Sinkholes Due to Groundwater Withdrawal in Tazerbo Wellfield,SE Libya

The desert of eastern Libya forms one of the most arid regions of the Sahara. The Great Man‐Made River Project (GMRP) was established. It transports millions of cubic meters of water a day from desert wellfields to the coastal cities, where over 80% of the population lives. The Tazerbo Wellfield is one of the wellfields designed within the GMRP, delivering water to the eastern coast of Libya through an underground pipe network. Tazerbo Wellfield consists of 108 production wells; each well was designed to pump 100 L/s. The planned total groundwater withdrawal from all wells is 1 million m³/d. The deep sandstone aquifer (Nubian sandstone) is covered by a thick mudstone‐siltstone aquitard and is being heavily pumped. The aquifer and fine‐grained sediments of the aquitard may be compacted resulting in land subsidence as a result of high exploitation. Local sinkholes have developed in the area of Tazerbo since the start of the pumping from the wellfield in 2004. These sinkholes have been caused mainly by lowering of the piezometric heads due to the withdrawal of groundwater. In this study, a hydrogeological investigation is presented about the effect of large groundwater pumping from the Nubian sandstone aquifer in Tazerbo Wellfield, SE Libya, based on physical parameters for 108 production wells and 23 observation wells.     

Application of a Bayesian approach to stochastic delineation of capture zones

This paper presents a Bayesian Monte Carlo method for evaluating the uncertainty in the delineation of well capture zones and its application to a wellfield in a heterogeneous, multiaquifer system. In the method presented, Bayes' rule is used to update prior distributions for the unknown parameters of the stochastic model for the hydraulic conductivity, and to calculate probability-based weights for parameter realizations using head residuals. These weights are then assigned to the corresponding capture zones obtained using forward particle tracking. Statistical analysis of the set of weighted protection zones results in a probability distribution for the capture zones. The suitability of the Bayesian stochastic method for a multilayered system is investigated, using the wellfield Het Rot at Nieuwrode, Belgium, located in a three-layered aquifer system, as an example. The hydraulic conductivity of the production aquifer is modeled as a spatially correlated random function with uncertain parameters. The aquitard and overlying unconfined aquifer are assigned random, homogeneous conductivities. The stochastic results are compared with deterministic capture zones obtained with a calibrated model for the area. The predictions of the stochastic approach are more conservative and indicate that parameter uncertainty should be taken into account in the delineation of well capture zones.     

Geological setting and tectonic subdivision of the Neoproterozoic orogenic belt of Tuludimtu, western Ethiopia

The N–S trending Tuludimtu Belt in the extreme west of Ethiopia has been subdivided into five lithotectonic domains, from east to west, the Didesa, Kemashi, Dengi, Sirkole and Daka domains. The Kemashi, Dengi and Sirkole Domains, forming the core of the belt, contain volcano-sedimentary successions, whilst the Didesa and Daka Domains are gneiss terranes, interpreted to represent the eastern and western forelands of the Tuludimtu Belt. The Kemashi Domain, which consists of an ophiolitic sequence of ultramafic and mafic volcanic and plutonic rocks together with sedimentary rocks of oceanic affinity, is interpreted as oceanic crust and is considered to represent an arc-continent suture zone. The Dengi Domain, composed of mafic to felsic volcanic and plutonic rocks, and a sequence of volcanoclastic, volcanogenic, and carbonate sediments, is interpreted as a volcanic arc. The Sirkole Domain consists of alternating gneiss and volcano-sedimentary sequences, interpreted as an imbricated basement-cover thrust-nappe complex. All the domains are intruded by syn- and post-kinematic Neoproterozoic granitoids. Structural analysis within the Didesa and Daka Domains indicate the presence of pre-Pan African structures, upon which Neoproterozoic deformation has been superimposed. The gneissic rocks of these two domains are regarded as pre-Pan African continental fragments amalgamated to West Gondwana during Neoproterozoic collision events. Unconformably overlying all of the above are a series of tilted but internally undeformed conglomerate–sandstone–shale sequences, regarded as post-accretionary molasse-type deposits, formed during gravitational collapse of the Tuludimtu Belt. The Tuludimtu Belt is interpreted as a collision orogenic belt formed during the assembly of West Gondwana prior to final closure of the Mozambique Ocean.     

Spatial estimation of long-term seasonal and annual groundwater resources: application of WetSpass model in the Werii watershed of the Tekeze River Basin,Ethiopia

Treelines are widely studied worldwide in relation to climate changes because they are hypothesized to be sensitive climate proxies. However, forest treeline expansion toward higher altitudes may be influenced both by a warming climate and by other factors, such as surface morphology and, in the European Alps, the decline in alpine farming. Our results from five valleys in the inner and peripheral regions of the Alps show that present-day treeline altitudes mostly depend on anthropogenic and orographicgeomorphologic factors. Climatic treelines are limited to steep and inaccessible slopes, and occur at higher altitudes and farther from mountain peaks in the inner regions than in the peripheral regions of the mountain range. Looking for sites in which to study treeline responses to climate change, we recommend investigating the inner regions of the Alps where treelines are farther from human disturbances and from geomorphologic constraints, potentially resulting in freer upward shifts under warmer temperature conditions. We also found that, in the valleys selected, human disturbance is mainly concentrated about 165 m below non-climatic treelines, suggesting a homogeneous influence on treelines, regardless of geographic position.     

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.     

Could sand mining be a major threat for the declining endemic Labeobarbus species of Lake Tana,Ethiopia?

Sand and gravel materials are often used in Ethiopia's construction sector. However, the impacts of sand mining on the water body's habitat and biodiversity are not yet considered in the country. In this paper, we study how sand mining activities at Lake Tana and its inflowing rivers affect the environment and spawning grounds of the endemic Labeobarbus species. We measured physico‐chemical parameters in‐situ and developed structured questionnaires to collect primary data on the fishery and sand mining. We found significant differences in conductivity, total dissolved solids and temperature among sampling sites (P < 0.05). Majority (>90 per cent) of the respondents confirmed the drastic physical changes in the rivers and a severe decline in fish production. The study revealed that the ecology of the mined rivers was seriously affected by sand mining, which interfered with migratory routes of fishes and resulted in loss of their spawning grounds. The unregulated sand mining also conflicted with the interests of the fisheries management and environment. Thus, urgent policy intervention is needed to protect the ever‐declining Labeobarbus species of Lake Tana and the environment.     

Predicting storm runoff from different land‐use classes using a geographical information system‐based distributed model

A method is presented to evaluate the storm runoff contributions from different land‐use class areas within a river basin using the geographical information system‐based hydrological model WetSpa. The modelling is based on division of the catchment into a grid mesh. Each cell has a unique response function independent of the functioning of other cells. Summation of the flow responses from the cells with the same land‐use type results in the storm runoff contribution from these areas. The model was applied on the Steinsel catchment in the Alzette river basin, Grand Duchy of Luxembourg, with 52 months of meteo‐hydrological measurements. The simulation results show that the direct runoff from urban areas is dominant for a flood event compared with runoff from other land‐use areas in this catchment, and this tends to increase for small floods and for the dry‐season floods, whereas the interflow from forested, pasture and agricultural field areas contributes to recession flow. It is demonstrated that the relative contribution from urban areas decreases with flow coefficient, that cropland relative contribution is nearly constant, and that the relative contribution from grassland and woodland increases with flow coefficient with regard to their percentage of land‐use class areas within the study catchment. Copyright © 2005 John Wiley & Sons, Ltd.     

Degradation of groundwater quality in coastal aquifer of Sabratah area,NW Libya

Overextraction of groundwater is widely occurring along the coast where good quality groundwater is at risk, due to urbanization, tourist development and intensive agriculture. The Sabratah area at the northern central part of Jifarah Plain, Northwest Libya, is a typical area where the contamination of the aquifer in the form of saltwater intrusion, gypsum/anhydrite dissolution and high nitrate concentrations is very developed. Fifty groundwater samples were collected from the study area and analysed for certain parameters that indicate salinization and pollution of the aquifer. The results demonstrate high values of the parameters electrical conductivity, sodium, potassium, magnesium, chloride and sulphate which can be attributed to seawater intrusion. The intensive extraction of groundwater from the aquifer reduces freshwater outflow to the sea, creates drawdown cones and lowering of the water table to as much as 30 m below mean sea level. Irrigation with nitrogen fertilizers and domestic sewage and movement of contaminants in areas of high hydraulic gradients within the drawdown cones probably are responsible for the high nitrate concentration towards the south of the region. Seawater intrusion and deep salt water upconing result in general high SO₄ ^2? concentrations in groundwater near the shoreline, where localized SO₄ ^2? anomalies are also due to the dissolution of sebkha deposits for few wells in the nearby sebkhas. Upstream, the increase in SO₄ ^2? concentrations in the south is ascribed to the dissolution of gypsum at depth in the upper aquifer.