Constructed wetlands

Another project in the Fresh Start for Freshwater Clean-up Fund work programme was trialling constructed wetlands. Funding support for this project was also received from DairyNZ and Living Water. Wetlands are great buffers between the land and waterways, helping to protect them from erosion, runoff and nutrients.

At the start of this project, NIWA was commissioned to identify the most appropriate locations and types of constructed wetlands that could be implemented in the Waituna catchment to intercept nutrients and sediments en route to streams and Waituna Lagoon.

Thirty different constructed wetland options at 14 different sites were investigated across the catchment. Cost estimates were provided, along with recommendations on wetland locations, size, and type, to get the most environmental benefit for the funding available. It was decided to trial a cost effective constructed wetland located in an area showing relatively high levels of nutrients in the waterway, and to use this wetland as a ‘proof of concept’ within the catchment to inform further work.

Construction on the wetland pond was completed in March 2014, this flows into two phosphorous filtration beds which were constructed in December 2014. The phosphorus beds contain locally available material, one of lime rock and the other oyster shell to see if there is a difference in the nutrient uptake. The wetland pond has been planted out with the native plant Eleocharis sphacelata or tall spike sedge to stimulate nitrate-removing bacteria.

Monitoring of the constructed wetland started in September 2017. This was a much later date than initially anticipated due to some unexpected setbacks. There were initially some issues with getting the Eleocharis established in the wetland pond due to Pukeko pulling them out, this was resolved with the use of cages to protect the plants. Another issue that arose was the slow growing nature of Eleocharis, and the lack of plant supply locally. A local nursery trialled different ways of growing stock, some were sourced from elsewhere in the country and others were also transplanted directly from a local landowner’s pond. As Eleocharis grows over time it spreads out to form a mat, so eventually the pond will be completely covered in it. Whilst this has not been achieved yet, there is now enough cover to enable monitoring to start.

An existing gravel pit was also one of the sites considered for a constructed wetland by NIWA. Often in Southland gravel is extracted from below the top soil for on-farm use, leaving open pits, frequently with standing water areas. These provide a ready situation for development into constructed wetlands to remove dissolved nutrients and suspended solids in the water as well as biodiversity benefits, like new habitat for wildlife.

In 2011 the farmer commented they had noticed sediment being carried in the flow from an artificial waterway which connected to Moffat Creek, one of the main tributaries for Waituna Lagoon. This artificial waterway flowed past the gravel pit, so it was decided to block off the artificial waterway and divert the flow through the now disused gravel pit, enabling the sediment to settle out before the water exited the gravel pit into Moffat Creek. Further enhancement involved riparian planting in February 2016 with the assistance of the Clean-up Fund.

In August 2017 the Waituna Landcare Group organised a fish survey at a number of sites across the Waituna catchment. One of the sites surveyed was the gravel pit, which revealed its providing fantastic fish habitat with the species found including common bully (7), inanga (48) and long fin eels (50).

A trial lime rock phosphorous filter bed was also constructed on this property, which will provide a good comparison to the other constructed wetland site as they are at different ends of the catchment on different soil types.