Nitrate in shallow fluctuating groundwater under sugarcane: Quantifying the lateral export quantities to surface waters

The contribution of groundwater (GW) pollutants, exported from via base-flow discharge, towards the loads in surface waters (SW) has been poorly characterised. This characterisation is needed for intensively cultivated agricultural catchments in which the base-flow discharges are substantial. In this study the flux theory based solute transport equations were used to clarify the issue of lateral transport of the nitrate in GW to SW and the contribution that it made towards the total load, in runoff plus base-flow, in the nearby creek. Water table levels under sugarcane and the concentrations of nitrate-N, ammonium-N (AN), and soluble P (SP), in six piezometers (10 m deep) installed along a 1.1 km transect perpendicularly crossing a creek were monitored over a period of two rainy seasons in paddocks in the northeast humid tropics of Queensland, Australia. Nitrate-N, AN, and SP concentrations in the GW increased with rising water table, decreased with receding levels, and varied spatiotemporally from 23 to 1500 μg L−1 for nitrate-N, 4–332 μg L−1 for AN, and 1–51 μg L−1 for SP. Nitrate was transported at rates ranging from 0.008 g m−2 d−1 to 0.012 g m−2 d−1 in base-flow discharge towards the creek. Based on a hypothetical equivalent surface land area vs. 1 m × 1 m sub-surface soil matrix area, we show that approximately 52% of the total anthropogenic loading in the creek was from GW. This suggested a need for the adoption of N-fertiliser management practices that will reduce or minimise nitrate leaching to GW.

► Pollutants in shallow groundwater (GW) may be laterally exported to surface waters (SW).
► Flux theory-based equations were used to quantify the lateral transport of nitrate from GW to SW.
► The results showed that nitrate was transported laterally in base-flow discharges towards SW.
► The transported quantity was approximately 52% of the total, in surface runoff plus base-flow, transported towards SW.
► It seems nitrate leaching under intensive sugarcane production was substantial and contributed significantly towards the total loading in SW.