Land Disposal of Wastewater
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 132
The specific study objectives were:
1. To identify the flux of nitrates/nitrites/nitrogen in the soil profile;
2. To quantify the amount of nitrate leaching to the water-table
3. To develop water and land management options to remove or minimize the risk of groundwater contamination by nitrate.
Objective 1 and 2 were fulfilled in part, with results identifying some nitrogen fluxes within soil and groundwater, and groundwater analyses identifying nitrogen leached to the water-table. Objective 3 was not addressed due to insufficient information on soil nutrient processes and water balance.
Key conclusions that can be drawn from this study are:
- Gaseous losses of nitrogen from the soil/water system are assumed to be very significant and may account for up to 50% of the applied nutrient load(assuming approximately 25% of the applied nutrient load is absorbed by the soil or taken up by plants);
- Deep drainage to the groundwater represents more than 50% of the inflow volume yet accounts for only 5% of the applied nutrient load.
- Groundwater levels respond rapidly to wastewater application. Hydrographs from all monitoring bores show a response of groundwater within minutes of wastewater application. This response may be a result of bypass flow.
- Raw wastewater irrigation resulted in measurable impacts on the groundwater at 2 m and 5 m depth. The variation in redox potential measured in groundwater and soil indicate that microbial processes are occurring that respond to irrigation events. The study, however, did not address the long-term impact of repeated flood irrigation events.
- The action of surface runoff down the pasture bay during flood irrigation resulted in the mobilization of soil nitrogen, mainly in the form of nitrite. This suggests that nitrite accumulation in the upper soil profile may result from flood irrigation practices.
This study did not assess the soil storage nutrient dynamics which is critical in order to confidently report nutrient balance closure. The partitioning between gaseous losses and increase in soil organic nitrogen depends on estimates of long term increases in soil organic nitrogen; the nutrient and phosphate balances are complicated by the absence of data about long term increases in soil nitrogen and phosphate.