Stable isotope fingerprints of agro-contaminant inputs in a nutrients-limited catchment, Western Australia

Modern agriculture utilises large quantities of fertilisers which are applied to maximise food production. However, these high loads of agricultural chemicals are not always fully utilised by plants or decomposed in the soil; therefore, they frequently leach out from agriculture systems and pollute surface and ground waters. Farms and facilities that process agriculture products and wastes also release high loads of organic pollutants to the environment. These environmental problems are a subject of concern in many regions around the world, including the Swan Coastal Plains of Western Australia where agricultural production has been intensive over the last 150 years.

The Nambeelup Brook catchment (~143 km²) of Western Australia, which is characterised by diverse land use, was selected as an experimental study site. On average, ~9.7 GL/y of water is discharged from this catchment to costal lagoons and contributes to their eutrophication. However, the surface water flows are highly seasonal and the nutrient loads vary widely. Apart from the traditional hydrochemical techniques, multi-tracer stable isotope analyses of water molecules [δ²H(H₂O) and δ¹⁸O(H₂O)], nitrates [δ¹⁵N(NO₃) and δ¹⁸O(NO₃)] and sulphate ions [δ³⁴S(SO₄) and δ¹⁸O(SO₄)] were used in the present study to partition different sources of pollutants in the catchment.

During the wet season of 2018, all surface waters were fresh, with Total Dissolved Solids varying between 100 and 1,000 mg/L and δ¹⁸O(H₂O) and δ²H(H₂O) between 0.8 and ‑4.2 ‰ and 4.5 and -19.4 ‰, respectively. These values reflected the time since the last rainfall and differences in evaporations with respect to differences between water retention times in different parts of the catchment. Sulphate concentrations ranged 6–140 mg/L and δ³⁴S(SO₄) 14.3 to 26.3 ‰, reflecting inputs from fertilisers, natural acid rock drainage and sulphur reduction. Nitrates had relatively low concentrations in surface waters (<1.3 mg/L, except one location 6.5 mg/L) but diverse δ¹⁵N(NO₃), ranging -5.4 to 15.3 ‰ and δ¹⁸O(NO₃) -6.7 to 19.4 ‰ and displaying general denitrification trend.

These stable isotope results provide important supplementary information about possible sources of agro-pollutants across the catchment, but they must be analysed in conjunction with water hydrochemical composition. The major challenge is to clarify the ambiguous signatures and to partition the mix from other processes, such as denitrification and sulphate reduction. These challenges, to a large extent, can be addressed using the multi-tracer approach and analysing oxygen stable isotope composition of various molecules: δ¹⁸O(H₂O), δ¹⁸O(NO₃) and δ¹⁸O(SO₄).