Tropical islands are classically defined by a small land mass surrounded by ocean with mountains catching rainfall and streams and rivers in well-defined and relatively short watersheds delivering the fresh water to coastal estuaries and beaches. Human settlements are typically established along the streams, using the fresh water for drinking, agriculture, fishing, and recreation. As the waterways pass through human settlements, the water can become polluted with agricultural runoff and human wastes from poorly designed or maintained onsite sewage disposal systems (OSDS) such as cesspools and septic systems. Polluted water then contaminates downstream sites including tidally influenced stream mouths, embayments, estuaries, coastal waters, and coral reefs. Communities living in island or rural mainland coastal areas are presented with public health risks from pathogens associated with both animal and human fecal matter.
Fecal indicating bacteria (FIB) e.g., Enterococci spp. (ENT), Escherichia coli (E. coli) and total coliforms (TC), have been classically used as test organisms to identify public health risk from human pathogens associated with human wastes, with enterococcus being the best surrogate for bacterial pathogens in tropical stream waters (Viau et al., 2011) and tropical marine recreational waters (Lamparellia et al., 2015), and have been adopted as criteria standards by states as recommended by federal government (U.S. E.P.A., 2018). Health risks include gastrointestinal illnesses as well as effects such as respiratory illness, skin rash, eye irritation, and ear infection (U.S. E.P.A., 2018)
In the tropics, routine monitoring of streams and coastal waters for FIB identifies areas that do not meet government standards but will not necessarily define the level of risk to the public from human-pathogen contaminated waters, as those bacteria may originate in fecal matter from homeothermic animals common to those watersheds, both feral and domesticated (Boehm et al., 2009; Fewtrell & Kay, 2015). Feces from cattle and dairy cows carry human pathogens and contaminate food crops through airborne transmission to soil and irrigation waters causing outbreaks of bacterial infections and product recall (Soller et al., 2010; Venegas-Vargas et al. 2016). Feral pigs are also known to carry human pathogens (e.g., Leptospires) that pass through feces and urine to surface waters (USEPA, 2009). Avian colonies are known to cause high concentrations of FIB (Vogel et al. 2013; Zimmer-Faust et al., 2020) mixed with human sources making Quantitative Microbial Risk Assessment problematic. Enterococci and E. coli bacteria are found in tropical soils and streams (Hardina & Fujioka, 1991; Luther & Fujioka, 2004; Goto & Yan., 2011; Viau et al. 2011; Byappanahalli et al. 2012; Ekklesia et al., 2015) and are capable of colonizing and growing in Hawai‘i’s soils (Byappanahalli et al., 2012).
Because human and non-human sources of FIB pose varying degrees of risk to public health and require different management strategies to alleviate that risk, it is important to determine if human wastewater is present in recreational waters. In fact, some regulations require different public notification procedures to be followed depending on whether human sources of FIB are determined
(Hawaii Department of Health, 2021). But pathogens themselves are both difficult and dangerous to culture and quantify. Organic micropollutants (OMP) including pharmaceuticals, personal care products, and artificial sweeteners from anthropogenic sources (e.g., on site disposal systems [OSDS] [cesspools, septic systems] or wastewater treatment plants [WWTP]) are frequently used as tracers for detecting and identifying sources of wastewater. A recent review of source tracking in tropical Hawaii using these methods is found in Johnson, 2020.
Beginning in 2014 studies began combining microbial (FIB) and chemical fecal indicators (artificial sweeteners) in analysis of recreational waters (Gue´rineau et al., 2014; Sima al., 2014; Ekklesia et al., 2015). Ideally the OMP indicator should be 1) source specific for raw wastewater or treated effluents, 2) ubiquitous (> 80% detection frequency) in contaminated waters of concern yet missing in background samples, 3) concentrated in samples with respect to levels of detection 4) persistent in subsurface groundwaters and 5) detected with rapid, inexpensive yet sensitive analysis (Oppenheimer et al., 2011; Soh et al. 2011; Spoelstra et al., 2013; Yang et al. 2017; McCance et al., 2018).
Sucralose (SUC) is one such OMP that has stood out, as it is ubiquitous in diets worldwide, present now in relatively high concentrations in wastewater, and easy to analyze. The use of SUC in human food products and beverages is historically well documented (Molinary & Quinlan, 2006; Brorström-Lundén et al., 2008). SUC is a man-made chemical (C12H19Cl3O8) introduced as an artificial sweetener approved by the U.S. Food and Drug Administration in 1998, used in Europe by 2003 (Loos et al., 2009; Robertson et al., 2016) and widely adopted worldwide thereafter as a substitute for sugar in food manufacture. Its use is increasing worldwide (Alves et al., 2021). SUC was shown to pass through the human body unchanged; 85.5% with feces and 11.2% with urine over five days (Roberts et al., 2000) making it an ideal indicator of human waste associated pathogens. Soon after introduction with food, its presence in wastewater, resistance to treatment in WWTP, and general suitability as a qualitative and quantitative tracer of human wastewater was examined and reviewed repeatedly (Mawhinney et al., 2011; Oppenheimer et al., 2011; Bernot et al., 2016; Yang et al., 2018; Biel-Maeso et al., 2019; Van Stempvoort et al., 2020).
SUC is hydrophilic (Bernot et al., 2016) refractory (Yang et al., 2018), and recalcitrant with less than 15% removal by adsorption, biodegradation (Badruzzaman et al., 2013), or photolysis (Tran et al. 2014; Sang et al., 2014). There are many studies that address its properties, including its low adsorption (Biel-Maeso et al., 2019), persistence in soils (Biel-Maeso et al., 2019; Van Stempvoort et al., 2020), low biodegradation in the environment (Tollefsen et al., 2012; Labare & Alexander, 1993) and source specificity (Oppenheimer et al., 2011; Yang et al, 2018).
Additionally, the utility of SUC as a human waste tracer is supported by straight-forward analysis and low minimum detection limits (MDL) using previously established combinations of solid-phase extraction (SPE) and liquid chromatography-tandem mass spectrometry (LC-MS-MS). Methods used for determining concentrations in water and wastewater matrices were first reported in early 2008 in Swedish studies (Brorström-Lundén et al., 2008) and subsequent studies (Loos et al., 2009; Scheurer et al., 2009; Minten et al., 2011; Morlock et al. 2011; Ordóñez et al., 2012; Batchu et al., 2013; Loos et al., 2013; Batchu et al., 2015; Arbelaez et al., 2015). There are reviews of LC-MS-MS methods specifically for sweeteners (Lange et al., 2012; Lorenzo Ferreira et al., 2018; Luo et al., 2019). New methods have been developed for SPE (Lakade et al., 2018) and for on-line high performance SPE-LC-Ms-MS (Henderson et al., 2020).
Although the use of SUC as a tracer of surface and groundwater contamination by human wastewater is well established in developed nations over the 20 years that SUC has been used as a food supplement and the concentrations have increased with more widespread use, no published reports were found of SUC concentrations for WWTP in small tropical islands. Studies have been done using SUC for identification of contamination by OSDS of groundwater (Edwards et al., 2019) because of concern for contamination of underlying drinking water aquafers, and of surface water (Edwards et al., 2017) on the Caribbean Island of Barbados, and post-hurricane drinking waters of Puerto Rico (Lin et al. 2020; Bradley et al. 2021). A study of coastal nutrient enrichment in Vatia Bay, Samoa, using the combination of caffeine and SUC, showed those tracers of human wastewater were present in both the stream and bay (Whitall et al., 2019). A United States Geological Survey (USGS) review of wastewater source tracking in Hawaii highlighted the use of pharmaceutical and organic waste compounds in 21 studies over the four main islands (Johnson, 2020), with caffeine most frequently used (Knee et al., 2010; McKenzie et al., 2021) and artificial sweeteners inexplicably neglected. This study on the small (1456 sq km) tropical island of Kauai followed the recommendation of combining FIB and wastewater OMP indicators in the tropics (Gue´rineau et al., 2014; Ekklesia et al., 2015). Concentrations of SUC were measured in municipal WWTPs and in surface waters from streams around the island, in combination with FIB, as an easy, relatively inexpensive, well-established method to determine which streams were polluted with human wastes and required further investigation as presenting a public health risk.