Predictors of Phosphorus Leaching from Bioretention Soil Media

The phosphorus saturation index (PSI) and P saturation ratio (PSR) were tested across a wide range of bioretention soil mixtures (BSMs) for their relationship to total and dissolved P in column leachate. The BSMs tested were made using different feedstocks including sand alone, food and yard waste compost, biosolids and yard compost, and high Fe biosolids. The PSI of the mixtures ranged from 0.23 ± 0.03 (biosolids and yard waste compost 15%, oyster shells 5%) to 1.26 ± 0.02 (biosolids and yard waste compost 80%). The PSR of the mixtures ranged from 0.05 ± 0 (100% sand) to 3.12 ± 0.12 (biosolids and yard waste compost 80%). A total of 12 storm events were staged using both synthetic rainwater (total P = 1.71 ± 0.3 mg L) and actual stormwater (total P = 0.08 ± 0.03 mg L). Excluding the sand-only mixture, all treatments were a source of P for the duration of the study. However both total and dissolved P concentrations decreased over time. Effluent concentrations of total P in the first event ranged from 0.59 to 75.55 mg L but decreased by the final event to between 0.15 and 10.73 mg L. The PSR was found to be a good predictor of P leaching from all BSMs with an of 0.73 for total mass of P leached across all leaching events. The PSI was a poor predictor ( < 0.3). The PSR also predicted total P in leachate for individual events more effectively than the PSI. Total P in the BSMs had no relationship to P in the leachate.