Phosphorus in soil treatment systems: accumulation and mobility

Area of Science:

• Environmental Science
• Soil Science
• Water Quality Management

Background:

• Septic tanks with soil treatment systems (STS) are widely used for domestic wastewater in rural areas.
• Phosphorus (P) leakage from STS can threaten surface water quality, especially near water bodies.
• The long-term behavior and P retention capacity of aging STS are not fully understood.

Purpose of the Study:

• To investigate phosphorus accumulation and leaching in aged soil treatment systems (STS) in Sweden.
• To characterize the forms of accumulated phosphorus and identify factors influencing P retention.
• To assess the risk of P leakage from STS to the surrounding environment.

Main Methods:

• Analysis of soil samples from six Swedish STS (11-28 years old) using batch and column experiments.
• Characterization of accumulated phosphorus using X-ray absorption near edge structure (XANES) analysis.
• Measurement of soil properties including pH, oxalate-extractable metals, and P sorption.

Main Results:

• Wastewater significantly altered soil properties, including decreased pH and increased oxalate-extractable metals.
• Accumulated P in STS ranged from 0.32 to 0.87 kg m⁻³, often representing less than 30% of the estimated P load.
• Column studies showed high P leaching potential with deionized water, varying by site; aluminum phosphates and organically bound P were key P sinks.
• Soils with high oxalate-extractable aluminum content exhibited lower vulnerability to P leakage.

Conclusions:

• Aged STS can retain significant amounts of phosphorus, but leakage remains a concern.
• The chemical form of phosphorus and soil properties, particularly aluminum content, influence P retention and leakage.
• Effective management of STS should consider soil characteristics to minimize phosphorus release into the environment.