Modelling effects of proposed Masterton land-nased effluent disposal scheme on receiving environment, Masterton New Zealand

The Masterton District Council is upgrading its Wastewater Treatment Plant (WWTP) at Homebush. The scheme, which has an estimated cost of $22.8 M, includes irrigation of treated wastewater from six new treatment ponds over an area of up to approximately 150 ha of land to the west of Ruamahanga River, a sensitive receiving environment.

Important issues for the scheme were the amount of groundwater level rise during land disposal and effects on water quality of the receiving environment (groundwater, Ruamahanga River and Makoura Stream). A three dimensional numerical groundwater flow and transport model was developed for the assessments.

The model showed that the groundwater level rise beneath the site during the disposal was estimated to range between 50 mm to 250 mm without causing any breakouts.

The 30 year transient simulations showed that for bacteria, the increase in concentration in groundwater would generally be negligible relative to the existing concentrations over most of the irrigated areas. For nitrate-N, the increase in groundwater concentration was estimated to be of a similar magnitude to the existing concentrations. The modelling showed that phosphorus concentrations in groundwater would likely increase throughout the life of the project, reflecting a depletion of the soil ability to retain phosphorus. The long-term (30-year) increase in phosphorus concentration in groundwater was indicated to range from the background level (0.02 mg/L) to a maximum of 0.5 mg/L adjacent to the river. In all cases the predicted increases were within the current range of natural fluctuations. The model showed a rapid drop-off of contaminant concentrations in groundwater outside of the irrigated area, with minimal effects indicated on nearby private wells.

For the Makoura Stream, which crosses the site, predicted concentration increases for nitrate-N and phosphorus, relative to background concentration for summer low flows, were 7 % and 50 %, respectively. For the river, predicted concentration increases were 6 % for nitrate-N and a long-term 30 % increase for phosphorus (for low flow). In all cases these predicted increases were within the current range of natural fluctuations.