Natural Attenuation of Perchlorate in Denitrified Groundwater

Abstract

Monitoring of a well‐defined septic system groundwater plume and groundwater discharging to two urban streams located in southern Ontario, Canada, provided evidence of natural attenuation of background low level (ng/L) perchlorate (ClO4−) under denitrifying conditions in the field. The septic system site at Long Point contains ClO4− from a mix of waste water, atmospheric deposition, and periodic use of fireworks, while the nitrate plume indicates active denitrification. Plume nitrate (NO3−‐N) concentrations of up to 103 mg/L declined with depth and downgradient of the tile bed due to denitrification and anammox activity, and the plume was almost completely denitrified beyond 35 m from the tile bed. The ClO4− natural attenuation occurs at the site only when NO3−‐N concentrations are <0.3 mg/L, after which ClO4− concentrations decline abruptly from 187 ± 202 to 11 ± 15 ng/L. A similar pattern between NO3−‐N and ClO4− was found in groundwater discharging to the two urban streams. These findings suggest that natural attenuation (i.e., biodegradation) of ClO4− may be commonplace in denitrified aquifers with appropriate electron donors present, and thus, should be considered as a remediation option for ClO4− contaminated groundwater.

Concentrations of Artificial Sweeteners and Their Ratios with Nutrients in Septic System Wastewater

“…The resultant median concentrations (generally falling in a range of 10 to 60 µ g/L for all four artificial sweeteners) may be used to represent concentrations in septic tanks where direct measurements are lacking, in modeling or mass balance calculations of septic wastewater environmental loadings in areas with similar usage of these artificial sweeteners. For example, Robertson et al ( 2016a ) and Spoelstra et al (unpublished data) used ACE concentrations in groundwater samples (wells, streambed profiler, and/or seeps) to estimate the fraction of water derived from septic system wastewater. As such, artificial sweetener analysis of water samples gives insight into potential health risks due to the possible presence of other wastewater constituents such as nitrate, bacteria and viruses, and pharmaceuticals.…”

“…Nineteen different septic systems from southern and central Ontario were sampled for this study, the details of which are outlined in Table 2 , with some also described by Robertson et al ( 2016a ). All wastewater samples were obtained directly from within the septic holding tanks, using a peristaltic pump connected to ¼-in polyethylene tubing.…”

“…Current analytical methods can detect part-per-trillion levels of these compounds in groundwater and surface water samples (Buerge et al 2009 ;Lange et al 2012 ;Gan et al 2013 ;Tran et al 2014 ). Furthermore, both ACE and SUC are fairly resistant to attenuation and degradation in municipal wastewater treatment systems (Scheurer et al 2009 ;Soh et al 2011 ;Kokotou and Thomaidis 2013 ;Burke et al 2014 ), rural septic systems (Robertson et al 2013(Robertson et al , 2016a(Robertson et al , 2016b, and drinking water treatment plants (Mawhinney et al 2011 ). Other sweeteners, such as saccharin (SAC) and cyclamate (CYC), are more readily degraded in wastewater treatment systems (Lange et al 2012 ;Lim et al 2017 ), but may still be useful wastewater tracers for nonquantitative detection and source identification (e.g., Van Stempvoort et al 2013 ).…”

“…The range of concentrations of artificial sweeteners in untreated and treated wastewater at municipal wastewater treatment plants (WWTP) has been provided in several studies (see references in Table 1 ). However, to the best of our knowledge there is currently only one published report (Oppenheimer et al 2011 ; only for SUC) that considers typical artificial sweetener concentrations in septic system wastewater, excluding the ACE data for several sites that are reported here for this study that were previously published by Robertson et al ( 2016a ). Wastewater from septic systems differs from that received by WWTP in that individual septic systems integrate waste from a much smaller number of people and will typically not have inputs from commercial or industrial activities or stormwater runoff.…”

“…We report here Raw data and additional statistics for this study are provided in Tables S1 and S2 in Appendix S1. A subset of the acesulfame data for multiple-dwelling sites (this study) were previously published by Robertson et al ( 2016a NGWA.org a compilation of concentrations of four artificial sweeteners: ACE, SUC, SAC, and CYC, from several different septic systems across Ontario, Canada. The septic systems chosen for this study have varying usage; from small single-family seasonal cottages to seasonal campground facilities and a resort, with no additional treatment systems (e.g., denitrifying filter).…”