As part of a trial in west Otago a wetland was established servicing a 49ha catchment that included water fed by a paddock drain.
The wetland was fenced and planted and water sampled every two weeks at the mouth of the drain and 50m downstream where the water left the wetland.
The samples showed a dramatic fall in E coli while nitrogen levels reduced by a third.
But ammonia levels remain high, possibly because of a nearby septic tank.
Simpson says an unexpected result is increased nitrite level, which they think is linked to ammonia.
The trap cost less than $2000 and each year they remove a cubic metre of sediment from it.
