The Niger Delta at the southernmost part of Nigeria, is a large and ecologically sensitive region. Natural and anthropogenic activities, such as subsidence, sea-level rise, upstream dams and reservoirs, dredging and overabstraction of groundwater, disturb this equilibrium, leading to adjustments in ecological conditions and boundaries. Ecological changes affecting large areas cannot be explained by local observation. Relating observed effects to causes of environmental behaviour therefore requires a regionalized data collection, that provides a broader perspective of the issues involved with resultant ease in the identification of common problems and formulation of effective solutions. Key words environmental impact assessment; ecological changes; human impacts; ecohydrology of deltaic areas; environmental sensitivity indices; saline water/freshwater interaction Perspectives de la recherche hydrologique régionale dans le delta du NigerRésumé Le delta du Niger, à l'extrême sud du Nigeria, est une vaste région écologiquement sensible au sein de laquelle diverses espèces biologiques, dans l'eau et dans le milieu souterrain environnant, sont en équilibre dynamique. Les processus naturels et les activités humaines, comme la subsidence, les marées, les barrages et réservoirs, le dragage et la surexploitation des eaux souterraines perturbent cet équilibre, et sont la cause de modifications des facteurs écologiques et de leurs conditions aux limites. Les modifications écologiques affectant de vastes régions ne peuvent pas être expliquées par des observations locales. La relation des effets aux causes implique donc la collecte de données à l'échelle régionale, qui fournit une perspective plus large et facilite l'identification des problèmes et la formulation de solutions efficaces. Mots clefs évaluation des conséquences environnementales; modifications écologiques; mpacts anthropiques; écologie des régions deltaïques; indices de sensibilité environnementale: nteraclion eau salée/eau douce

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Lime stabilisation is one of the traditional methods of improving the engineering properties of lateritic soils for use as subgrade and foundation materials for the construction of road pavements and highway embankments. Understanding the mechanical performance of lime-stabilised lateritic subgrades in terms of their durability under continuous water ingress will improve environmental sustainability by conserving scarce natural resources and reducing the environmental impacts of repair and replacement of pavements. However, there are several conflicting reports on the durability of lime-stabilised soils subjected to continuous water ingress and harsh environmental conditions. Therefore, this paper evaluates the influence of leaching on the physicochemical behaviour and durability of lime-stabilised lateritic soil under continuous water ingress, simulating the typical experience in a tropical environment. Variations in the strength and durability of the lateritic soil at various lime contents (0, 2.5, 5, 7.5, 10, 15, and 20 wt.%) and soaking periods (3, 7, 14 and 28 days) were evaluated by performing the California bearing ratio tests before and after subjecting the lime-lateritic soil (LLS) samples to continuous leaching using two modified leaching cells. Furthermore, physicochemical analysis was performed to assess the variation of cation concentrations and changes in the physical properties of the pore fluid as the leaching time progressed from 3 to 28 days. The results show that the minimum strength reduction index of the soil corresponds to its lime stabilisation optimum (LSO). Electrical conductivity decreased monotonically and almost uniformly with an increase in leaching time, irrespective of lime content. So, too, was calcium concentration and to a lesser degree for pH and potassium concentration. Adverse changes in the physicochemical behaviour of the LLS samples occurred at lime contents below and slightly above the optimum lime content of the soil. Whereas permanent pozzolanic reactions occurred at lime contents above the LSO and thus resulted in a 45-fold increase in strength and durability. The results are significant for reducing the detrimental effect of the leaching-induced deterioration of flexible pavements founded on tropical floodplains.

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