Recently, coastal aquifers have been found to be increasingly exposed to seawater intrusion (SWI) due to climate change and anthropogenic activities. Various method exists for coastal aquifer vulnerability mapping and the one most commonly used is GALDIT because of its simplicity. The present study modified the original GALDIT ratings and weights using Shannon’s entropy theory to study the seasonal vulnerability of coastal aquifer in the coastal region of Benin, West Africa. Thus, the monthly GALDIT index for the study region was computed using 5 years of (2015–2019) average data of GALDIT dynamic input parameters. The original and modified GALDIT approaches were validated using total dissolved solid (TDS) concentration. Pearson’s correlation and Spearman coefficient correlations were calculated, and generally the modification of the GALDIT parameters’ relative weight using entropy has improved the method as this gave a better correlation with TDS concentration (0.739). From the calculated monthly GALDIT index, the most vulnerable period was identified using TOPSIS method. Based on TOPSIS results, the coastal aquifer of Benin is more vulnerable to seawater intrusion in February due to the decrease of groundwater level in that period and less vulnerable in July. The performed sensitivity analysis showed that height of groundwater level above the mean sea level, distance from shore, and thickness of the saturated aquifer have the most influence in vulnerability to SWI assessment in the study area.

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<p>            In recent decades, urban population growth in Africa, Asia and Latin America is changing on an important rate and scale. The urban development is causing changing in surface runoff and groundwater recharge by modifying the existing mechanic. Urbanization increases the impervious area, which tends to lower the evaporation and direct infiltration of rainfall but increases surface runoff. This does not only influence the direct rainfall infiltration rate but also some urbanization practices cause water quality degradation such as increase of nitrogen, salinity, TDS and faecal contamination.</p> <p>Most of countries in arid and semi-arid regions have their water supply system relying on groundwater resource, this make the resource more important and its protection and conservation require particular attention. In areas where groundwater resource is threatened by urban development combined with climate change, Low Impact Development/best practices (LID/BP) are required for storm water treatment and infiltration, to increased direct deep infiltration of rainfall, better management of surface water which in turn can affect groundwater discharge or recharge.</p> <p>            LID has emerged recently in the last 30 years, it is promoted as suitable management practice for stormwater in urban area. Its purpose is to restore water balance in the postdevelopment site to the predevelopment conditions. LID stormwater management practice use in urbanized catchment could help to restore or increase groundwater recharge and help mitigate water scarcity issues. Many researches have investigated the effect of LID practices on surface water resource for better management but few of them has investigated the effect of LID stormwater management feature on groundwater recharge.  </p> <p>            The southern coastal sedimentary basin of Benin is recently under a demographic pressure, impervious area is increasing due to constructions and the water use is increasing proportionally to the population growth. In the region, more than 87% of the population is supplied with groundwater resource. The present study has coupled SWMM with groundwater flow model MODFLOW to investigate the influence of LID practices on groundwater recharge in the study region which is under a residential development. The hydrologic model SWMM was used to estimate groundwater recharge, the infiltration was then used to evaluate the potential effect of the development on groundwater availability. The main goal of this study is to produce a numerical model that can be used to evaluate the deficit groundwater recharge caused by a site development in the southern region of Benin and design LID structure to restore groundwater recharge as in predevelopment conditions. The study has also attempted to develop an excel sheet which will be used for groundwater recharge estimation in specific regions of Benin and the developed database can be used to estimate groundwater recharge deficit caused by a site development.</p>

In the present study, a three-dimensional SEAWAT model was developed to generally simulate the impact of climate change and anthropogenic activities on seawater intrusion (SWI) in the coastal region of Benin by the end of 2050. The model was calibrated and validated from 2015 to 2020, considering groundwater head and salt concentration measured in 30 wells. After calibration, a sensitivity analysis was performed with the model parameters (hydraulic conductivity, recharge, storage coefficient and boundary conditions). For the calibration, model computed and observed values displayed good correlation, approximatively 0.82 with a root mean square error (RMSE) of 0.97 m and 13.38 mg/L for groundwater head and salt concentration, respectively. The simulation results indicate that freshwater head had declined by 1.65 m from 2015 to 2020 (taking reference from the average groundwater head in 2015: 27.08 m), while the seawater intrusion area increased in the same period by an average of 1.92 km2 (taking reference from the seawater intrusion area in 2015: 20.03 km2). The model is therefore used to predict groundwater level decline and seawater intrusion area increase by the end of 2050, considering the predicted sea level rise (SLR) and estimated groundwater pumping rate. Furthermore, the interface fresh groundwater–saltwater change was studied using the SHARP interface developed by USGS in 1990. The interface variation was found to be influenced by the distance from shoreline, sea level, groundwater level and geological formation hydraulic conductivity. Finally, the 3D model was used to simulate the effect of a managed aquifer recharge system on reducing SWI rate in the study region.

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