Long-Term Orthophosphate Removal in a Field-Scale Storm-Water Bioinfiltration Rain Garden
- 1 October 2012
- journal article
- Published by American Society of Civil Engineers (ASCE) in Journal of Environmental Engineering
- Vol. 138 (10), 991-998
- https://doi.org/10.1061/(asce)ee.1943-7870.0000566
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 (), the bioavailable form of phosphorus, as the storm water infiltrated into the infiltration bed of the rain garden. The median concentration decreased from in the ponded water to in the pore water at the bottom of the infiltration bed. Overall, the rain garden showed no sign of decreased removal performance over 9 years of monitoring. In addition to monitoring dissolved concentrations over time, soil samples were collected throughout the rain garden to quantify the accumulation of in the soil. Results show that was uniformly distributed throughout the top layer of the ponded area of the infiltration bed ( dry soil, ) and then decreased with depth between 0 and 10 cm. The sorbed concentrations remained relatively constant between the depths of 10 to 30 cm throughout the infiltration bed ( dry soil, ). A mass balance comparing the mass of entering and leaving the rain garden to the mass sorbed to the soil suggested that the extraction procedure used to remove the from the soil (0.5 N HCl for 24 h) provided a rough estimate of the that accumulated during the 9 years of operation. Comparison of the sorbed to the first 30 cm of soil over the 9 years (1.58 kg or ) to the maximum amount of that the soil can hold if in equilibrium with dissolved concentrations typical of the rain garden ( dry soil, based on batch sorption experiments) indicated that the top 10 cm of the infiltration bed was saturated with but saturation of deeper depths would not occur for . This led to the conclusion that, in regards to the soil, infrequent maintenance is needed with respect to removal during the long-term operation of the rain garden.
Keywords
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