Characterising groundwater and surface-water interconnections using hydrogeology,hydrochemistry and stable isotopes in the Ouémé Delta,southern Benin

The Mio-Pliocene aquifer of the coastal sedimentary basin of Benin is the most exploited aquifer for water supply to the urbanised region in the southern part of the country. The population explosion is putting increasing pressure on quantitative and qualitative aspects of the groundwater resources. Preventing groundwater contamination caused by surface waters requires a thorough understanding of surface-water/groundwater interactions, especially the interactions between the Mio-Pliocene aquifer and surface waters. This study aimed to investigate the interactions between groundwater and surface waters along the major rivers (Sô River and Ouémé Stream) and brooks in the Ouémé Delta. Field campaigns identified 75 springs located in the valleys which feed the rivers, and thus maintain their base flow. The piezometric results indicated, through flow direction assessment, that the Mio-Pliocene aquifer feeds Ouémé Stream and Sô River. Chemical analyses of groundwater and surface waters show similar chemical facies, and changes in the chemical composition in groundwater are also observed in the surface waters. Moreover, the isotopic signatures of surface waters are similar to those of the groundwater and springs, which led to the identification of potential groundwater discharge areas. As a result of groundwater discharge into surface waters, the fraction of groundwater in the surface water is more than 66% in the brooks, regardless of the season. In the Ouémé Stream and Sô River, the fraction of groundwater is 0–21% between June and September, while from October to March it is 47–100%.

     

Optimal tonal detectors based on the power spectrum

The detection of tonals embedded in noise is an important sonar function and the traditional power spectrum analysis method has been widely used for this purpose. Wagstaff et al. (1997) proposed the WISPR (Wagstaff's Integration Silencing PRocessor) family processors, which perform a nonlinear integration or combination of the power spectrum observations. In this paper, we analyze the statistical property of the power spectrum observations and develop novel tonal detectors by optimally integrating the spectrum observations. The optimal detectors are derived by using the method of maximum likelihood hypothesis test. The results from simulations and real sea trial data have shown that the proposed detectors are promising in detecting tonals     

An Evidential Reasoning Approach to Land Degradation Evaluation: Dempster-Shafer Theory of Evidence

This study explores the combination of expert and local knowledge for an integrated evaluation of land degradation in southern Mauritania. The expert knowledge uses two imprecise information and uncertainty management tools, Fuzzy measure theory and Dempster‐Shafer theory, mediated by expert knowledge, to integrate data collected using local knowledge. Fuzzy measures were found to be an efficient way of handling uncertainty of data compiled from local environmental knowledge and uniformly scaling all land degradation indicator variables to the same numerical range of values. The two techniques allowed scaling and integration of 13 evidential themes characterizing southern Mauritania. The cartographic outputs of this study show the potential of Fuzzy measure theory and Demspter‐Shafer theory as powerful complementary environmental resource management tools. On a scale of 0.0 to 1.0, the maps indicate the degree of degradation. The seriousness of land degradation decreases from settlements, roads, and watering points outward. As such, they constitute an excellent guide for resource allocation and rehablitation plans.     

Spatial and temporal variability of salinity in Casamance River Basin,Southern Senegal,West Africa

Salinity in the Casamance River Basin has been on the rise for quite some time. This is believed to be further accentuated by declining rainfall. The salinity patterns in time, space and frequency domains are analysed. These patterns are useful in designing measures not only for arresting the rise in salinity but also for mitigating it. © 1998 John Wiley & Sons, Ltd.     

Capacity for water conservation in invasive (Gerbillus nigeriae) and declining rodents (Taterillus pygargus and Taterillus gracilis) that exhibit climate-induced distribution changes in Senegal

Following the southward shift of rainfall isolines in the Sahel at the end of the 1960s, Gerbillus nigeriae appeared in northern Senegal in the mid-1990s, and two resident Gerbillidae (Taterillus pygargus and Taterillus gracilis) subsequently declined. We investigated the causal role of the capacity to conserve water in such climate-related shifts in the distribution of these Gerbillidae by comparing the effects of a water-poor diet on the water-efflux rate (W_−out) of freshly trapped adults pre-acclimatized to a water-rich diet. During the 12-day period of water restriction in all species, 30-50% of individuals became hyperactive and showed greater weight loss and higher W_−out than the remaining inactive individuals. Such emergence of migratory strategists within populations could accelerate the expansion of G. nigeriae. On a water-poor diet, T. gracilis showed a lower capacity to conserve water (higher W_−out) than T. pygargus and G. nigeriae, in both inactive (W_−out = 44.5 ± 1.8 vs 29.6 ± 0.8 vs 27.4 ± 0.7 ml kg^−0.82.day⁻¹, respectively) and hyperactive individuals (W_−out = 60.4 ± 1 vs 45.4 ± 0.7 vs 44 ± 0.8 ml kg^−0.82.day⁻¹, respectively). We propose that the capacity to conserve water accounted for both expansion of G. nigeriae and decline of T. gracilis, whereas competition between T. pygargus and G. nigeriae could account for the decline of T. pygargus.     

Reduced sulfur and iron species in anoxic water column of meromictic crater Lake Pavin (Massif Central, France)

The vertical distribution of reduced sulfur species (RSS including H₂S/HS⁻, S⁰, electroactive FeS) and dissolved Fe(II) was studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (∑H₂S_MBRS) were in the range from 0.6 µM to 16.7 µM and were substantially higher than total reduced sulfur species (RSS_V) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 μM. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of FeS colloidal species.Dissolved Fe was high (> 1000 µM), whereas dissolved Mn was only 25 µM, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, “free” sulfide, H₂S/HS⁻, was low; and about 80% of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeS_am phase. The upper part of monimolimnion layer is characterized by higher concentrations of S(0) (up to 3.4 µM) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide species.     

Biogeochemical modelling of anaerobic vs. aerobic methane oxidation in a meromictic crater lake (Lake Pavin, France)

Methane is a powerful greenhouse gas and its concentration in the atmosphere has increased over the past decades. Methane produced by methanogenic Archae can be consumed through aerobic and anaerobic oxidation pathways. In anoxic conditions found in freshwater environments such as meromictic lakes, CH₄ oxidation pathways involving different terminal electron acceptors such as , , and oxides of Fe and Mn are thermodynamically possible. In this study, a reactive transport model was developed to assess the relative significance of the different pathways of CH₄ consumption in the water column of Lake Pavin. In most cases, the model reproduced experimental data collected from the field from June 2006 to June 2007. Although the model and the field measurements suggest that anaerobic CH₄ oxidation may contribute to CH₄ consumption in the water column of Lake Pavin, aerobic oxidation remains the major sink of CH₄ in this lake.     

Sustainable development consequences of European Union participation in Senegal's Marine Fishery

The article evaluates the impact of EU fishing activities for the sustainable development of Senegal's maritime fishery. An evaluation of Senegalese fisheries’ policy discourse introduces the idea that Senegalese fisheries’ policies have not been fully compatible with the sustainable development of the country's fishery sector. A comparative analysis of the sustainable development impacts associated with the activities of different fishery user groups shows that managed activities of heterogeneous user groups can benefit the sustainable development of the sector. Conclusions suggest that the heterogeneity of the user groups frequenting Senegal's marine fishery system can be capitalised upon through allocating fishery access in accord with each group's ability to generate sustainable development benefits. On the basis of these findings the article suggests that the presence of an EU fleet in Senegalese waters could play a meaningful role in ensuring the sustainable development of Senegal's marine fishery.     

Benchmarks for validating range-dependent seismo-acousticpropagation codes

The availability of fast and relatively low-cost computing power has resulted in radical changes to the role of seismo-acoustic modeling. With the increase in the number of models available, there is the inevitable question of how can one go about validating all these numerical schemes. Recently, the issue of establishing reference solutions for range dependent ocean acoustic problems was addressed within the Acoustical Society of America. This has resulted in a set of well-defined benchmarks for range-dependent fluid problems. However, to date, there is no consistent set of benchmarks for the range-dependent seismo-acoustic codes. In this paper, we present a collection of problems intended for general use by the modeling community for validation of new computational schemes. A number of new seismo-acoustic codes are applied to produce reference solutions for these benchmarks