The potential for phosphorus loss to groundwater from soils irrigated with dairy factory wastewater

ABSTRACT

Soils irrigated with phosphorus (P)-rich wastewater can increase the risk of P losses from land to water. We investigated if wastewater-irrigated soils can leach P through the soil profile and enrich groundwater. Soils were sampled annually for 20 years to 7.5 cm depth and for one year to 2.2 and 15 m. A mass balance showed that topsoils >200 mg L⁻¹ Olsen P (and <50% anion storage capacity, ASC) were no longer accumulating P and had very high concentrations of water and CaCl₂-extractable P (designed to indicate loss to runoff and leaching, respectively). Samples in five paddocks showed substantial P leaching to 2.2 m, while sampling to 15 m showed leaching to 4.5 m depth. Deeper layers, especially in the 15 m deep cores, had a coarse texture and low capacity to sorb P, which caused high P concentrations (1.9 mg L⁻¹) at one groundwater site. Based on these data preventing soils from reaching the thresholds of 200 mg L⁻¹ Olsen P and 50% ASC is necessary to prevent excessive P losses.

KEYWORDS:

• Grassland
• leaching
• subsurface flow
• trend
• dairy
• leaching
• sorption
• thresholds

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data are available on request of the corresponding author.

Additional information

Funding

Funding for this study was provided by Fonterra and the Our Land and Water National Science Challenge via the Ministry for Business, Innovation and Employment (contract C10X1507).