Soil retention of phosphorus from on-site wastewater treatment systems

Hence, in certain cases, a high P removal rate (up to 90%) of OWTSs is required by Swedish authorities. Since these requirements may have costly consequences to property owners, it is debated whether they are too strict. In this debate, it is often claimed that the soil retention of P occurring in the natural environments may be underestimated by authorities.

Soil retention is the inhibition of the transport of P through the ground, due to different chemical, physical and biological processes occurring there. These processes make the P transport slower, which may reduce the unwanted impact on receiving water bodies. However, the efficiency of soil retention of P remains unclear.

In a systematic map, we collected, coded, organised and elucidated the relevant evidence related to the topic, to be able to guide stakeholders through the evidence base, and to support future research synthesising, commissioning, and funding. Further, we examined P adsorption properties of 14 subsoils.

Based on this, we came up with the following recommendations:

• When specific data for the soil in question are lacking, we propose a conservative estimate of 20 g P m-3 for phosphorus retention in the unsaturated zone. 
• For the groundwater zone, where phosphorus sorption is poorly understood, we suggest a conservative estimate of 8 g P m-3. 

If oxalate extraction data are available, higher estimates may be justified. A simplified Excel tool was developed to illustrate expected P retention patterns over time and as a function of distance from the source. Finally, the need for improved knowledge on phosphorus retention in the groundwater zone is emphasized.