It has been widely documented that municipal wastewater treatment plant effluents are a major source of artificial sweeteners to surface waters. However, in rural areas, the extent to which septic systems contribute these same compounds to groundwater aquifers is largely unknown.
In rural areas, municipal water supplies and wastewater treatment facilities are generally not available. Households, therefore, rely on private wells for water supply and treat their wastewater using septic systems. By design, these systems discharge treated wastewater to groundwater. In Canada and the United States, 11 and 14% of households, respectively, get their water from private wells, and private well owners are not subject to the same regulations and monitoring that municipalities are with respect to the quality of their water supplies.
We hypothesize that a significant fraction of shallow groundwater in unconfined aquifers in rural settings, where septic systems are common, is affected by septic system effluent and that analysis of artificial sweeteners in groundwater will indicate this. Furthermore, in areas where intensive agriculture activities also take place, we suspect that artificial sweetener concentrations will indicate that septic effluents are not a significant source of nitrate and other nutrients in regional groundwater, compared with what originates from agriculture (e.g. fertilizer application to crops).
Methodology
This study examines the occurrence and concentration of four commonly used artificial sweeteners – acesulfame (ACE), sucralose (SUC), saccharin (SAC) and cyclamate (CYC) – in the Lake Algonquin Sand Aquifer in the Nottawasaga River Watershed, located approximately 50 kilometers north of Toronto, Ontario, Canada. Being a surficial aquifer, it functions as a water supply for rural residents, receptor of treated domestic wastewater from septic systems, and source of baseflow to the Nottawasaga River. For groundwater wells and seeps (springs) containing artificial sweeteners, we used the concentration of ACE to estimate the fraction of water derived from septic effluents and to see if nutrients in groundwater are related to the degree of septic influence. Groundwater from the aquifer was collected from private domestic wells and groundwater seeps discharging along the banks of the Nottawasaga River.
In addition to analyzing major anion samples, and sub-samples of soluble reactive phosphorus and ammonium, the concentrations of the four above artificial sweeteners were analyzed using suppressed ion chromatography, coupled to a tandem mass spectrometer (IC-MS/MS). The domestic well samples and the groundwater seep samples collected in 2010-11 were analyzed using the above technique, which had method detection limits of: ACE = 0.008, SAC = 0.021, CYC = 0.003, SUC = 5.0 μg L−1. The groundwater seep samples collected in September 2013 were run with a slightly updated IC-MS/MS method, lowering method detection limits for all but CYC, and especially for sucralose (ACE = 0.002, SAC = 0.002, CYC = 0.003, SUC = 0.020 μg L−1).
Outcomes
At least one of the four artificial sweeteners was found in 32% of the 149 groundwater seep samples collected along the banks of the Nottawasaga River. ACE was the most commonly found artificial sweetener, with 21% of all seep samples collected having detectable (>mdl) ACE (Fig. 1A and 1B). ACE also had the highest concentration of the artificial sweeteners measured, with a maximum value of 1.7 μg L−1 (Fig. 1B). The high prevalence and concentration of ACE is likely due to its common use as an artificial sweetener in Canada, its strong resistance to degradation in the environment, and its mobility in groundwater.
Overall, approximately 30% of samples had detectable levels of one or more artificial sweeteners (Fig. 1C), indicating the presence of water derived from septic system effluents. Using ACE concentrations to estimate the fraction of septic effluent in groundwater samples, ~3.4 to 13.6% of the domestic wells had 1% or more of their well water coming from septic system effluent. Similarly, 2.0 to 4.7% of the groundwater seeps had a septic effluent contribution of 1% or more. No significant relationship was found between the concentration of ACE and the concentration of nitrate, ammonium, or SRP in the groundwater, indicating that septic effluent is not the dominant source of nutrients in the aquifer.
Figure 1: Concentrations of acesulfame (ACE), saccharin (SAC), cyclamate (CYC), and sucralose (SUC) in groundwater samples collected as groundwater seeps discharging along the banks of the Nottawasaga River in (A) 2010 and 2011, (B) 2013, and (C) from private wells at rural residences in 2011. The ends of the boxes correspond to the 25th and 75th percentile of the data, and the whiskers represent the 10th and 90th percentiles. The line within the box is the median. The percentage of samples with concentrations above the method detection limit for each artificial sweetener are given within each box. The number of samples analyzed for each set is indicated below the x-axis. Sucralose is not reported for the 2010 and 2011 samples because of a relatively high detection limit for the analytical method used at that time.
Conclusions
The four artificial sweeteners measured in this study are all approved for human consumption by Health Canada, and therefore their presence in rural groundwater and private wells might not be considered a public health issue in and of itself. However, artificial sweeteners in groundwater indicate the presence of wastewater, which might be the source of other contaminants of concern (e.g. pathogens, nitrate or pharmaceuticals). Thus, artificial sweeteners can serve as a screening tool to identify groundwater that should receive additional testing, especially if the water is used for potable supplies. From an aquatic ecosystem perspective, the effects of environmentally relevant concentrations of artificial sweeteners in rivers and lakes are unknown for most organisms.
Given that 11% of the Canadian population obtains their domestic water from private wells, and that these same households usually also have septic systems, the occurrence of artificial sweeteners in groundwater is likely widespread in rural areas across the country. In rural areas, artificial sweeteners are a potential way of apportioning nutrient contributions between agricultural and septic sources, especially when used in combination with other novel tracers such as stable isotope ratios.
Although the Nottawasaga River has several municipal wastewater discharges, we show that groundwater discharging directly to the Nottawasaga River is also a source of artificial sweeteners to the river. Therefore, surface water bodies (rivers, streams, lakes) that do not have municipal wastewater discharges can still contain artificial sweeteners derived from groundwater affected by septic system effluents. Given that ACE is so recalcitrant and mobile in groundwater, it is possible that even after other wastewater compounds such as pathogens and pharmaceutical compounds are completely removed, the presence of septic-derived water could still be detected using ACE.
Spoelstra, J., Senger, N.D., & Schiff, S.L. (2017). Artificial sweeteners reveal septic system effluent in rural groundwater. Journal of Environmental Quality, 46(6), 1434-1443.
Contact: John Spoelstra, Department of Earth and Environmental Sciences/Canada Centre For Inland Waters, Environment and Climate Change Canada.
For more information about the WaterResearch, contact Amy Geddes.
