Urban green stormwater infrastructure (GSI) facilities are designed to reduce peak discharges to streams, while also retaining pollutants in soils and plants. While these structures have been shown to function well hydrologically, the pollutant retention aspects are poorly known. The target pollutant in this study was soil phosphorus (P), with the objective of understanding whether P is vulnerable to mobilization in GSI facilities. We characterized the fractions of inorganic and organic P in soils as a function of infiltration rates. We assessed the effects of drying and flooding on P mobilization from soils, as reducing conditions during flooding can cause P release from soil minerals, while oxidizing conditions during dry periods can increase mineralization of organic P, with either scenario potentially mobilizing orthophosphate. We sampled bioretention soils from municipally managed “Green Streets” facilities in Portland, OR, across an infiltration rate gradient. Sequential P extractions and tests for degree of P saturation (DPS) and P sorption index were performed on samples of two soil depths (0-10 cm, 10-15 cm) at each of 16 facilities. Experimental treatments were applied to soils to determine if flooding or drying over time mobilizes water-extractable phosphate, thus representing a risk of orthophosphate loading to receiving waters.
The 16 sites showed a wide range in total P (756-1662 ppm P), organic matter (range: 6%-24%) and P binding fractions. Differences in total P, water-extractable inorganic P, NaOH-extractable inorganic P, and organic P between sites were significant, but depth differences within sites were not statistically significant. Water-extractable Pi was correlated with TP (r2=0.47), NaOH-extractable inorganic P (r2=0.68), DPS (r2=0.56) and water holding capacity. Because DPS represents P associated with poorly-crystalline Fe and Al oxides, a strong relationship between water-extractable inorganic P and DPS suggests that phosphate is being released from associations with these minerals. Flooding and drying treatments showed some evidence of phosphate release, suggesting that both inorganic P sorption to redox-sensitive Fe oxides and mineralization of organic P could be important mechanisms. No strong correlations between infiltration rates and individual P fractions were observed. Nonetheless, these findings show that hydrological patterns can stress the P retention capacity of some bioretention soils that experience extended drying and flooding cycles.