Chemical And Isotopic Composition of Rainwater in The Coastal, Forest And Mountainous Ares of Volta Region of Ghana

ABSTRACT 

Harvesting rainwater for domestic use is one of the oldest technologies developed by man. In Ghana, groundwater utilization challenges posed by dry wells and high levels of contaminants such as iron, fluoride and other minerals in some geological formations makes harnessing of rainwater for household use inevitable. Whiles researchers are looking for efficient ways of dealing with contaminants associated with these water sources, focus is gradually shifting to the development of enhanced methods for rainwater harvesting. In this study, rainwater samples were collected from Akatsi ( a coastal zone), Amedzofe (high altitude zone) and Kpando ( a forest zone) in the Volta region of Ghana and analyzed for the physico-chemical parameters such as pH, electrical conductivity (EC), total dissolved solids (TDS), colour, turbidity, alkalinity, Ca2+, K+ , Na+ , Mg2+ , NO3 - , NH4 + , pH, Cl- , and SO4 2-. The results showed that, pH values of the Akatsi, Amedzofe and Kpando rainfall ranges from 5.40 to 6.85, 4.3 to 6.3 and 4.76 to 6.00 respectively. Amedzofe showed a relatively low pH with 13.70% ranging from 4 to 5, 82.40% of the rainfall events fell within pH of 5 to 6 and 3.90% had pH less than 7 but greater than 6. About 96.10% of the total rainfall events had pH less than 6. EC, TDS, turbidity and colour were generally low with higher values recorded at Akatsi while Amedzofe recorded the lowest values. The concentration of major ions and element-to-sodium ratios showed clear variations with higher levels found in Akatsi (coastal zone) and lower levels found in Amedzofe. Anion order of dominance in Akatsi, Kpando and Amedzofe are HCO3 - >SO4 2- >Cl- >NO3 - , HCO3 - >SO4 2- >Cl- >NO3 - and HCO3 - >Cl- > SO4 2- >NO3 - respectively, while cation order of dominance are Na+ >Mg2+>Ca2+, Ca2+> Na+ >Mg2+and Na+ > Ca2+>Mg2+ for Akatsi, Amedzofe and Kpando respectively. Isotopic composition of rainwater was also determined for the three areas. Akatsi (coastal zone) presents high values of δ 18O xiii and δ 2H, whereas rainwater in Amedzofe was characterized by low values of δ 18O and δ 2H, Kpando on the other hand was intermediately high. The Local Meteoric Water Line for the study area using rainfall greater than 20mm was defined as δ 2H =7.453 δ18O + 8.621 (n=45; R2 =0.907, rain > 20mm). Isotope fractionation in precipitation, including depletion in both δ18O and δ2H is primarily due to the rainout and the altitude effects with temperature effect being insignificant. The calculated altitude effect shows a gradual lapse rate of δ 18O and δ 2H of about -0.2‰/100 m and -1.5 ‰/100 m for Akatsi and Amezofe respectively. Amedzofe and Kpando showed an altitude effect of -0.15‰ /100 m. and -2 ‰/100 m for δ 18O and δ2H respectively. The groundwater in the study area is mainly of meteoric origin with different months of recharge. The plot of δ18O and EC shows that, mineral dissolution is the main factor controlling the groundwater salinization in Akatsi while Amedzofe and Kpando groundwater chemistry are mainly controlled by evaporation.