Minimising surface water pollution resulting from farm-dairy effluent application to mole-pipe drained soils. II. The contribution of preferential flow of effluent to whole-farm pollutant losses in subsurface drainage from a West Otago dairy farm

To evaluate the role of artificial drainage systems in the transfer of nutrients and faecal organisms from soil to waterways, mole-pipe drainage flows were monitored from two large (27 x 40 m), hydrologically isolated field plots that were part of a long-term dairy pasture in West Otago, New Zealand. One plot was grazed only whilst the other plot was spray irrigated with untreated dairy-shed effluent on seven occasions, shortly following grazing events in spring, summer or autumn. Monitoring throughout the 4-year study showed that relatively large amounts of ammonium-N, total-P, and Escherichia coli (E. coli) bacteria were transported through the artificial drainage system via direct drainage of effluent following three of the seven effluent irrigation events. These effluent drainage events occur-red when the maximum depth of effluent application exceeded the soil water deficit measured in the 0-45-cm layer. A pronounced non-uniform pattern of effluent application was observed, with areas to the outside of the irrigator run effectively receiving double the average application depth, and at an instantaneous rate greater than 100 mm h(-1). Although the volumes of effluent transported in drainage flow were relatively small, the concentrations and loads of ammonium-N, total-P, and E. coli bacteria in the resulting drainage were large. Based on the measured irrigation uniformities and soil water balance calculations, a simple model of effluent flow through mole-pipe drained soils was developed. Model outputs indicated that two key management strategies could avoid or reduce the transport of pollutants from these effluent-treated soils, specifically (i) increasing irrigator groundspeed and (ii) storing effluent when soil conditions are wet. Failure to implement these management practices is likely to result in the delivery of large amounts of pollutants to surface water-ways when flows in the receiving surface waters are low and temperatures relatively high.