Innovative techniques for removing nitrate from groundwater

Woodchip can help reduce nitrate in water systems, through building woodchip bioreactors and through installation of woodchip permeable ‘walls’ buried into groundwater. 

These two nitrate mitigation practices have been studied in Aotearoa New Zealand, and they have different applications and designs which must be carefully considered.   

Principal Scientist Murray Close and a large team from ESR (Andy Pearson, Theo Sarris, Louise Weaver, Phil Abraham, Laura Banasiak and others) and DairyNZ (Lee Burbery) are monitoring the installation and long-term performance of denitrification systems.

The problem     

Nitrate leaching, particularly from agricultural land use, can cause water contamination, and traditional mitigation practices may not be effective in all landscapes. Nitrate is a highly mobile contaminant that can easily travel from the surface to groundwater. As nitrate discharge can have adverse effects on both environmental and human health, it is crucial to implement effective measures to mitigate its impact.

The solution 

ESR researchers studied the results from a woodchip bioreactor trial in an open drain in South Canterbury, and from a woodchip denitrification wall trial in a shallow alluvial, fast-flowing gravel aquifer on the Canterbury Plains.

The bioreactor was effective at reducing nitrate, but it also had the potential to initially export dissolved organic carbon to the farm drain, which could be an environmental hazard. Careful planning and monitoring are both required to mitigate this issue at the start of the operation.

Despite the woodchip wall initially leaching dissolved organic carbon, the denitrification wall was effective at nitrate reduction. The nitrate removal rate of the woodchip wall was higher than previously predicted, and while it slightly increased the emission of methane gas, the emission of nitrous oxide was less than for pasture.

Both woodchip bioreactors and denitrification walls can be effective nitrate-mitigation practices, but they require careful design and monitoring for their specific applications. The woodchip bioreactor was effective in reducing nitrate in an open drain, while the woodchip denitrification wall was effective in a heterogeneous, fast-flowing gravel aquifer. Further long-term studies are needed to evaluate their effectiveness and potential environmental impacts.

Woodchip denitrification

Woodchip is a carbon-rich material that supports the growth of microorganisms that transform nitrate in water into nitrogen gas, a process known as denitrification. When water containing nitrate flows through a bed of woodchip, microorganisms use the carbon in the woodchip as an energy source and reduce the nitrate to nitrogen gas through a series of biochemical reactions. The nitrogen gas is then released into the atmosphere, effectively removing it from the water.

The process of denitrification requires a specific set of environmental conditions, including the presence of carbon, the absence of oxygen, and an appropriate temperature range. In a woodchip bed, the carbon is provided by the woodchip itself, and the absence of oxygen is achieved by controlling the flow of water through the bed.

Woodchip bioreactors offer an effective and sustainable way to remove nitrate from agricultural runoff and other sources of contaminated water. However, proper design and management are important to ensure their effectiveness and to avoid potential environmental risks, such as the release of dissolved organic carbon into the water.