Feasibility Analysis of MERIS as a Tool for Monitoring Lake Guiers (Senegal) Water Quality

ENVISAT/MERIS scenes of Lake Guiers covering the period 2003-2010 were processed for concentration retrieval of chlorophyll a (CHLa), suspended particulate matter (SPM) and colored fraction of dissolved organic matter (CDOM), i.e. the three main parameters relevant to the water quality management of the lake. Estimates in the range of 30 - 117 μg CHLa L-1 (average 62.13 μg·L-1), 0.10 - 29.0 mg SPM L-1 (average 22.01 mg·L-1), and 1.10 - 1.90 CDOM m-1 (average 1.33 m-1) were recorded, suggesting the possibility of occasional poor quality waters in some compartments of the lake. The values calculated as part of this study are consistent with literature data. On the basis of these estimates, interpretations were made as to the feasibility of applying MERIS data for synoptic environmental monitoring purposes. The data were subjected to statistical analysis, including regression analysis and significance tests. Estimates of CHLa and CDOM revealed some level of correlation, which suggests that phytoplankton biomass degradation may account for nearly 47% of the dissolved optical compounds CDOM. Notable areas of high CHLa and CDOM concentrations are found in the southern inshore zone, an environment with less water agitation. In contrast, SPM concentrations tend to increase in environments of very shallow water marked by high water turbulence and bottom mobility. However, it was not possible to fully assess the model performance and detection accuracy of the results due to lack of ground truths. Nonetheless, the results show concentrations that compared well with the insitu data from earlier studies and data reported elsewhere from other lacustrine systems. Therefore, it can be inferred from this study that MERIS data present a useful low-cost (i.e. cost effective and readily available) approach for environmental monitoring of Lake Guiers waters with excellent spatial coverage. In addition, the study highlighted the minimal effect of the so-called “bottom effect” on model predictions, despite the small depth of the lake.

Hydrogeochemical Relationships between Spring and Subsurface Waters in the Dindefello Area of South Eastern Senegal

The analytical results of water samples collected from the Dindefello Plain area revealed the dominant major ions of bicarbonate, silica, calcium, magnesium and sodium, in order of decreasing relative abundance. Spring water exhibited a very low, i.e. similar to rainwater, mineralization status, unlike groundwater. To better understand the origin of mineralization, the data were further analyzed for interrelationships between parameters and for mineral-water relationships using speciation calculations. This provided various tenuous lines of evidence for speculating various modes of groundwater mineralization. Hydrochemical evaluation of major ions suggested an early stage of mineralization which could be attributed to mineral breakdown driven by pH in the vadose zone under the leaching action of H+. Conceivably the soil in recharge areas supplies CO2 to infiltrating rainwater, thus adding to the amount of aqueous CO2 (H2CO3) already entrapped from the atmosphere. Then, H+ ions, produced from the dissociation of aqueous CO2 ( ), reacts along groundwater flow paths with the carbonaceous-sulfated and silicated minerals, resulting in the observed major ion chemistry. The spatial variations in the groundwater chemistry indicated that the type of bedrock aquifer has an effect on the water chemistry.

Study on the Applicability of Landsat-8 Images as a Tool for Monitoring the Trophic State of Lake Guiers (Senegal)

In this study, an algorithm to transform LANDSAT 8 images of Lake Guiers into chlorophyll-a concentration maps was developed. The objective was to see the feasibility of this method as a mapping tool for monitoring the trophic state of the Lake. The available data are 15 in-situ measurements of the CHL-a content of Lake Guiers’ water using a Micro FLU device and images of the lake acquired in 2013 and 2016 by the LANSAT 8 OLI sensor. A series of correlation tests (simple linear functions, multi-variables, or containing two- order terms) were performed between the spectral sensor signal of June 19, 2016 LANSAT 8 image (“corrected” and converted into reflectance) and the CHL-a content of Lake waters measured on the same day of the satellite record. This was, to find the best regression equation that can be inferred to develop an algorithm to transform LANDSAT 8 image into CHL-a concentration maps. The best equation obtained by correlating the ground truths and the reflectance ratios of the visible and near infrared bands (B2, B3, B4 and B5), is as follows: where B2 = Blue band, B3 = Green band, B4 = Red band, B5 = NIR band. The obtained R2 is 0.703 (e.g., a correlation coefficientof 0.84), revealed that the measured concentrations are well reproduced by the regression model. Applying this model to the 2013 and 2016 image data revealed the monthly variations of the lake CHL-a contents and their seasonal evolution and interannual variations. The obtained results are quite consistent with those from similar studies and provide valuable information regarding Lake Guiers’ vulnerability to algal development. The calculation and analysis of the variability of the CHL-a contents over the study period made it possible to evaluate the trophic state of the Lake, which shows a general tendency for eutrophication process, leading to a progressive degradation of the quality of the water resource.

Chemical Evolution of Groundwater in the Dindefello Plain Area in South-Eastern Senegal

This study was to clarify the main mechanisms of shallow groundwater mineralization in the Dindefello Plain Area. Water composition data obtained in this study were subjected to aqueous speciation calculations together with data plotting on key diagrams so as to create work assumptions. Hypothesized reaction models of the processes of chemical weathering of carbonates and silicate minerals under the carbon dioxide regime were proposed and tested by selecting two water sample analyses interpreted as “starting” and “ending” water composition along a hydrologic flow line, and then running the PHREEQC (version 2) batch modeling procedure, to simulate chemical balances and compositional variations of groundwater within the geochemical system. For the flow path data discussed here, there was close agreement between the model results and the observed hydrochemistry, and so the proposed geochemical evolution model was deemed reliable.

Diagnosis Study of the Louga-Ouarack-Ndoyene R31 Regional Road (Senegal)

The degree of structural damage of the Louga-Ouarack-Ndoyene R31 regional road was surveyed using Lacroix deflectographe and geotechnical testing, in order to recommend possible rehabilitation measures aimed at maintaining the high level of service over the projected life span of the road. The data processing and interpretation were essentially based on the French standards and specifications, through which synthetic variables were generated with a qualitative significance?and on the basis of which the extent of repairs and rehabilitation works to be undertaken were suggested. The decision grid thus produced, revealed road structure quality indices of mostly Q3, Q4, and Q5 types for a segment length of 36.954 km (equivalent to some 69.44% of the total road section), which justifies some major reinforcement works. The remaining part of the road section (nearly?30.56% with road quality indices ≤ Q2) is still in acceptable condition, requiring only some?minor maintenance works. The rehabilitation costs were estimated to be of the order of?5,352,000,000 (five billion three hundred fifty-two million) CFA francs, based on local market price conditions.