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%.

     

Spatial and temporal variability of water quality and zooplankton in an artisanal salina

Conditions in artisanal salterns, or salinas, vary depending on their physical structure, seasonal effects, and the two periods of the salt production cycle. They therefore constitute unique wetland habitats, with particular communities that are still poorly studied. Water quality and zooplankton of a working artisanal salina were studied to evaluate spatial variability between sections and temporal variability between the salt production and flooded periods and between seasons. Zooplankton communities were related to environmental conditions. Sampling took place every second week, during almost two years, at six sites of the Tanoerias salina in Aveiro, Portugal: at the beginning and centre of the salina inlet channel, in a supply pond, in two evaporator and one crystalliser ponds. Since salt production occurs in the warm season only, seasonal effects concur with those caused by the salt production cycle. Results showed significant differences in temperature, dissolved oxygen (DO) and biochemical oxygen demand (BOD₅) between salt production and flooded periods, i.e. summer and spring/winter seasons. Salinity and alkalinity were linked to salt production with extreme values in the salt-producing crystalliser pond. Zooplankton communities varied between seasons, but also with the salina section and the salt production cycle. Zooplankton densities and diversity tended to be lower during salt production, particularly under the extreme living conditions in the salina crystalliser pond. Variation in zooplankton communities between samples was mainly influenced by temperature, i.e. season, at the salina inlet, by BOD₅ or DO, also related to temperature and season, in the first ponds and by nutrients throughout the system.     

Mapping Lichen Diversity as a First Step for Air Quality Assessment

The evaluation of air quality is an important topic. It is well known that lichens have a set of characteristics that make them well suited for biomonitoring purposes. Sampling lichen diversity is not as expensive as chemical analysis, allowing a dense sampling grid and reducing the costs. Lichen diversity can be used to identify more disturbed areas, resulting from pollution, land use or ecological variables. In recent years, in order to enable extended use of lichens and to reduce ambiguities, i.e., variations due to unwanted environmental variables, efforts have been made to develop a feasible protocol for lichen sampling for biomonitoring purposes. This work aims at providing the information needed a priori for an air quality assessment study, in the form of a map showing areas where lichen diversity and abundance is lower. This study was done by sampling foliose and fruticulose lichen diversity and frequency, in a region in southwest Portugal (Sines) with large industrial facilities. A long-term study has been underway in the same area since the 1970s using lichens as bioindicators to evaluate air quality. In this work, we used a standard protocol to determine a lichen diversity value (LDV), to be used as an indicator of environmental quality. In order to reduce uncertainty concerning the type of disturbance affecting lichens, sampling sites were restricted using well-defined criteria. Whenever possible, sampling site variables were quantified. This method allowed us to reduce the many sources of variability affecting lichen diversity.     

Assessment of the effect of a planned marina on groundwater quality in Enfida plain (Tunisia)

The Enfida aquifer system is of importance to the economic activity of the eastern center of Tunisia. The planning of a marina is likely to present a significant risk on groundwater. Classical physically based modeling is used to better understand salty water intrusion in the aquifers. The transport model (MT3DMS) is coupled with the groundwater model (Modflow). Model calibration was carried out over the period 1972–2005. Four scenarios were then simulated for a 50-year period, to assess the effects of both planned marina and future abstraction regime. We predict a rise in the groundwater salinity generated by the planned pumping infrastructure. The impact of the planned project will be observed only near the marina. However, limited measurements of transmissivity may affect the model’s results. Thus, the second part of the paper is aimed to assess the models output error due to the uncertainty in transmissivity, using a stochastic approach. Hundred realizations of a log-normal random transmissivity field had been performed. According to the most pessimistic realizations, the uncertainty may reach 49 % in the sector of an important pumping field. Accordingly, the calculated concentration may reach 6.5 g/l in 2055 instead of 3.2 g/l.