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

Abstract

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 (PO43−P), the bioavailable form of phosphorus, as the storm water infiltrated into the infiltration bed of the rain garden. The median PO43−P concentration decreased from 0.21–0.25 mg/L in the ponded water to 0.03 mg/L in the pore water at the bottom of the infiltration bed. Overall, the rain garden showed no sign of decreased PO43−P removal performance over 9 years of monitoring. In addition to monitoring dissolved PO43−P concentrations over time, soil samples were collected throughout the rain garden to quantify the accumulation of PO43−P in the soil. Results show that PO43−P was uniformly distributed throughout the top layer of the ponded area of the infiltration bed (0.13±0.03 mg/g dry soil, n=4) and then decreased with depth between 0 and 10 cm. The sorbed PO43−P concentrations remained relatively constant between the depths of 10 to 30 cm throughout the infiltration bed (0.05±0.02 mg/g dry soil, n=20). A mass balance comparing the mass of PO43−P entering and leaving the rain garden to the mass sorbed to the soil suggested that the extraction procedure used to remove the PO43−P from the soil (0.5 N HCl for 24 h) provided a rough estimate of the PO43−P that accumulated during the 9 years of operation. Comparison of the PO43−P sorbed to the first 30 cm of soil over the 9 years (1.58 kg or 176 g/year) to the maximum amount of PO43−P that the soil can hold if in equilibrium with dissolved PO43−P concentrations typical of the rain garden (0.05 –0.11 mg/g dry soil, based on batch sorption experiments) indicated that the top 10 cm of the infiltration bed was saturated with PO43−P but saturation of deeper depths would not occur for >20 years. This led to the conclusion that, in regards to the soil, infrequent maintenance is needed with respect to PO43−P removal during the long-term operation of the rain garden.