About 73–90% of total phosphorus in the bioretention outflow was
present as bioavailable ortho-phosphate (PO4
3−). Like nitrates, total
and ortho-phosphate decline gradually over successive storms, with
the Silver Springs bioretention system releasing more phosphorus
than Royal Classics (Fig. 3), and with significantly higher concentrations
in the first storm. By the end of the seventh storm, about 5–6 mg/L of
total phosphorus was still being leached. The bioretention systems
were the sole source of phosphorus in the leachate; the influent
stormwater had phosphorus concentrations below detection limits
(b0.01 mg/L). Phosphorus export was also reported by Mullane et al.
(2015), who studied composts of different ages, and Hunt et al.
(2006), who used bioretention systems whith soils saturated with
phosphorus