Nitrate in groundwaters of intensive agricultural areas in coastal Northeastern Australia

Abstract

Water containing high concentrations of nitrate is unfit for human consumption and, if discharging to freshwater or marine habitats, can contribute to algal blooms and eutrophication. Nitrate pollution of groundwaters in coastal Northeastern Australia is of particular concern because of its proximity to environmentally sensitive areas (e.g. the Great Barrier Reef) and the large number of people (in cities and rural areas) relying on groundwaters for drinking water. This study determined the extent of nitrate contamination in groundwater in this region based on information from 1454 wells, and examined the likely source of the nitrate by comparing δ 15 N values of groundwaters to those of possible industrial or organic nitrogen contaminants. In wells where nitrate concentrations were elevated, and therefore, likely to be a result of human activities, concentrations were subsequently monitored to provide an assessment of temporal trends in nitrate concentrations. Overall, groundwaters were relatively free from excessive nitrate contamination, with nitrate concentrations in only 3% of wells above the maximum permissible limit for drinking water (50 mg/l). However, a further 11% had elevated nitrate concentrations (≥20 mg/l) with the greatest occurrence (14–21% of wells affected) of elevated nitrate concentrations in the Burdekin, Mackay and Bundaberg areas. These percentages are similar to those found in many other intensive agricultural areas. Nitrate in approximately half of these wells was likely to have come directly from fertilizer. Nitrate in only eight wells was likely to have come from organic sources, such as sewage, septic or feedlot overflows. Nitrate in the remaining wells could not be attributed to a particular source. Thus, improvement of nitrogen fertilizer management practices is a key activity in managing groundwater nitrate concentrations. In the Burdekin and Mackay areas, there was no general trend in groundwater nitrate concentrations over 2 years of monitoring of wells with initial nitrate concentrations >20 mg/l. However, there was considerable variability within wells between sampling times and further monitoring is required. In Bundaberg, nitrate concentrations in 40% of wells significantly declined over 6 years (1993–1999) of regular monitoring. The results for this area suggest that nitrogen fertilizer inputs during the monitoring period were not excessive relative to the local aquifer’s nitrogen balance. The opportunities for improved management of nitrogen fertilizer in the study region are discussed.