Wetland plants grown on a floating base help reduce nutrients in waterways
The Hawkes Bay Regional Council is trialing floating wetlands to remove nutrients in the pastoral Tukipo catchment in Central Hawkes Bay.
Floating wetlands consist of emergent wetland plants growing in a buoyant mat on the surface of the water and are therefore not restricted to shallow water. The plant roots hang down beneath the floating wetland and provide a large surface area for the growth of beneficial micro-organisms (biofilm) that can enhance nitrogen removal.
Because the plants are floating, they are forced to take their nutrients from the water rather than from the sediments.
Floating wetlands can also inhibit algal growth through shading, reduce wave erosion of the lake banks, and provide additional habitat for fish and birdlife.
The floating wetlands are an innovative approach that makes use of the well-known water purifying effects of natural wetlands. They have a concentrated wetland effect.
They can be used to improve water quality in lake embayments and in small water bodies such as ponds, dams and rivers.
This is a full-scale trial on council owned land in the Tukipo catchment, which forms part of the upper Tukituki flood protection scheme. Virtually all the water entering the rivers in this scheme flow through 78 drainage channels maintained by the Council.
Water is this generations gold. Increasingly intensive land use and climate change mean that more demands are being placed on water availability with the intensifying land use impacting adversely on the quality of our water. Because of this one of the Hawkes Bay Regional Councils long-term focuses is on water quality and quantity.
We are trying to encourage growth while maintaining a clean and healthy environment, and these dont necessarily go hand in hand.
The Tukipo has the poorest water quality in the Ruataniwha Plains, and its catchment is landlocked on the floodplain and therefore doesnt benefit from flushes produced by storm events in the mountains. It is surrounded by intensive farming.
Measurements showed that there are high loadings of dissolved nitrogen in the trial waterway. The acceptable limit for slightly degraded stream is between 0.4 and 1mg/litre depending on definition.
This particular stream, spring fed and permanently flowing, drains 250ha.
We thought about building a conventional wetland to strip out nutrients but it would have needed to be 15,000 square metres in size, and would have required significant earthworks and tree clearing.
Also compared to the floating wetland a conventional nutrient stripping wetlands may cost significantly more to set up and maintain.
Instead, we found out about NIWAs work with Kauri Park nursery to build floating wetlands.
They are designed to strip out 90% of the nitrogen and 15% of the phosphorus from the water in our trial, which is taking 12 months to collect all the data.
And instead of 15,000 square metres, this new floating wetland is only 750 square metres in size, a significant reduction.
And it can be built into the existing drainage network, and didnt need a lot of special earthworks. Its long and narrow.
If the trial is successful we will be looking at the possibility of application to incorporate this concept into on-farm waterways.
This means huge wetlands dont need to be created. With a bit of landscaping, these floating wetlands will fit into the existing drainage network.
Why are these wetlands so efficient? A normal wetland uses plants to strip out nutrients: the nutrient is absorbed into the leaf of the plant.
The plant leaves take up 5-10% of the nutrients in the floating wetlands, but the majority of it happens like this: the mats themselves are made of pet plastic, which is recycled bottles etc. It is injected with marine foam to make the mat buoyant.
The mat itself has a large surface area: for every square metre of mat, there is 200 square metres of surface area for the biofilm to grow on. Biofilm is a slimy kind of substance created by bacteria and fungi which eats the nutrients in the water.
The mat has Carex and rush plants plugged into it: they were planted in October.
Then every square metre of plant roots growing beneath the mat provides a further 300 square metres of area for the biofilm to develop and then absorb nutrient.
Its like hydroponics out in the open.
The technology is versatile in that the mats are transportable. Some of these mats could be unbolted and used somewhere else. Possibly a cash crop could be grown on them which is harvestable, watercress for example.
The mats are working really well now, but we cant get definitive results for a year. We are very optimistic, and there are good indications it is working well.
We hope it will take out 90% of the nitrogen which will virtually eliminate nitrogen from this waterway based on the Tukipo nitrogen levels.
The trial is fully funded by the Regional Council, and its the biggest in NZ. However, there are many more planned around NZ, the idea is really taking off.
I am really excited we are trying out this technology, and it is likely floating wetlands will become a significant eco tool in managing pollutants in our waterways: like using a five pound mallet in a toolbox of 12oz hammers.
It has the potential, along with farming best practices, to be a significant contributor to cleaning up our water.
There has been quite a bit of work done overseas on these ideas, and NIWA was approached by Floating Islands International which is based in Montana, USA.
That got us interested in them, and the more we started exploring potential applications, we saw they had quite a lot of advantages and a real niche in various situations.
In 2005 I went to Montana, and when I came home identified certain applications particularly in urban stormwater for removal of urban contaminants such as metals and oil.
In this case, nutrient removal from rural run-off, the water flows are much more even with spring fed ground water flows.
Its a gel-like gloop, of bacterial and algal slime. The furry feeling that develops on your teeth when they havent been cleaned is an example of a bacterial biofilm. Slippery rocks in a stream or river are an example of an algal and bacterial biofilm.
Bacteria and algae live in the gel-like biofilm. Little particles adhere to it because it is so sticky, and it also adsorbs dissolved substances like a very fine sponge.
Some of the bacteria can convert dissolved forms of nitrate into nitrogen gas. 98% of the atmosphere is nitrogen, so its a good sustainable removal mechanism.
To achieve this bacteria need a surface to grow on, a low oxygen environment, and some organic matter which is produced by the plants in the system.
The plants release exudates from their roots and the dead plant material decomposes. It is really the bacteria and the plants working together forming a beneficial combination.
The floating wetlands are miniature ecosystems with protozoa, invertebrates; a little ecosystem in its own right.
There arent many disadvantages: the cost, and if they arent moored properly they could float away and become a problem. The idea is to try and form a pretty resilient mat that can deal with water fluctuations and constant movement.
These are flexible, and not rigid. And the way they are made means there is a lot of internal surface area because of the dense mesh of polyester thread. That means a lot of beneficial bacteria and biofilm can form within the mat itself.
There is a lot of interest around the country because it is a treatment concept that people can relate to. It is novel.
Up until now there hasnt been any good data on what they really remove, and we didnt know if they would be better than other wetlands, and what they would actually be able to remove.
We expect they have a lifetime of at least 10 years, with the UV rays in sunlight the major factor likely to degrade them. But as the plants grow they will shade the surface of the mats. And as time goes on the dense mat of plant roots and rhizomes will themselves become the mats.
Previously there have only been small scale experimental trials, so larger scale field trials are very important to test out how they work in practice.
We are helping assess the performance of the systems with data collected by HBRC staff. We had input into designing the monitoring programme. Nutrient levels are moderately high into these spring fed streams.