Landholders are leading the way in trialling bioreactors in the Wet Tropics, in an effort to treat nitrogen flowing to the Great Barrier Reef.
A denitrification bioreactor was installed on a cane farm last week as part of the Wet Tropics Major Integrated Project – an up to $15 million reef water quality initiative funded by the Queensland Government through the Queensland Reef Water Quality Program.
Bioreactors are strategically-positioned trenches filled with woodchips that work by converting excess dissolved inorganic nitrogen (DIN) in groundwater into nitrogen gas. Proven internationally but untested in the unique landscape conditions of the Wet Tropics, they are one of a series of treatment initiatives being trialled to reduce nutrient, sediment and pesticide loads flowing to the Reef.
Wet Tropics Major Integrated Project catchment repair officer Suzette Argent said it was estimated the first bioreactor would remove 125 to 400kg of nitrogen feeding into the Reef lagoon each year.
“This bioreactor is treating groundwater draining from a five-hectare paddock,’’ she said.
“It follows months of community engagement, rigorous site assessments, topographical surveys, soil tests and design works. Willing landholders and correct landscape positioning are a critical combination for success, along with the right soil and groundwater conditions – DIN levels need to be evident in groundwater.”
Excess amounts of dissolved inorganic nitrogen in the Reef lagoon impact water quality and reduce reef resilience. They have also been linked to increased outbreaks of the coral-eating Crown of Thorns starfish.
Ms Argent said sub-surface water flows through the bioreactor and, under the anaerobic conditions, naturally-occurring bacteria convert the DIN into a gas which is then dispersed into the atmosphere.
Basin Coordinator Sandra Henrich said it was challenging for land managers to know their relative DIN contribution.
“Current water quality monitoring aggregates water scores from a large area with lots of different users and land uses,’’ she said.
“This makes it difficult to know what an individual farmer needs to do on their property, so an important part of the design of this project involves water monitoring to help with farm decision making.
“Monitoring bores placed at the bioreactor’s groundwater entry and exit points will help determine if there has been a change in nitrogen levels as a result of the treatment. This helps us build a more comprehensive story both about nitrogen contributions and the effectiveness of different treatment systems.”
Ms Henrich said that if successful, bioreactors could be implemented across the landscape as small-scale groundwater treatment systems.
“These are an ‘on-farm’ technology but no land has to be taken out of production and they require little maintenance,’’ she said. “Once a bioreactor is installed, it is business as usual for farming operations.”
The Wet Tropics Major Integrated Project will also trial other treatment systems like wetlands and high efficiency sediment basins.
Source: Terrain NRM