New Zealand has adopted the Healthy Rivers approach to environmental restoration and control landuse change with an intent to steer waterways towards pristine conditions that provide social, economic and environmental returns. Nutrients, in particular, have been increasing at all monitoring sites for many decades, reflecting on-going land-use change, specifically the varying fortunes of the dairy industry. Highly transmissive volcanic soils result in seasonal surges in nitrogen and phosphorous loads in baseflow to creeks and rivers, with dairy farms leaching more than 50 kg/ha/a nitrate, compared to 2-5 kg/ha/a from forests and plantations. This nitrate derives from excessive fertiliser use (>250 kg/ha/a) and high returns from animal excreta.
Whilst management practices have significantly improved over the last few decades, reducing leaching by half through efficient fertiliser use and judicial stock management, significant nitrates still permeate the soils and drain to rivers and creeks, resulting in algal blooms, pollution of water supplies and disruption of ecosystems.
The potential exists, however, to further attenuate nitrogen levels through microbial metabolism of facultative anaerobes (primarily Pseudomonas denitrificans) in oxygen-poor groundwaters. These bacteria use nitrate as an electron receptor during denitrification and can rapidly reduce nitrate to negligible levels in baseflow to creeks. Rates of denitrification, however, are highly variable, dependent on the oxygen levels in groundwaters and rates of transport through the aquifer. Multiple lines of evidence are required to evaluate the efficacy of denitrification impacts at any given site.
We have use a combination of agricultural (APSIM) and groundwater (MODFLOW_MT3DMS) modelling and local monitoring to elucidate the nitrate story across the Upper Waikato catchments of New Zealand and we use this to help dairy farmers spatially and temporally manage land use change and develop stock management strategies that incorporate denitrification as a principle component of nutrient discharge mitigation.