Phosphorus retention and hydraulic performance in borrow sand-based wastewater soil treatment units in impermeable settings: Case study in Abitibi-Témiscamingue, Québec

On-site wastewater treatment systems (OWTS), including soil treatment units (STUs), rely on effluent infiltration into local soils for treatment. However, in regions with impervious soils, this process becomes challenging. As a result, borrow sand is necessary for STU construction. For example, in the Abitibi-Témiscamingue region (western Québec, Canada), impervious sediments from the Clay Belt necessitate the use of such systems. This study evaluates the hydraulic performance and phosphorus retention capacity of borrow sand-based STUs through field inspections, laboratory experiments, and numerical modelling. Field inspections of 26 septic systems revealed that only 2 systems exhibited surface flooding, suggesting minimal hydraulic failure. However, sampling from 12 systems showed that 9 released phosphorus into the environment, with phosphorus concentrations exceeding 35 mg/L in the downstream ditches in some cases, which were far above Québec's surface water limit of 0.03 mg/L. Laboratory sorption tests demonstrated that the sands used in STUs had limited maximum P sorption capacities (88.49 and 17.12 mg/kg). Numerical simulations using COMSOL Multiphysics further indicated that P retention in the sand layer is likely to be exhausted within a year, leading to P migration into the environment. Simulations also indicated that systems with properly designed outlets to a drainage ditch could maintain hydraulic performance during extreme rainfall events. These findings highlight the inadequacy of current borrow sand-based STU designs in ensuring long-term phosphorus retention. To mitigate environmental contamination, improved design strategies and management practices should be considered.