Attenuation of effluent-derived faecal microbes in grass buffer strips

A series of field experiments assessed the ability of sloping (8degrees) 5-m-long by 2-m-wide grass buffer strips to trap the faecal microbes Escherichia coli and Campylobacter. The microbes, applied within dairy-farm effluent, were washed into the strips by surface runoff generated at rates of 4-13 litres/min using a water sprinkler system. The effluent and surface and subsurface outflows at the lower end of each plot were sampled for microbial analysis. Flow rate influenced the timing of peak microbial concentration in outflow and the recovery of both microbes. Under high flow, recovery rates varied from 15-100%, and hence entrapment was often minimal. Under the slowest rate of water application, entrapment was much greater (greater than or equal to95%), at least over the 40 min of water application. During large runoff events, and where preferential flowpaths occur, buffer strips need to exceed 5 m in length in order to markedly reduce the delivery of faecal microbes to waterways. Of those microbes trapped in the grass strips under fast flow rates, some were remobilised and washed out following a subsequent runoff event, 5 days later. On occasion, a considerable volume of flow was observed to bypass beneath the subsurface collecting troughs, probably reducing the effectiveness of the buffer strips.