Fencing drains, seeps and wetlands

Drains, seeps and wetlands will only work as filters if they are not:

• drained – nitrogen removing bacteria (denitrifying bacteria) need wet conditions to grow
• grazed – particularly during the wettest parts of the year.

To exclude cattle from drains, seeps and wetlands, use an electric fence. It’s particularly important to keep cattle out when soils are wet. This:

• protects the soil structure so it can act as a filter
• protects these areas from direct contamination from stock effluent
• stops the soil from being pugged and becoming a source of dirty runoff

Results from a NIWA (National Institute of Water and Atmospheric Research) study show that well vegetated open drains can remove up to:

• 70 percent of nitrate
• 40 percent of phosphate.

However, poorly managed drains can act as ‘highways’, particularly for the movement of:

• nutrients – such as nitrogen and phosphate
• faecal organisms – for example, the bacteria E. coli
• sediment – such as eroded topsoil.

Many drains empty directly into waterways and can be a major source of pollutants in some areas. For open drains to be effective in removing farm pollutants make sure:

• they have vegetation, for example, grasses, sedges and rushes – to trap and absorb farm nutrients and other pollutants
• they are fenced from stock – to protect vegetation and soil structure, and to avoid direct input of stock effluent
• you control weeds with herbicide, rather than digging – grassed drains are effective filters
• you dig drains during maintenance in sections, rather than the whole lot at once. This ensures part of the drain remains vegetated, and still effective in removing farm pollutants and sediment.

Many subsurface drains flow directly into waterways, bypassing the ‘sponge zone’ or waterway margin. One way to get around this is to construct a small wetland area between the end of a drain and the receiving waterway. Alternatively, drain outlets can be redirected to a wetland area. This will act as a natural barrier, filtering out any farm pollutants.