Fonterra is backing a farmer-led project to protect water quality in Bay of Plenty lakes.
Even without planting, fencing streams reduces faecal coliforms reaching the water by 35% and prevents banks eroding.
In ideal conditions, fenced grass riparian strips can reduce:
- sediment entry to streams by at least 80%.
- dissolved P entry by at least 50%.
- nitrate-N by at least 60%.
Planting riparian strips also helps restore the ecology of streams by casting shade that cools the water and suppresses aquatic weed growth. When combined with weed and pest killing, planting can enhance habitat on land for native insects and bird life.
However, riparian planting is less successful in reducing nitrate entry to streams where there are no wet soils bordering the stream channel from which N can be released back to the air.
This is because nitrate tends to pass into groundwater from the paddock surface through the soil, rather than moving over the surface attached to particles as phosphorus does. So riparian planting alone will not be sufficient to solve the problem of nutrient build-up in Waikato waterways.
More farmers in the region and nationwide are protecting streams on their property with riparian fencing and plants.
Seepage areas and wetlands
Seeps and wetlands act like the ‘kidneys’ of the land. They naturally remove pollutants from farm run-off by filtering and cleaning the water that flows through them. Sediment and faecal material are trapped in the wetland vegetation, and the microbes living in wetland soils remove nitrate from farm run-off.
Very high levels of nitrate removal have been recorded for wetland areas (up to 90%). However, for wetlands to work effectively they need to be retained (unaffected by drainage), adequately sized for the catchment area and fenced to allow growth of wetland vegetation that will slow water flow.
A carbon-rich form of vegetation increases the effectiveness of wetland bacteria that act to remove nitrogen. Abundant carbon also reduces the proportion of harmful greenhouse gases emitted by wetland areas.
Wetlands can be constructed at drain outlets, or at the base of catchment areas, to remove nitrogen before drainage water flows into downstream waterways.
A wetland of 2-5 % of the total catchment area draining into it is recommended for significant N-removal. Constructed wetlands are getting noticed as an effective mechanism for protecting sensitive downstream receiving waters such as lakes.
Retaining and fencing naturally occurring wetlands and seeps is much cheaper than a constructed wetland. Fencing out swampy areas can also prevent stock losses. If weeds are controlled these areas can quickly become attractive features in the farm landscape, and provide valuable habitat for fish and bird life as natural wetland vegetation re-establishes. Tree-planting is generally not recommended around wetland margins as trees can dry out the soils and shade the carbon-rich wetland species. Locally occurring reeds, rushes, and sedges are more suitable.
Fenced and grassed drains
Many drains on farms empty directly into waterways and can be a major source of pollutants in some areas. Poorly managed drains can act as contamination ‘highways’, providing shortcuts into waterways for nutrients, bacteria and sediment. A well-managed drain can aid nutrient removal and form an important habitat for fish and other water life.
Keeping drains fenced and well vegetated can greatly reduce the amount of nutrients discharged to waterways. Fencing banks prevents stock trampling and erosion, reducing the frequency of drain maintenance required. Vegetation such as rough grass will trap farm nutrients and slow the water flow so that nutrient removal processes can occur (in this case drains can act like mini-wetlands).
Mechanical drain cleaning should be kept to a minimum, and spraying of drain vegetation done only along a portion of the drain at any one time.
Tracks and raceways
Poorly designed and maintained tracks and raceways can collect and channel farm run-off into the nearest waterway, carrying sediment, nutrients and effluent into the water.
Well-designed raceways divert run-off from the track onto paddocks rather than into waterways. As well as helping the environment, this also prevents channelling and eroding, saving on track maintenance.
An important consideration for track design and minimal maintenance is the shaping of the track to direct water off the track surface and into rough grass areas (for example, with a crown on a flat slope, or correct cambering and culverts on a slope). Regular cut-offs (small channels on the side of the track) will remove water, again directing it into the paddock or rough grass alongside. This prevents scouring of the track surface and excessive erosion.
Approaches to waterways are particularly important: if there is an area of fenced vegetation or small wetland alongside the waterway, track run-off should be diverted into this area so that dirty water does not run directly into the stream.
• Article sourced from Waikato Regional Council publication ‘The condition of rural water and soil in Waikato region’.