Influence of Salinization and Freshening on the Retention of Phosphorus in Non-Amended and Al-WTR Amended Bioretention Media

Bioretention systems can remove pollutants including phosphorus (P) from urban stormwater but in cold climates P retention is complicated by factors including seasonal inputs of de-icing road salts (typically sodium chloride [NaCl]). The objective of this study was to evaluate the influence of stormwater with variably high NaCl concentrations on soluble reactive P (SRP) retention in bioretention media considering the effects of salinization and freshening. Column experiments were conducted using three field bioretention media with and without an aluminum water treatment residual (Al-WTR) amendment added. Columns were exposed to synthetic stormwater influent with NaCl concentrations switching between 10 and 1000 mg/L. Overall, there was net SRP release from the columns with no amendment added, whereas Al-WTR amended columns showed net SRP retention. For non-amended columns, effluent SRP concentrations were significantly higher during the freshening periods compared to the salinization (high salt) and regular (low salt) periods with the timing of maximum SRP concentrations delayed relative to the onset of freshening. The high SRP release during freshening periods co-insided with significantly higher effluent and porewater pH (pH > 9). Iron (Fe), manganese (Mn), Al, pH and oxidation–reduction potential (ORP) data provide insight into possible contributing processes but overall the data highlight the complexity of processes controlling SRP retention and release when media is exposed to variable salt concentrations. Finally, the impact of freshening on SRP retention was limited for Al-WTR amended columns. This study provides important new evidence of the impact of seasonally high NaCl stormwater concentrations on SRP retention in bioretention systems.

Graphical Abstract