Owing to a lack of efficient solid waste management (SWM) systems, groundwater in most developing countries is found to be contaminated and tends to pose significant environmental health risks. This review paper proffers guidelines on the application of isotopic techniques to trace groundwater pollution sources from data spanning from 2010 to 2020 within developing countries. Earlier groundwater studies in those countries were mainly focused on using hydrochemical and geophysical techniques. The limitation of these techniques is that they can only monitor the concentration of pollutants in the water bodies and possible leachate infiltration but cannot determine the specific sources of the pollution. Stable isotopes of δ¹⁸O, δ²H and δ¹³C can confirm leachate migration to water bodies due to methanogenesis. The high tritium in landfill leachates is useful to identify leachate percolation in groundwater. The δ¹⁵N technique has been used to distinguish between synthetic and organic nitrogen sources but its application is limited to differentiating between atmospheric vs. inorganic nitrogen sources. The use of a dual isotope of δ¹⁵N–${\mathrm{NO}}_3^{-}$ and δ¹⁸O–${\mathrm{NO}}_3^{-}$ is beneficial in terms of identifying various sources of nitrogen such as atmospheric and inorganic fertilizers but is yet to be used to differentiate between nitrogen pollution sources from dumpsites, sewage and animal manure. The coupling of the ¹¹B isotope with δ¹⁵N–${\mathrm{NO}}_3^{-}$ and δ¹⁸O–${\mathrm{NO}}_3^{-}$ and other hydrochemical parameters has proven to be effective in distinguishing between nitrate fertilizer, animal manure, seawater contamination and sewage. Therefore, in areas affected by agricultural activities, landfill leachates, domestic or sewage effluent and seawater intrusion, it is incumbent to couple hydrochemical (${\mathrm{Cl}}^{-}$, ${\mathrm{NO}}_3^{-}$, B, DO) and isotope techniques (δ¹⁸O, ²H, δ¹³C, δ¹⁸O–${\mathrm{NO}}_3^{-}$, δ¹⁵N–${\mathrm{NO}}_3^{-}$, δ¹¹B and ³H) to effectively determine pollution sources of groundwater in developing countries. The foregoing review will provide guidelines for studies that may aim to critically distinguish between seawater intrusion, dumpsites, sewage and septic leachates.

Consumption of water contaminated with heavy metals has serious health consequences and these include hypertension, kidney disease, cancer, anaemia, brain damage, etc. However, groundwater pollution with heavy metals may occur naturally or anthropogenically as a result of water-rock interaction, dumpsite leachate, agricultural activities, or sewage percolation. Therefore, the apportionment of the specific sources of heavy metals to groundwater is vital to prevent and control its pollution to groundwater. The present work assessed thirty-four (34) groundwater samples from the vicinity of the Granvillebrook and Kingtom dumpsites, Freetown, Sierra Leone. The objective of the research was achieved by integrating geospatial, indexical, and multivariate statistical methods. The results showed that heavy metals such as Cu, Cr, Fe, and Mn exceeded the permissible levels in the majority of the groundwaters. As and Pb were below the detection level of the instrument in all the groundwaters analyzed. Seasonal variations revealed that the concentration of heavy metals is relatively higher in the dry season due to the effect of dilution. The heavy metal evaluation index and degree of contamination analyses respectively showed that 16.7 and 41.6% of the groundwaters in the Granvillebrook dumpsite are highly polluted by heavy metals. All the groundwater samples in the Kingtom dumpsites except well KW17 are unaffected by heavy metals. The principal component analysis showed that fuel combustion, brake abrasion, and discharge of municipal and industrial waste from the dumpsites are some of the anthropogenic sources that impact groundwater pollution in the study areas. The findings of this research revealed the urgent need to develop strategies to control and mitigate the occurrences of heavy metals and thus serve as a base for future research in the study areas.