Internal phosphorus load in a Mexican reservoir through sediment speciation analysis.

ABSTRACT

Since the sequential extraction of phosphorus (P) in sediment makes it possible to determine the P potentially available for release, in this paper, we evaluate the fractions of P in sediment profiles from Valle de Bravo reservoir, a eutrophic lake in central Mexico to determine the contributions of each fraction to the internal P load (IPL). The P fractionation scheme employs sequential extractions of sediment with O2-free water (MilliQ), bicarbonate-dithionite (BD), sodium hydroxide (NaOH), hydrochloric acid (HCl), and potassium persulfate (K2S2O8-) to obtain five P fractions. A monitoring of redox potential (Eh), pH, and total phosphorus (TP) in the bottom water of the reservoir indicated variations of these parameters during the year, observing that as Eh decreased, the P concentration increased, it was also observed that when increasing pH, P concentration also increased. Analyzing the behavior of fractions of P in sediment profiles, we found that the dominant fractions are those bound to iron and aluminum oxides, corresponding to approximately 50% of total P since P concentrations of these fractions were twice as high in the top 5 cm of the sediment profiles and decreased with increasing depth. Considering the variations of Eh and pH in the bottom water of the reservoir and that these parameters are factors that control the release of P with the fractions of P bound to Fe/Mn and Al/Fe oxides, we concluded that these fractions contribute most to P potentially available for release in the reservoir, representing a possible IPL of 23.5 ± 1.4 t/year.