Decoupling anthropogenic vs. natural impacts at a wastewater treatment plant situated on acid sulfate soils

Decoupling natural and anthropogenic impacts on the subsurface environment can be difficult, particularly when it has been subject to a wide range of influences over time and space. In this work we show how the use of hydrogeochemical plotting tools, time-series analysis of key contaminants of concern, and targeted isotopic analysis can be used to better understand the contamination sources/processes in a complex environment - a Wastewater Treatment Plant (WWTP) located on coastal acid sulfate soils (ASS). Analysis of soil profiles for potential oxidisable sulfur, acid neutralising capacity (ANC), and pH_fox along with groundwater chemistry, revealed that oxidation of pyritic sediments, initially deposited during the mid-Holocene, have led to significant pH declines and the secondary mobilisation of metals into the groundwater environment. This is further complicated by historic anthropogenic inputs associated with the WWTP (e.g., effluent leakages) and the surrounding agricultural land uses. There is distinct separation between spatial and temporal trends in the nutrient and heavy metals data in groundwater, suggesting these reflect different contaminant sources and/or processes. Isotopic data indicate nutrients are largely derived from the WWTP, whereas time-series analysis of key contaminants of concern and hydrogeochemical plotting tools indicate metals are largely derived from the secondary mobilisation of ASS due to acidity generated during sulfide oxidation. This work highlights the importance of understanding the hydrogeological environment and need for careful planning and ongoing management of WWTP sites, particularly those constructed on potential acid sulfate soils (PASS), which, if disturbed or exposed, can lead to impacts beyond the area of ASS via groundwater discharge to nearby surface water bodies (in this case the site is adjacent to a Ramsar-listed wetland). The outcomes of this work have significant global application in the identification, assessment, and control of ASS, the practice of contaminant source attribution, and the siting and design of future WWTPs, which will continue to be sited in coastal areas to meet population needs.