NITRATE DYNAMICS IN SHALLOW GROUNDWATER AND THE POTENTIAL FOR TRANSPORT TO OFF-SITE WATER BODIES

摘要

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 NO_3-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 NO_3-N is potentially transportable to creeks/streams. The NO_3-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 11.5m above the piezometer bottom. The NO_3-N concentration in the fluctuating GW also showed spatio-temporal dynamics and it ranged from 0.60 to 3.70 mg L~(-1). The NO_3-N adsorbed at anion exchanges sites, up to 10m depth, ranged from 154 to 3956 kg ha~(-1), compared with 21 kg ha~(-1) under rainforest. In the fluctuating GW, the NO_3-N concentration increased with increasing GW table height and the NO_3-N adsorbed at anion exchange sites (R~2 = 0.96). The NO_3-N load in the GW ranged from 40 to 110 kg ha~(-1) and it increased with increasing GW table and GW NO_3-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 NO_3-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 NO_3-N in the GW was transported to creeks/streams when the GW table receded.