Nitrate and ferrous iron concentrations in the lower Burdekin aquifers: assessing denitrification potential

Abstract

The lower Burdekin is one of Australia’s premier irrigation districts with more than 80,000 ha of sugarcane and other crops. Because it is located adjacent to the Great Barrier Reef Lagoon, there is strong interest in understanding the fate of nitrogen applied as fertiliser. Natural denitrification is increasingly recognised for its ability to reduce nitrate concentrations in groundwater and we therefore analysed water samples for a range of constituents (nitrate, ferrous iron, dissolved oxygen, redox potential, pH) from 57 monitoring bores to investigate the denitrification potential of the lower Burdekin aquifers. Nitrate concentrations ranged from <0.1 to 14.4 mg/L NO3-N (three times the ANZECC environmental standard of 5 mg/L). Ferrous concentration varied from 1 to 360 mg/L, dissolved oxygen was < 2 mg/L, redox potential varied from -120 to +235 mV, and pH ranged from 5.9 to 7.6. Elevated nitrate levels were observed in 16% of the bores located mostly in the Home Hill area. These areas also had low ferrous levels. High ferrous levels were found mostly in the Ayr area and at shallow depths. The data in general showed an inverse relationship between nitrate and iron, and between nitrate and ammonium. Furthermore, undetectable amounts of nitrate were measured in the nested bores located along the coast. These geochemical conditions favour the presence of ferrous iron and a reduced environment conducive to denitrification. These results suggest that denitrification is one of the mechanisms involved in reducing the amount of nitrate in the lower Burdekin aquifers and hence reducing the potential for nitrate to move from the groundwater into the near-shore marine environment.