Long-term effects of drinking-water treatment residuals on dissolved phosphorus export from vegetated buffer strips

The export of dissolved phosphorus (P) in surface runoff from agricultural land can lead to water quality degradation. Surface application of aluminium (Al)-based water treatment residuals (Al-WTRs) to vegetated buffer strip (VBS) soils can enhance P removal from surface runoff during single runoff events. However, the longer-term effects on P removal in VBSs following application of products such as Al-WTR remain uncertain. We used field experimental plots to examine the long-term effects of applying a freshly generated Al-WTR to VBSs on dissolved P export during multiple runoff events, occurring between 1 day and 42 weeks after the application of Al-WTR. Vegetated buffer strip plots amended with Al-WTR significantly reduced soluble reactive P and total dissolved P concentrations in surface runoff compared to both unamended VBS plots and control plots. However, the effectiveness of Al-WTR decreased over time, by approximately 70% after 42 weeks compared to a day following Al-WTR application. Reduced performance did not appear to be due to drying of Al-WTR in the field. Instead, the development of preferential flow paths as well as burying of Al-WTR with freshly deposited sediments may explain these observations. Better understanding of the processes controlling long-term P removal by Al-WTR is required for effective management of VBSs.