Long-Term Orthophosphate Removal in a Field-Scale Storm-Water Bioinfiltration Rain Garden

Unabated runoff from impervious surfaces after rain events is considered a major source of impairment of receiving water bodies. Bioinfiltration storm-water control measures (SCM) have been shown to be effective in reducing runoff and pollutants from urban areas and thus provide a mechanism for protecting downstream sources from erosion and contamination from suspended solids, metals, and nutrients. However, less is known about the loss mechanisms responsible for contaminant removal and the long-term performance of such SCMs. Even less is known of the performance from a vadose zone perspective. The research presented herein examines the long-term (9 year) performance of a bioinfiltration rain garden with specific emphasis on the removal of orthophosphate. Field data indicated clear removal trends for orthophosphate (${\mathrm{PO}}_4^3\text{−}\mathrm{P}$), the bioavailable form of phosphorus, as the storm water infiltrated into the infiltration bed of the rain garden. The median ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ concentration decreased from $0.21–0.25\mathrm{mg}/\mathrm{L}$ in the ponded water to $0.03\mathrm{mg}/\mathrm{L}$ in the pore water at the bottom of the infiltration bed. Overall, the rain garden showed no sign of decreased ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ removal performance over 9 years of monitoring. In addition to monitoring dissolved ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ concentrations over time, soil samples were collected throughout the rain garden to quantify the accumulation of ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ in the soil. Results show that ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ was uniformly distributed throughout the top layer of the ponded area of the infiltration bed ($0.13\pm 0.03\mathrm{mg}/\mathrm{g}$ dry soil, $n=4$) and then decreased with depth between 0 and 10 cm. The sorbed ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ concentrations remained relatively constant between the depths of 10 to 30 cm throughout the infiltration bed ($0.05\pm 0.02\mathrm{mg}/\mathrm{g}$ dry soil, $n=20$). A mass balance comparing the mass of ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ entering and leaving the rain garden to the mass sorbed to the soil suggested that the extraction procedure used to remove the ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ from the soil (0.5 N HCl for 24 h) provided a rough estimate of the ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ that accumulated during the 9 years of operation. Comparison of the ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ sorbed to the first 30 cm of soil over the 9 years (1.58 kg or $176\mathrm{g}/\text{year}$) to the maximum amount of ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ that the soil can hold if in equilibrium with dissolved ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ concentrations typical of the rain garden ($0.05–0.11\mathrm{mg}/\mathrm{g}$ dry soil, based on batch sorption experiments) indicated that the top 10 cm of the infiltration bed was saturated with ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ but saturation of deeper depths would not occur for $>20\text{years}$. This led to the conclusion that, in regards to the soil, infrequent maintenance is needed with respect to ${\mathrm{PO}}_4^3\text{−}\mathrm{P}$ removal during the long-term operation of the rain garden.