Dynamics of sediment phosphorus in the middle and lower stretch of River Ganga, India: insight into concentration, fractionation, and environmental risk assessment of phosphorus

Despite continuous efforts, eutrophication is still occurring in freshwater and phosphorus (P) is the most important nutrients that drive the eutrophication in rivers and streams. However, little information is available about the distribution of P fractions in river sediment. Here, the sequential extraction approach was used to evaluate the sediment P fractionation and its content in the anthropogenically damaged river Ganga, India. Different sedimentary P fractions viz. exchangeable (Ex-P), aluminum bound (Al–P), iron bound (Fe–P), calcium bound (Ca–P), and organically bound phosphorus (Org-P), were quantified. Significantly higher level of total P was recorded in pre-monsoon season (438.5 ± 95.8 mg/kg), than other [winter (345.7 ± 110.6 mg/kg), post-monsoon (319.2 ± 136.3 mg/kg), and monsoon (288.6 ± 77.3 mg/kg)] seasons. Different P fractions such as Ex-P, Al–P, Fe–P, Ca–P and Org-P varied from 2.88–12.8 mg/kg, 7.64–98.8 mg/kg, 32.2–179.2 mg/kg, 51.97–286.1 mg/kg and 9.3–143.7 mg/kg, respectively, which correspondingly represented 0.5–10.54%, 3.41–20.18%, 17.27–37.82%, 37.35–60.2%, 4.15–25.88% of the Total P with a rank order of P-fractions was Ca–P > Fe–P > Org-P > Al–P > Ex-P. Bio-available P contributes a considerable portion (37.9–46.0%) of total P which may increase the eutrophication to overlying water. Results demonstrate that inorganic P species control the P bio-availability in both time and space. However, an estimated phosphorus pollution index based on sediment total P content showed no ecological risk of phosphorus to Ganga River sediment.