This study focuses on one of South East Water’s Water Recycling Plants (WRPs), located adjacent to an area of significant agricultural activity (market gardens), where multiple potential sources of groundwater contamination (nutrients) have been identified. The primary objective was to examine the use of stable and radiogenic isotopes in conjunction with contaminants of emerging concern (e.g. pharmaceuticals) to gain a greater understanding of the underlying hydrogeochemical processes and separate wastewater-derived contamination from other potential sources e.g., agriculture.
Design and methodology
This project combined long term groundwater monitoring data (collected since 1997) with in-situ isotopic tracers sampled in 2018-2019 including: δ2HH2O and δ18OH2O, δ13CDIC, 3H and 14C, analysed at ANTSO’s Lucas Heights facility, and δ18ONO3 and δ15NNO3, analysed at China’s Key Laboratory of Agricultural Water Resources. The isotopic data were combined with other lines of evidence, including contaminants of emerging concern (e.g. pharmaceuticals) analysed at RMIT, as an additional metric to better delineate wastewater impacts in a mixed source environment.
Original data and results
Groundwater residence time indicators from groundwater wells located adjacent to or upgradient of the treatment plant, contain 14C and 3H activities (>73 pMC and >0.68 TU) which indicate a large component of modern recharge (i.e. <50 years old). Groundwater near suspected leaky infrastructure contains elevated levels of these radiogenic isotopes compared to those considered more indicative of background conditions (>97 pMC and >1.4 TU compared to 72-85 pMC and 0.68-1.03 TU, respectively), indicating potential anthropogenic inputs. Water stable isotopes, plot along a mixing line between regional groundwater (e.g. outside the known plume) and evaporated groundwater (e.g. recharge from former and current sludge lagoons). End-member mixing calculations indicate a significant proportion of effluent in groundwater downgradient of treatment infrastructure. Elevated nutrient concentrations in groundwater occur throughout the region (e.g., 0.6 to 160 mg/L nitrogen (total)). Nitrate isotopes show clear differences between nutrients derived from agriculture (values between 5 and 29‰, median = 12.0‰), and those impacted by the treatment plant (values between 2.9 and 41.2 ‰, median = 18.0‰), with effluent values between 20.2‰ and 39.1‰). Further work is being undertaken to quantify pharmaceuticals as an additional metric to better delineate wastewater impacts.
The findings show how isotopic tracers can assist in overcoming the difficulties associated with delineating multiple similar contamination sources. This will enable wastewater treatment plant operators to more accurately assess and manage their impacts on groundwater.