Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania

Abstract

Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer overexploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na+ and Cl- near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na-Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca-HCO3 and Ca-Mg-HCO3 groundwater types. Further along flow paths, Ca2+ and Na+ ion exchange causes groundwater evolution to Na-HCO3 type. From the PHREEQC simulation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system

Assessment of Long-Term Evolution of Groundwater Hydrochemical Characteristics Using Multiple Approaches: A Case Study in Cangzhou, Northern China

“…Accordingly, the water in this region may derive from salt water (leakage from the salt water aquifer overlying the deep groundwater aquifer, Figures 2 and 3) or seawater intrusion [13,53]. Most of the shallow groundwater plots in Zone II are spread over the middle part of the diamond area, suggesting that mixing of fresh water and salt water occurs [5]. The majority of deep groundwater plots are located in the bottom right portion of the diamond area, indicating that the dominant ions of deep groundwater are Na In both shallow and deep aquifers, sediments are mainly composed of quartz, calcite, dolomite, albite, fluorite, halite, gypsum and sylvite [53,55]; therefore, the dissolution of these minerals can be a possible source of major groundwater ions.…”

“…The majority of deep groundwater plots are located in the bottom right portion of the diamond area, indicating that the dominant ions of deep groundwater are Na In both shallow and deep aquifers, sediments are mainly composed of quartz, calcite, dolomite, albite, fluorite, halite, gypsum and sylvite [53,55]; therefore, the dissolution of these minerals can be a possible source of major groundwater ions. Moreover, Ca Cation exchange, which is one of the most important hydrogeological process occurring in aquifers, affects the content of cations in groundwater and modifies the groundwater quality [5,13,22]. According to the stratum structure and lithology characteristics revealed by drilling holes in the NCP [34], sediments in the study area mainly consist of silt, silt clay and clay (Figures 2 and 3), and Ca The ratio of Na can be estimated based on the ratio of their milligram equivalents, with 1:1 indicating that they originate from dissolution of gypsum [40].…”

“…Groundwater is of particular importance for public drinking water supply, especially for the residents of arid and semi-arid regions [1,4,5]. However, rapid urbanization and increasing populations have accelerated the consumption of groundwater resources and caused serious environmental problems in the last few decades [6,7], and numerous studies have shown serve groundwater pollution, as well as adverse effects caused by exploitation, such as land subsidence, water ecosystem degradation, land desertification, drinking water pollution and associated human health risk, etc.…”

“…Groundwater availability and quality has become the source of concern for researchers worldwide. Most studies of these issues focused on the saltwater intrusion [5,11], groundwater salinization [12][13][14][15], arsenic and other heavy metals in groundwater [16][17][18][19], the sustainability of aquifer exploitation [20] and groundwater quality evaluation [21]. The stability and safety of water supply associated with long-term groundwater exploitation are essential to sustainable development.…”

Hydrogeochemical assessment of a coastal aquifer using statistical and geospatial techniques: case study of Mombasa North Coast, Kenya

“…For instance, Mtoni et al (2012) employed a hydrochemical approach in investigating the interaction between the seawater and the Quaternary aquifer of Dar es Salam and classifying the water types based on the prevalent ions in the water samples. A further groundwater study was conducted in the same region to identify the main hydrogeochemical processes controlling the quality of groundwater as well as the possibility of anthropogenic pollution and seawater intrusion (Mtoni et al 2013). Another study was conducted on the coastal aquifer of South East Tanzania to detect the groundwater sources and the main factors influencing the groundwater quality with the aid of multivariate statistical analyses (Bakari et al 2011).…”