Edge-of-field solutions for phosphorus

Phosphorus is a significant issue in New Zealand waterways, especially in rivers, streams and lakes. Runoff from agricultural land and livestock are major sources of phosphorus entering into waterways.

Browse through the page below to discover what edge-of-field mitigation solutions are available.

If you would like more information about ‘in-field’ mitigation practices, then follow the links at the bottom of this page.

An artificially created wetland is an area where the flow of drainage and/or runoff is slowed, and so the soil is very wet and there is often ponding. Plants growing in wetlands need to be tolerant of these conditions. Wetlands can be either natural i.e. they have developed naturally, or they can be purpose built or constructed. During this time of relatively slow movement of drainage and runoff through the wetland, contaminants are removed from the water. 

The plants will uptake nutrients in the water such as nitrate and inorganic phosphorus, the wet conditions will encourage denitrification (the conversion of nitrate to dinitrogen gas), the larger sediments will settle, and the finer sediments are likely to be trapped on biofilms that form on the litter and plants under water. Therefore, wetlands can effectively improve water quality as it leaves the farm and before it arrives in streams and rivers. A constructed wetland is often the best edge-of-field mitigation where there are large volumes of drainage and/or runoff with smaller concentrations of contaminants. 

A  Detainment Bund PS120© is a special type of sediment trap to temporarily detain stormwater runoff from pasture lands. It intercepts stormwater run-off by briefly ponding it on pasture to reduce significant amounts of sediment, phosphorus, and pathogens. Runoff ponding is achieved by constructing low earth bunds across valley floors where water flows during high-intensity rain storm events. Water ponding is controlled by a flow structure and a spill way in case of a very high runoff event. Pasture growth in the ponding area is not compromised due to temporary storage and then release of the ponded water.

If however you are more interested to find out about in-field mitigation solutions, the following links may be of interest:

• Mitigation and best practice options NIWA 
• Manging farm phosphorus