RETENTION OF NITRATE AND PHOSPHATE IN SOILS OF THE DARLING PLATEAU IN WESTERN-AUSTRALIA - IMPLICATIONS FOR DOMESTIC SEPTIC-TANK SYSTEMS

Streams in mixed rural and residential areas on the Darling Plateau near metropolitan Perth in Western Australia were monitored for nitrate and phosphate. Concentrations of nitrate in streams are proportional to surveyed rates of inputs of nitrogen in water catchments dominated by agricultural land use. In catchments dominated by unsewered residential land use, stream concentrations of nitrate are much lower than expected from rates of input. A comparison of mass balances of inorganic nitrogen and bromide, added as a conservative tracer, indicates that at least 80% of nitrogen leaching from domestic septic tank systems into soils is lost within a distance of 10 m from the leach drain. Concentrations of dissolved phosphate in streams are low (mostly <5 mug/L P-PO4) and are not significantly related to type of land use. Adsorption of phosphate to soils was measured after batch equilibration of soil slurries for periods up to 12 months and the isotherms were described by a time-dependent Freundlich type adsorption equation. Adsorption capacities for phosphate are high and reflect high levels of iron and aluminium oxides in the soils. Soil adsorption data together with data from leaching experiments in small columns of soil were fitted to an empirical equation, describing the effect of flow rate and distance travelled on the mobility of phosphate in a saturated soil. A minimum travel time for a distance of 5 m in these soils of between 1 and 8 years is predicted with this equation for phosphate leaching from a septic tank. Monitoring of ground water near 6-7 year old domestic septic tank systems showed that phosphate had not yet moved beyond 5 m from the leach drain.