Table 5 Primary conclusions of relevance to policy and management

• It is not possible to derive valid generic guideline values for the efficiency of soil retention of P occurring between OWTS and recipients from available empirical data

• There are too few studies conducted over sufficiently long periods of time (at least decades) that cover the complete system from OWTS to recipients

• It is not possible to derive generic estimates of the efficiency of soil retention of P occurring in the vadose zone within or below OWTSs from available empirical data

• Almost all studies in the vadose zone are conducted without replication

• Very few studies report dilution-corrected P retention data

• Few studies were performed long enough to be able to evaluate the soil retention of P in the long run

• Few studies quantify soil properties relevant to the adsorption of P

• The evidence base does not allow answering the question of the magnitude of the potential impact of OWTSs on the concentration of phosphorus in recipients on a general basis, nor under what conditions soil-based OWTSs generally have such an impact

• The studies addressing this question are far too few, given the complexity and variability of the systems

• In most cases (for practical reasons) there are methodological issues, making the causal inference dubious, also in the specific cases

• The evidence base may allow for creating model simulations and forecasts for certain specific environmental conditions. However, the practical utility of such models would be restricted

• The hydrogeological and biogeochemical conditions underground vary considerably, and site-specific information about these conditions is often unknown

• There are too few studies to create valid model simulations and forecasts for a broader range of conditions