Nitrate dynamics in shallow groundwater and the potential for transport to off-site water bodies

Large quantities of the nitrate leaching below the crop root-zone, particularly during the wet season, in the tropical Far North Queensland (FNQ) of Australia, may be entering off and on-site the water bodies. The objectives of this study were to (i) provide quantitative information on NO₃-N in the shallow fluctuating groundwater (GW) that develops during the rainy season (January through May) in the Johnstone River Catchment (JRC) of FNQ and (ii) determine whether this NO₃-N is potentially transportable to creeks/streams. The NO₃-N concentration and GW table heights were monitored, at least at weekly intervals, in 6 piezometers, installed to 8.5-12.0 m depth in 6 different soil types under fertilized sugarcane (Saccharum Officinarum-S) in the JRC during the 1999 rainy season. Depending on the location of piezometers on the landscape and time of water sampling, the GW table fluctuated between from 1.5 to 11.5 m above the piezometer bottom. The NO₃-N concentration in the fluctuating GW also showed spatio-temporal dynamics and it ranged from 0.60 to 3.70 mg L^-1. The NO₃-N adsorbed at anion exchanges sites, up to 10 m depth, ranged from 154 to 3956 kg ha^-1, compared with 21 kg ha^-1 under rainforest. In the fluctuating GW, the NO₃-N concentration increased with increasing GW table height and the NO₃-N adsorbed at anion exchange sites (R² = 0.96). The NO₃-N load in the GW ranged from 40 to 110 kg ha^-1 and it increased with increasing GW table and GW NO₃-N concentration. The estimated N-load in the GW that was discharged into creeks/streams when the GW receded ranged from 21 to 81 kg ha^-1. The results provide evidence that a (i) a major proportion of the NO₃-N that was leaching below the sugarcane root-zone entered the shallow GW that developed during the rainy season, and (ii) a significant proportion of the NO₃-N in the GW was transported to creeks/streams when the GW table receded.