Soluble phosphate in fluctuating groundwater under cropping in the north-eastern wet tropics of Australia

Soluble reactive phosphorus (SP) present in groundwater (GW) is generally considered insignificant, and therefore of little consequence to the quality of waters receiving base-flow discharges. In this study we investigated whether: (i) significant quantities of SP were present in GW (GW-SP); (ii) potential existed for GW-SP to be exported to streams via base-flow discharge; and (iii) the exports are a health risk to ecosystems receiving base-flow discharges. Investigations were carried out at two sites in the Tully River Catchment (TRC) during three consecutive rainy seasons, and 24 wells in the Johnstone River Catchment (JRC) were also monitored during one rainy season, in the north-eastern wet tropics of Australia. In the TRC, the GW-SP varied temporally, within and between rainy seasons, from 2 to 158 μg P/L at Site 1 and from 3 to 31 μg P/L at Site 2. The temporal variations in GW-SP were driven by fluctuating water-table at Site 2, but no such trend was observed at Site 1. The SP in drain-water (DW-SP) varied temporally from 0.6 to 110 μg P/L at Site 1, compared with 2–83 μg P/L at Site 2. The positive association between DW-SP and GW-SP at Site 2 indicated an export of SP from GW to a drain via base-flow discharge. In the JRC, the GW-SP in the 24 wells varied temporally from 0 to 300 μg P/L with the means across the wells ranging from 5 to 190 μg P/L, with the variations driven by fluctuating water-tables. More than 50% of the GW-SP or DW-SP concentrations in TRC were significantly higher than the P trigger values, 2–5 μg P/L, proposed to sustain the health of aquatic ecosystems in this region; a similar result was observed in the JRC. Speciation analysis via filtering (i.e. P passing through a 0.45-μm filter) for selected GW samples indicated substantial quantities of soluble organic P in some wells, ranging from 5 to 89% (mean 38%) of the total soluble P (SP plus soluble organic P). Because the soluble organic P was not included in GW-SP determinations, the hazard/risk mentioned above is an underestimate. The GW-SP exported during rainy seasons, from both catchments, ranged from 0.16 to 0.43 kg P/ha. Our findings indicate there were significant quantities of SP and soluble organic P in GW, it was exported to streams, and there is a health risk to receiving surface water bodies.