Nitrate contamination of groundwater in the Lower Volta River Basin of Ghana: Sources and related human health risks

A significant population within the Lower Volta River Basin of Ghana relies solely on untreated groundwater (GW) and surface water (SW) for various purposes. However, negative practices associated with increasing human activities pose threats to particularly GW quality in the basin. Using NO3- as a proxy, this study mainly focused on the status of GW contamination, origins of NO3- and potential human health risks through integrated hydrochemistry, correlation analysis, isotopes (N-15, delta O-18), Bayesian and USEPA human health risk models. Slightly acidic to alkaline GW and SW environments were observed. Electrical conductivity (EC) values above 1000 mu S/cm were recorded in 45% of the GW with a maximum of 19370 mu S/cm. NO3- in GW ranged from 0.12 to 733 mg/L with average 59.6 mg/L and positively correlated with K+, Ca2+, Mg2+ Cl-, Na+ and EC. In SW, a maximum of 5.3 mg/L of NO3- was observed. Largely, 75% of the GW exceeded local background NO3- value of 2.1 mg/L, while 35% were above the WHO recommended value of 50 mg/L. Bivariate and correlation relationships elucidated human contributions to sources of NO3-, Cl-, SO42- and K+ to GW in the basin. From NO3-/Cl- ratio, 43% of the GW and 21% of SW were affected by effluents and agrochemicals. Values for delta N-15-NO3- and delta O-15-NO3- ranged from + 4.2 parts per thousand to +27.5 parts per thousand and +4.5 parts per thousand to + 19.9 parts per thousand for GW, and from +3.8 parts per thousand to + 14.0 parts per thousand and +10.7 parts per thousand to +25.2 parts per thousand for SW. Manure, septic effluents and mineralized fertilizers are sources of NO3- contamination of water in the basin. The Bayesian model apportioned 80% of GW NO3- contamination to sewage/manure. Hazard index indicated 70%, 50% and 48% medium to high-risk levels for infants, children and adults respectively, with 79% high-risk of SW NO2- contamination to infants. Immediate measures for GW and SW quality protection are recommended.