WaterSubject

Nitrate resulting from nitrogen fertilisers used in Agriculture is a widespread contaminant of shallow groundwater and causes adverse effects on human, animal and ecosystem health. In order to evaluate the full extent of groundwater nitrate contamination, and how it might evolve in time, it is essential to understand controls on aquifer assimilative capacity. This level of understanding will also help to better target policies and incentives aimed at controlling the amount of nitrate entering downstream water systems.

The potential for nitrate attenuation in groundwater was assessed by examining the concentration and distribution pattern of electron donors such as dissolved organic carbon (DOC), ferrous iron, and redox indicators such as dissolved oxygen (DO) and Eh in 57 monitoring bores on the lower Burdekin coastal floodplain, one of Queensland’s and Australia’s premier irrigation districts. Nitrate concentrations ranged from 0.1 to 14.4 mg/L NO₃-N but were mostly undetectable in bores close to the coast. Groundwater age dates suggest that while there are nitrate ‘hot spots’ in certain areas, some or most of the nitrate is being consumed on its way to the ocean. Low nitrate concentrations were coupled with high ferrous concentrations.

The low DO concentrations (<2 mg/L) and high ferrous concentrations found in 55% of the bores indicate that redox conditions are suitable for nitrate attenuation by either denitrification or dissimilatory nitrate reduction to ammonium. The reducing environment may be associated with the high DOC concentrations (up to 82 mg C/L) found in these groundwaters. Furthermore, high levels of ferrous iron found in the Ayr area combined with the wide spread geographical distribution of DOC indicate that these areas have a high potential for sustaining geochemical processes that reduces nitrate levels. The distribution of geochemical indicators also suggests that the shallower depths (<15 m) of the groundwater systems have more potential for nitrate reduction than the deeper depths. The map identifying areas within the lower Burdekin with most potential for denitrification is a valuable first step in helping to understand and manage the fate of nitrate entering the groundwater.

Nitrate is the most ubiquitous groundwater contaminant in the world today. Groundwater with elevated concentrations of nitrate is unfit for human consumption and, if discharging to freshwater or marine habitats, may pose a threat to the stability and diversity of aquatic biota. Achieving higher efficiency and effectiveness of policies or targets aimed at controlling nitrate that enters fresh and/or coastal water systems, needs a sound understanding of the level of contamination, processes and conditions effecting the concentration, and hence the fate of nitrate in groundwater. This knowledge will also help the farming community by ensuring that money is not wasted on the application of excess nitrogen fertiliser.

Spatial and temporal patterns in groundwater nitrate concentrations are associated with a change in nitrogen source, dispersion, dilution with groundwater of low nitrate concentrations and patchy nitrate attenuation processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Denitrification or nitrate reduction has also been attributed to the loss or decline in nitrate concentration in riparian zones, wetlands and in some shallow groundwater. Denitrification is a microbially mediated process where nitrate is used as a terminal electron acceptor to produce N₂ or N₂O.

Sophisticated research techniques, especially measurements of isotopic (¹⁵N, ¹⁸O) and dissolved gas (N₂, Ar) composition are of help to provide clear evidences for the existence of denitrification processes, the mechanism involved, and for quantification. In many situations nitrate reduction processes have been delineated from the dilution processes by tracking changes in ambient chloride to nitrate concentration ratios along the flow path. This approach will be of limited use in coastal areas where groundwater has high ambient concentrations of chloride (e.g. concentrations up to three times that of seawater are found in the lower Burdekin).

The denitrification process requires an abundant supply of electron donors and suitable geochemical conditions. For heterotrophic denitrification, dissolved and particulate organic carbon (DOC and POC) are the preferred electron donors. The process can occur in the absence of organic carbon with reduced inorganic species such as ferrous iron or sulfur, which is referred to as autotrophic denitrification. Assuming that the electron donor is organic carbon, there is an ideal sequence of redox reactions based on the energy available. In this sequence, organic carbon is degraded by the electron acceptors in the following order; O₂, nitrate, manganese oxide, iron oxide then sulfate. This sequence coupled with knowledge of the spatial distribution of electron donors may allow determination of the redox-state of the aquifer, evaluation of the extent of contamination, and how it might evolve with time.

Within the lower Burdekin aquifer of North Queensland, Australia 12% of the 397 bores have been reported to have nitrate concentrations above 5 mg/L NO₃-N, the Australian and New Zealand Environment and Conservation Council (ANZECC) guideline for long-term environmental sustainability. Studies have also shown that there has been little change in concentration in about 90% of the bores monitored over a 2–6-year period. A simple mass balance for the lower Burdekin shows that this area receives approximately 16,000 tonnes of nitrogen a year of which about 40–60% is removed by the sugar crop. The difference of about 6400–9600 tonnes of nitrogen per year is not accounted for, and it is the fate of this unused component that we are particularly interested in.

In this paper, we characterise the geochemical conditions in the lower Burdekin aquifer, particularly its redox status, and discuss the potential for nitrate attenuation. Examination of the distribution of the various geochemical indicators identifies areas and depths with significant potential for nitrate reduction.