Examining reduction of phosphorus and copper release from bioretention soil media components using phosphorus saturation ratio

Abstract

Bioretention is an effective means of treating urban runoff, however, organic soil components are known to export P and Cu. Drinking water treatment residuals (WTRs) are cost-effective P sorbents and may also be useful in Cu retention. To optimize bioretention soil mixtures (BSMs), predictive measures of P and Cu mobility are needed for application across a range of materials. A series of batch studies was conducted with WTRs and organics to determine the relationship between P and Cu export and soil physiochemical characteristics. Materials were incubated separately and in combination, and net release was calculated on a mass basis from the media. P release from combinations increased with increases in either phosphorus saturation index (PSI) or phosphorus saturation ratio (PSR), determined by oxalate or Mehlich 3 extraction, respectively. Variation in P release increased at higher PSI and PSR values, but decreased at low values (PSI < 0.5, PSR < 1). Keeping the PSI and PSR of BSMs low should help reduce or eliminate P release from bioretention systems and improve water quality. Other factors, including total C of media and specific surface area of WTRs, were also significant but contributed only moderately to an explanation of the variation seen. Total Cu was a predictive factor of Cu release from the combinations, however, variation in this response was high and the predictive strength of total Cu as an explanatory variable was low.