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 pHfox 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.

Combining environmental isotopes with Contaminants of Emerging Concern (CECs) to characterise wastewater derived impacts on groundwater quality.

The potential for Wastewater Treatment Plants (WWTPs) to cause adverse impacts to groundwater quality is a major global environmental challenge. Robust and sensitive techniques are required to characterise these impacts, particularly in settings with multiple potential contaminant sources (e.g. agricultural vs. site-derived). Stable (δ2HH2O, δ18OH2O, δ15NNO3, δ18ONO3 and δ13CDIC) and radioactive (3H and 14C) isotopes were used in conjunction with three Contaminants of Emerging Concern (CECs) - carbamazepine, simazine and sulfamethoxazole - to discriminate between multiple potential contamination sources at an Australian WWTP. The radioactive isotope tritium provided a sensitive indicator of recent (post-1990s) leakage, with groundwater activities between 0.68 and 1.83 TU, suggesting WWTP infrastructure (activities between 1.65 and 2.41) acted as a recharge 'window', inputting treated or partially treated effluent to the underlying groundwater system. This was corroborated by water stable isotopes, which showed clear demarcation between δ18OH2O and δ2HH2O in background groundwater (δ18OH2O and δ2HH2O values of approximately -5 and -28‰, respectively) and those associated with on-site wastewater (median δ18OH2O and δ2HH2O values of -1.2 and -7.6‰, respectively), with groundwater down-gradient of the plant plotting on a mixing line between these values. The CECs, particularly the carbamazepine:simazine ratio, provided a means to further distinguish wastewater impacts from other sources, with groundwater down-gradient of the plant reporting elevated ratios (median of 0.98) compared to those up-gradient (median of 0.11). Distinctive CEC ratios in impacted groundwater close to the WWTP (∼3.0) and further down-gradient (2.7-9.3) are interpreted to represent a change in composition over time (i.e., recent vs. legacy contamination), consistent with the site development timeline and possible changes in effluent composition resulting from infrastructure upgrades over time. The data indicate a complex set of co-mingled plumes, reflecting different inputs (in terms of both quantity and concentration) over time. Our approach provides a means to better characterise the nature and timing of wastewater derived impacts on groundwater systems, with significant global implications for site management, potentially allowing more targeted monitoring, management and remedial actions to be undertaken.

Contaminants of Emerging Concern as novel groundwater tracers for delineating wastewater impacts in urban and peri-urban areas.

Management and treatment of environmental impacts from wastewater treatment plants (WWTPs) is a major, worldwide, sustainability challenge. One issue associated with WWTP operation is the potential for groundwater contamination via leaking or infiltration of wastewater, particularly with inorganic nutrients (ammonia and nitrate) as well as persistent organic compounds. Despite the potential for such contamination to create environmental and health risks, conventional methods, such as the assessment of major ions, nutrients, bacteriological indicators and conventional tracers (such as stable and radiogenic isotopes) are often unable to provide accurate delineation of multiple potential sources of contamination. This is particularly important for WWTPs which often occur in urban, peri-urban or intensively farmed agricultural areas where multiple potential sources (such as livestock, fertilisers, wastewater irrigation, and domestic septic systems) may contribute similar contaminants. This review explores the applicability of promising novel groundwater tracers, such as Contaminants of Emerging Concern (CECs) and isotopic tracers, which can be used in conjunction with conventional tracers (i.e. 'co-tracers') to provide a more definitive assessment of contaminant sources, plume delineation and even (potentially) indicating the age of contamination (e.g., recent vs. legacy). The suitability of the novel groundwater tracers is evaluated according to four key criteria: (i). sufficient presence in raw wastewater and/or treated effluents; (ii) diagnostic of WWTP impacts as opposed to other potential off-site contamination sources; (iii) persistence in the subsurface environment; and (iv) amenable to rapid and sensitive analysis. Further analysis of various classes of CECs along with improved detection limits associated with improvements in analytical methodologies should allow for future application of promising groundwater tracers, providing WWTP operators and regulatory authorities a more definitive toolbox with which to assess groundwater contamination associated with site operations. These include: persistent pharmaceuticals and personal care products (carbamazepine, crotamiton, primidone, atenolol and sulfamethoxazole), artificial sweeteners (acesulfame, sucralose, saccharin and cyclamate) and potentially, certain pesticides (atrazine and simazine).