Groundwater remains an important source of freshwater to meet daily water demand, especially in the developing country of sub-Saharan Africa [1
]. It complements the water supply shortage experienced as a result of failed infrastructure. This water source must meet a certain quality standard to ensure safe consumption, based on the physicochemical composition of groundwater controlled by specific hydrochemical processes such as oxidation-reduction, evaporation, precipitation, and dissolution. The chemical composition of groundwater is not restricted to only its source(s), but also by the biogeochemical processes occurring along the groundwater flow path, particularly within the aquifers where freshwater interacts with several other inorganic and organic substances either geogenic or of anthropogenic origin [5
Floods and drought are the common impacts of climate change on water availability [8
]. In shallow coastal aquifers, flooding can also impact the quality of groundwater, especially in the unconfined aquifer with porous overburden characterised by high hydraulic conductivities offering little or no attenuation to contaminants coming from polluted surface water from the flooded river and lagoons. Recently, substantial rainfall has triggered seasonal flooding in the coastal area of the Eastern Dahomey basin, and poses a threat to groundwater quality in shallow coastal aquifers [9
Climate change is expected to result in sea level rise, ocean surge [11
], and flooding of the coastal plains. This combined with contamination from urbanization and waste disposal, results in a mixture of waste dumped in open refuse sites being carried along river channels [9
]. The result is mineralization and nutrient loading in surface water with dissolution, advection, and absorption, eventually percolating into shallow aquifers [12
]. In most cases, the recharge of floodwater to groundwater takes between a few days to a few weeks, which facilitates biogeochemical activities that result in ion and metal concentrations that could degrade the quality of groundwater. Notably, among these biogeochemical processes are oxidation and reduction, which drive ion and metal precipitation in surface and groundwater. Nutrient loads in surface water are expected to increase during precipitation and redox process with climate-induced flooding [12
]. It may also be possible, via field or modelling experiments [12
], to assess the impacts of climate change on individual components contributing to eutrophication.
The relationship between climate change and drinking water is casual and complicated. This is due to the climate change-related effects of extreme precipitation, temperature rise, and waterborne diseases being systematically interrelated with different types of microorganisms, geographical area, season, type of water supply, water source, and watershed characteristics [15
] (Figure 1
). There are substantial knowledge gaps regarding systemic causes and effects with considerable uncertainty about how heavy rainfall, microbial pollution of water supplies, and increased turbidity are interrelated. Recently, several outbreaks of waterborne diseases were attributed to extreme hydrological events in sub-Saharan Africa and other parts of the world [16
]. This risk is enhanced in the densely populated coastal areas of a developing country like Nigeria, as waste management is a challenge due to indiscriminate dumping of waste from municipal, industrial, and agricultural activities across the cities [9
The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report failed to capture, in detail, the impacts of climate change on water quality. Researchers have established the impact of climate change-driven flooding on surface water quality, especially in the coastal zones of the world where extreme precipitation occurs [20
], whereas research in the area of climate change impact on groundwater quality is still in its early stage [18
]. This is probably due to the invisibility of groundwater and the fact that groundwater quality monitoring data in the study area is rarely conducted. Previous studies [2
] have identified elevated concentrations of ions, heavy, and traced metals in groundwater in shallow aquifers of the coastal cities of Nigeria, many of which attributed groundwater contaminations to indiscriminate waste disposal, effluents from sewage, industrial, and agricultural waste.
The study aimed to delineate the possible origin of a high elevated concentration of some ions that could impact the quality of groundwater within this basin, to serve as a fingerprint of the impact of climate change driven flooding on groundwater of the shallow coastal freshwater aquifer of Eastern Dahomey Basin (EDB). Understanding will guide policy for the Sustainable Development Goals (SDGs), management of these resources, and promotion of resilience in the wake of future climate change impacts.
The objectives of this study were to investigate the impacts of climate change through flooding on groundwater quality of the shallow coastal groundwater of the Eastern Dahomey basin, using seasonal redox biogeochemical process of some selected redox-sensitive ions such as Fe, Mn, NO3
, and SO4.
Understanding the possible impacts of this seasonal flooding on the groundwater quality of the freshwater aquifer is critical to the management of this essential resource. Oxidation and reduction processes have been shown to have a significant influence on the hydrochemical status of groundwater by either removal or addition of dissolved material which controls its overall quality [13
]. This process also controls the solubility and stability of many dominant species in groundwater. Groundwater redox process can be influenced by various factors such as recharge, presence of contaminants, local groundwater flow, and well construction configuration and development, which determines the available electron acceptors [6
]. The redox variation within a basin could establish a relationship with the spatial variability of some of these ions which could be linked to the redox condition. As a result of this, groundwater from areas with a similar redox condition will generally exhibit chemical stability for redox, while groundwater from areas with varied redox conditions relating to different controlling factors display various characteristics regarding redox-influenced groundwater chemistry [28
This study examined possible links between climate change and groundwater chemistry in the shallow coastal aquifers of Eastern Dahomey basin. There is evidence of chemical changes due to seasonal flooding resulting from extreme precipitation. Thus, we propose the biogeochemical processes of redox and other hydrochemical reactions, which might be influenced by future climate change. These processes are responsible for metals and ions mobilization/remobilization in surface and groundwater of this shallow coastal aquifer. Spatial distribution maps of SO42−, NO3−, Fe, and Mn revealed higher concentrations within the Alluvium and Coastal Plain Sands aquifer and some wells with the river flood plains of the Basin. The generalized Eh-pH diagram revealed groundwater of the study area are within the transitional (aerobic to anaerobic) phase of the geochemical thermodynamics. It also revealed that Nitrate is a dominant species in the groundwater from both wet and dry seasons, while Iron occurrence is restricted to the alluvium and coastal plain sands aquifer. This study also revealed that the reduction in concentration and nutrient enrichment of groundwater with the increase in well depth can be attributed to the loss of aeration and possible natural attenuation by the soil and aquifer medium. The linear regression model using both Eh and pH as independent variables against other selected variables such as EC, salinity, SO42−, Cl−, NO3−, Fe, and Mn for both season groundwater samples established predictability for both Eh and pH. A possible indication of ions and metals enrichment in groundwater resulting from the effect of seasonal flooding. This enrichment rendered some of the sampled groundwater unsuitable for drinking as the concentration of some of the water quality parameters were above the WHO standard for drinking water. Accumulation could lead to severe contamination of surface and groundwater and render it not potable for drinking or other purposes. This study recommends policy and management enhancements for proper waste management to support SDG planning for sustainable groundwater resource management in the Eastern Dahomey Basin that limits further contamination of the coastal groundwater system in the face of seasonal flooding, especially as predictions suggest more extreme precipitation in the future resulting from climate change.