Sources, pollution, and ecological risk assessment of polycyclic aromatic hydrocarbons (PAHs) in Porto-Novo Lagoon, Benin Republic

The Porto-Novo Lagoon is influenced by agricultural discharges and human activities. In order to evaluate the impact of wastes and human activities on Porto-Novo Lagoon, the sources and ecological risks of sixteen polycyclic aromatic hydrocarbons (PAHs) were assessed. The physicochemical and biological parameters of the water were also determined. The result showed that between the sampling sites, the mean concentration of dissolved oxygen ranged from 4.8 +/- 0.5 to 5.1 +/- 0.2 mg/L; biochemical oxygen demand varied from 12.6 +/- 2.0 to 77.9 +/- 81.9 mg/L; biological oxygen demand ranged from 2.8 +/- 2.6 to 5.6 +/- 0.9 mg/L; total phosphorus varied between 4.7 +/- 2.7 and 15.3 +/- 9.5 mg/L; total dissolved solids ranged from 183.0 +/- 115.8 to 337.5 +/- 413.3 mg/L, and Escherichia coli varied from 495.0 +/- 542.9 to 1920.0 +/- 2676.5 UFC/100 mL. Water parameter values obtained were not within World Health Organization (WHO)-recommended limits except pH and TDS. Total PAHs ( n-ary sumation PAHs) concentration varied from 38.8 to 123.9 mg/L. The mean n-ary sumation PAH concentration was 83.2 +/- 20.3 mg/L. Benzo[b]fluoranthene, benzo[g,h,i]perylene, and benzo[k]fluoranthene were the most dominant PAHs and contributed to 55.9%, 15.3%, and 4.5% of the n-ary sumation PAHs concentration, respectively. Douane-Tokpa and Djassin recorded the lowest and highest concentrations. PAHs of four to six rings were the most abundant across the sampling sites. Naphthalene showed the lowest risk in the lagoon. Acenaphthene showed low risk at Djassin, while Indeno(1,2,3 cd)pyrene showed low risk at Benin Industry Body Fat. Except for those that were not detected, all the PAHs at individual or complex mixture levels showed high risk at all the sites. The highest total concentration was recorded in Djassin followed by Beaurivage. The high level of PAHs pollution was attributed to both human and goods traffic, runoff, and the complex hotels close to the lagoon. Molecular diagnostic ratios and principal component analysis suggest that the target hydrocarbons were from both petrogenic and pyrogenic sources with predomination of vehicular emission and coal/woods combustion. n-ary sumation LWM/HWM confirmed also the predominance of pyrolytic sources of PAHs in Porto-Novo Lagoon. The predominance of the vehicular emission may be due to the position of the complex Porto-Novo Lagoon-Nokoue Lake which is between the two big cities of the country.