semi-confining cover of the Bartonian Clay. Besides cation exchange which is the major influencing process for the evolution of groundwater chemistry (particularly in the Bartonian Clay), also mixing with the original porewater solution (fossil seawater) occurs in the aquifer. Recharge temperatures were based on noble gas measurements. Potential infiltration water compositions, for a range of possible pCO₂, temperature and calcite dissolution system conditions, were calculated by means of PHREEQC. Then the sampled groundwaters were modelled starting from these infiltration waters, using the computer code NETPATH and considering a wide range of geochemical processes. Fitting models were selected on the basis of correspondence of calculated δ¹³C with measured δ¹³C. The ¹⁴C modelling resulted in residence times ranging from Holocene to Pleistocene (few hundred years to over 40 ka) and yielded consistent results within the uncertainty estimation. Comparison was made with the δ¹³C and Fontes and Garnier correction models, that do not take climate conditions at recharge into account. To date these are considered as the most representative process-oriented existing models, yet differences in calculated residence times of mostly several thousands of years (up to 19 ka) are revealed with the newly calculated ages being mostly (though not always) younger. Not accounting for climate conditions at recharge (pCO₂ and temperature) is thus producing substantial error on deduced residence times. The derived ¹⁴C model ages are correlated with He concentrations measured in the groundwater of the aquifer. The obtained residence times show a gap between about 14 and 21 ka indicating possible permafrost conditions which inhibited any groundwater recharge.     

Condemning the public: Design and New York's new 42nd Street

This paper examines the redevelopment of Times Square in New York City and brings together two discourses, the discourse of design and the discourse of eminent domain case law. I argue that both were inextricable parts of the Times Square redevelopment process and served similar functions: defining a public for The New Times Square. By determining what was `in the public interest', eminent domain case law set out two opposing publics: the criminal Times Square public and an idealized general public. By selectively editing the Times Square public's desires and behaviors, design helped define and represent new moral norms.     

Exploring the hydrological effects of normal faults at the boundary of the Roer Valley Graben in Belgium using a catchment-scale groundwater flow model

Hydrogeology Journal - Faults may impact regional groundwater flow and transport, so it is important to include them during aquifer-system conceptualization and while constructing groundwater flow...     

Consequences of Water Level Drops for Soft Sediment Deformation and Vertical Fluid Leakage

Both water level drops and erosion have previously been suggested as causes of fluid overpressures in the subsurface. Quantification of the relevance of these processes to supra-lithostatic fluid pressure formation with a wide selection of input parameters is lacking, and thus desired. The magnitudes and drop times that are required for water level drops to result in supra-lithostatic pore pressures in a variety of situations are calculated. Situations with pore fluids consisting of water, water with dissolved methane, water with a gas hydrate layer and dissolved methane in the underlying sediments, and water with dissolved methane, a gas hydrate layer, and free gas accumulation below the hydrate layer are separately addressed. The overpressure formation from reservoir gas expansion is also simulated. The simulation results demonstrate that high fluid overpressures can develop in a rock as a response to a water level drop without the presence of gas, provided that the rock has a sufficiently low compressibility. The contribution to fluid overpressuring is however dramatically increased if the pore water is saturated with methane prior to the water level drop, and is further amplified by the presence of gas hydrates and free gas accumulations beneath such hydrates. Gas expansion in reservoirs should be expected to significantly increase the fluid overpressures in shallow, sealed pressure compartments that experience erosion or water level drops, even if the water level drop duration exceeds one million years. Enough relationships between the calculated overpressure formation and the main controlling factors are provided in order to enable readers to make inferences about a variety of geological settings. Analyses of simulation results prompt us to suggest that pockmarks are likely to be triggered by gas expansion in vertical fluid migration pathways, that the giant craters at the seabed west of Albatross South in the Barents Sea result from hydrate dissociation, and that overpressure build-up due to gas expansion has contributed to reservoir overpressuring in many eroded basins, including the Hammerfest Basin in the Barents Sea.     

Geological and geotechnical constraints for urban planning and natural environment protection: a case study from Mekelle City, Northern Ethiopia

The study was conducted in the rapidly expanding city of Mekelle located in the northern part of Ethiopia, East Africa. An integrated approach including geomorphological, geological and engineering geological, geotechnical and hydrogeological methods were used to characterize the natural urban environment to be used as a baseline for protection and rational planning of development. Conventional field observation, test pits and drilled boreholes were used to collect soil, rock and water samples and standard laboratory procedures were applied. The results revealed that the main geologic constraints to sustainable development of the city are active erosion, instability of slopes, flood hazard, cyclic/alternating hard and weak rock layers, high swelling–shrinkage soils, shallow and unconfined groundwater with variable composition and high susceptibility to corrosion and pollution. It is recommended to avoid development and expansion of the city in areas of steep land forms, at the foot of steep slopes and in areas covered by black alluvial soil deposits. Development of a systematic multidisciplinary database aided by GIS, with continuous monitoring and updating is also highly recommended and will be useful for further refined geotechnical microzonation. It is the authors’ belief that this study highlights the basic constraints and hazards the city is facing, and provides baseline information and awareness to the city planners, decision makers, geo-environmentalists, engineers and the community for future expansion and rehabilitation plans and to tackle the existing hazards.     

Effects of multi-annual climate variability on the hydrodynamic evolution (1833 to present) in a shallow aquifer system in northern Belgium

Abstract

Using a groundwater flow model and long historical meteorological time series data, the evolution of the groundwater flow regime in a multi-layered groundwater flow basin in northern Belgium during the last one and a half centuries (since 1833) is reconstructed. Model output parameters such as piezometric levels, depth to water table, seepage fluxes in the valleys and calculated baseflow to the river system are presented and inter-annual and decadal variations are evaluated against seasonal fluctuations. The main time-varying boundary condition in the model is the aquifer recharge which was estimated using the method of Thornthwaite and Mather based on precipitation and temperature data. The model does not take into account changes in boundary conditions due to changes in land use (deforestation, drainage of cultivated land) or groundwater exploitation. Variations in model output parameters are therefore only due to climatological forcing. Only the natural non-exploited state of the aquifer is considered. Although few historical piezometric measurements are available to verify model output, the results give an indication of the natural hydrodynamic variations on a time scale of decades.

Citation Van Camp, M., Coetsiers, M., Martens, K. & Walraevens, K. (2010) Effects of multi-annual climate variability on the hydrodynamic evolution (1833 to present) in a shallow aquifer system in northern Belgium. Hydrol. Sci. J. 55(5), 763–779.