Groundwater assimilative capacity - an untapped opportunity for catchment-scale nitrogen management?

Abstract

Not all nitrate leached out of the soil zone ultimately pollutes the groundwater system and groundwater-fed surface waters; some nitrate can be assimilated in the subsurface environment. How much nitrate can be assimilated without exceeding water quality limits depends on a combination of biogeochemical and hydrological factors. Denitrification, i.e. the conversion of nitrate to gaseous forms of nitrogen (N₂, N₂O), is the key process determining the biogeochemical component of a catchment’s assimilative capacity for nitrate. Denitrification is the only attenuation process that actually removes nitrogen from the subsurface rather than just storing or diluting it. Saturated zone denitrification is an environmentally benign process, as it predominantly returns inert N₂ to the atmosphere. Three requirements must be met for denitrification to occur: oxygen-depleted conditions, availability of suitable electron donors, and existence of a microbial community with the metabolic capacity for denitrification. The second major attenuation process at the catchment scale is the dilution of nitrate-rich groundwater, typically recharged from agricultural land, with clean groundwater originating from low land use intensity areas (e.g. mountains, forests). This process is particularly relevant where different groundwater flowpaths converge in the lowland discharge zone of the large alluvial aquifers that occur in many eastern areas of New Zealand (e.g. Canterbury Plains). Provided the groundwater flowpaths and the biogeochemical processes occurring along them were known, this knowledge could be used to optimise spatial land use intensity patterns in a catchment within agreed water quality limits. Rather than relying on root zone leaching estimates alone, the acceptable land use intensity for a given piece of land would take the subsurface system’s assimilative capacity into account. Consequently, land uses with higher nitrate leaching losses would be possible where the assimilative capacity allows, while only lower losses would be acceptable on land with lower assimilative capacity. It is anticipated that this approach would result in spatial land use intensity patterns that better protect environmental, economic, social, and cultural values than current practice and recently introduced approaches that are exclusively based on root zone leaching estimates. Statutory environmental standards and nutrient limits will in the future constrain development in some catchments. Comprehensive assimilative capacity assessments across catchments or sub-catchments would thus help to guide investment in land development and to allocate clean-up funding more effectively, and enable land to be directed towards its optimum use.