The potential of treated municipal wastewater irrigation to cause aggregate instability and pore sealing on Banks Peninsula soils.

Abstract

The discharge of Treated Municipal Wastewater (TMW) into surface waters can degrade water quality and represents a waste of potentially valuable irrigation water and plant nutrients. While the application of TMW to soil can enhance plant growth, TMW containing high sodium (Na) concentrations can degrade soil structure resulting in decreased permeability and increased runoff. TMW from Banks Peninsula, New Zealand is currently discharged into Akaroa Harbour, however, a legal injunction requires that discharge into water be discontinued and therefore land application is being investigated. This thesis aimed to determine whether TMW application to Banks Peninsula soils would result in significant degradation to soil structure. The TMW contained 40 mg/L Na. Soil columns (0.1m x 0.19m) containing the Pawson Silt Loam and intact lysimeters (0.5m x 0.7m) containing both Pawson Silt Loam and Barry’s soil (a silt loam) were irrigated with a total volume of TMW of up to 1500 mm. The TMW irrigated onto the soil columns was spiked with Na up to 325 mg/L. Infiltration occurred unimpeded on all the soils, indicating that irrigating TMW would not degrade soil structure in the short term. Irrigation with TMW resulted in a significant increase in Na in the soil profile (from 330 mg/kg to 1760 mg/kg), however, there was sufficient native Ca and Mg in these soils (6540 mg/kg and 4130 mg/kg) to offset this increased Na. It is likely that in the long term, lime or gypsum will need to be added to maintain soil structure.