Stephen Collins is a groundwater scientist for Horizons Regional Council where his job involves managing the monitoring and research aspects of the region’s groundwater resources. A particular research interest of his is understanding land use effects on water quality, which has culminated into him undertaking a PhD part-time.
Stephen’s research is on refining estimates of subsurface nitrogen attenuation. At LandWISE 2021, he discussed nitrogen attenuation and how more accurate predictions of it can help improve overall catchment management.
Abstract:
Subsurface nitrogen attenuation – a key unknown in modelling catchment-scale water quality scenarios
Collins, S.,1, 2 Singh, R.,1 Horne, D.1 & Roygard, J.2
1Massey University, Farmed Landscapes Research Centre (FLRC), Palmerston North
2Horizons Regional Council (HRC), Palmerston North
Subsurface nitrogen attenuation is a measure of how much nitrogen is removed in the subsurface environment below the farm root zone, varying from very low rates in some places, to very high rates elsewhere. In other words, some land units appear more resilient in dealing with nitrogen losses compared to others.
An effective understanding of where and when subsurface nitrogen attenuation occurs, and by how much, can have a significant impact on the way nitrogen losses to waterways are modelled and managed in a catchment, especially when water quality improvements are needed.
Where water quality improvements are required, catchment-scale water quality scenarios need to be developed that take account of attenuation dynamics. However, subsurface nitrogen attenuation appears to be spatially variable across different hydrogeological settings, leading to uncertainty in the modelling of those scenarios. Therefore, a robust method of predicting the spatial variability, and potential rate, of subsurface nitrogen attenuation across different hydrogeological settings in agricultural catchments is needed.
The primary nitrogen attenuation pathway in subsurface environments is denitrification, a microbially-facilitated biogeochemical process that reduces nitrate into predominantly ‘benign’ di-nitrogen gas. This process is governed by a range of physical, chemical and biological characteristics that influence ‘reduction-oxidation’ (redox) conditions in hydrogeological settings. Subsurface redox conditions can broadly be described as either reduced (where dissolved oxygen is low) or oxidised (where dissolved oxygen is abundant). Denitrification mainly occurs in reduced conditions.
Though the science of subsurface denitrification processes is being increasingly studied and better understood, the challenge yet remains to achieve a robust method of predicting the spatial variability, and potential rate, of subsurface nitrogen attenuation across different hydrogeological settings.
Direct measurements of denitrification is practically complex and resource intensive, and therefore not easily measurable over large spatial scales. However, the opportunity to meet this challenge potentially comes from the ability to use easily measurable groundwater quality parameters to assess groundwater redox conditions as a suitable proxy for nitrogen attenuation in subsurface environments. Further, groundwater redox conditions show a strong relationship with some landscape attributes such as soil drainage, permeability, rock type and soil texture.
To aid this research, Massey FLRC and HRC are collaboratively investigating how to refine, measure and account for subsurface nitrogen attenuation in New Zealand landscapes to inform modelling of catchment-scale water quality scenarios.
Listen to a Summary Sound Clip here:
