Long Point Nitrate
Figures 1a and S3a show the presence of highly elevated NO3−-N of up to 103 mg/L in the proximal plume zone in October 2010, which is the result of oxidation of the wastewater NH4+ in the vadose zone below the tile bed. However, at the base of the aquifer and beyond 35 m from the tile bed, NO3−-N is attenuated to ≤0.1 mg/L at most locations in the plume core (Figures 1a and S3a). This is consistent with previous studies at this site which showed that NO3− is attenuated by denitrification that utilizes trace quantities of OC and reduced sulfur (S) from pyrite present in the aquifer sediments, as electron donors (Aravena and Robertson 1998). Dissolved OC present in the plume (1 to 7 mg/L, Aravena and Robertson 1998) was insufficient to account for the amount of nitrate attenuation observed. In addition to denitrification, some NO3− is attenuated with NH4+ at this site, by anaerobic ammonium oxidation (anammox, Robertson et al. 2012); however, denitrification is likely dominant over anammox in most plume zones, particularly beyond 17 m from the tile bed where NH4+ is absent but NO3− attenuation continues (Robertson et al. 2013).
In this study, NO3−-N concentrations in many of the background wells, both upgradient of the tile bed and overlying the plume, were also low (≤0.1 mg/L, Figure 1a). These wells were also sub-oxic (DO < 1 mg/L at depths greater than 0.5 m below the water table) and thus were likely similarly affected by denitrification, utilizing the same electron donors (aquifer OC and pyrite) as the plume water. Nitrate concentrations measured in the proximal zone in 2008 (Figure S4) were similar to the 2010 values as shown in Figures 1a and S3a.
Long Point Perchlorate
The septic tank effluent had relatively low ClO4− concentrations (2 to 76 ng/L, n = 13) which were generally lower than the value measured in the single sample of tap water (64 ng/L). Figures 1b and S3b show generally higher ClO4− of 13 to 802 ng/L present in the shallow proximal plume zone. Elevated values of up to 392 ng/L were also found in some of the groundwater that was sampled from above the septic plume in the area immediately downgradient of the tile bed (Figures 1b and S3b). Additionally, elevated ClO4− of 309 ± 172 ng/L was found in seven time-series samples taken at the water table below the tile bed (wells 120, 121, and 122) in April and May 2011, just prior to opening of the campground, and these samples also had very low Cl− (5.6 ± 2.4 mg/L). The low Cl− values indicated that these samples originated not from the wastewater, but from natural precipitation recharge that had occurred during the November to May nonuse period. These findings indicate that the high ClO4− concentrations in the plume are likely augmented by elevated values present in natural precipitation recharge, rather than being entirely derived from the high Cl− wastewater. Perchlorate concentrations measured in the proximal zone of the plume in 2008 (Figure S4) were similar to the 2010 values shown in Figure 1b. These elevated concentrations point to an additional source, or sources, of ClO4− at the study site.
In a recent nation-wide survey of atmospheric deposition (wet only) at 26 monitoring stations in the United States (Rajagopalan et al. 2009), a relatively low mean ClO4− value of 14 ng/L was measured. However, in another study, which measured both wet and dry deposition at six sites on Long Island, New York (Munster et al. 2009), a much higher mean ClO4− value of 210 ng/L was obtained, with a distinct peak associated with periods of fireworks use (up to 2780 ng/L). These concentrations are similar to the concentrations observed in the shallow groundwater at Long Point and we note that fireworks use also occurs periodically at Long Point. In fact, spent fireworks casings have been found lying directly on the tile bed and this grassed area could be a focused area of more intensive fireworks use. Similar high values could be present near the water table in other areas, but the monitoring network generally does not capture this shallow groundwater elsewhere at the site, except in the two upgradient wells where denitrification appears complete even in the shallowest points. Thus, we conclude that atmospheric deposition, with some contribution from fireworks residuals, are likely the main sources of the ClO4− found in shallow groundwater at Long Point.
Despite the persistent occurrence of elevated ClO4− in the groundwater near the septic tile bed, both in and overlying the plume, ClO4− was virtually absent (<30 ng/L) from the distal portion of the plume (Figures 1b and S3). To assist in understanding ClO4− behavior within the plume, normalized concentrations of the two wastewater indicators, Na+ and the artificial sweetener, acesulfame, which are relatively mobile and conservative within the plume (Van Stempvoort et al. 2011b; Robertson et al. 2013), were compared to normalized NO3−-N and ClO4− values (Figure 2). For this comparison, initial values (C0) were calculated as the mean values in the four proximal monitoring wells located immediately below the tile bed (wells 120, 121, 122, and 123; total of 20 monitoring points, Figure 1). Sodium and acesulfame declined along the plume in a relatively uniform manner as a result of hydrodynamic dispersion and possibly other factors, but both still remain at approximately 25 to 50% of the initial values 200 m downgradient. In contrast, both NO3−-N and ClO4− declined abruptly beyond 35 m from the tile bed and, except for a single monitoring point from well 7 (50 m distance) that is elevated in both NO3−-N and ClO4−, NO3−-N declined to <1% of the initial value, and ClO4− declined to <10% of the initial value. This suggests that ClO4− and NO3− experienced similar fates within the waste water plume, despite their different sources.
Figure 2. Normalized groundwater concentrations along the plume core zone and in the upgradient wells, October 27, 2010: (a) Na+, (b) acesulfame, (c) NO3−-N, and (d) ClO4−. All values downgradient of the tile bed are from the plume core zone (Na+ >10 mg/L and/or acesulfame >8 µg/L). Initial values (C0) are the mean October 2010 values of the monitoring points located directly below the tile bed (wells 120, 121, 122, 123, n = 20). Values greater than two times the initial value are plotted as C/C0 = 2. Note, 2008 ClO4− values are shown twice (Figure 2d); original values sampled in 2008 (Figure S3) and same values projected 46 m downgradient (and adjusted −14% for dispersive dilution) to reflect 2 years of groundwater flow during the period 2008 to 2010.
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Normalized concentrations of the 2008 ClO4− samples, which were all collected within 17 m of the tile bed (Figure S3), are also shown in Figure 2d. Perchlorate concentrations measured in subtile wells 120, 121, 122, and 123 in 2008 (117 ± 69 ng/L, n = 22) remained similar to values measured in the same wells in 2010 (119 ± 113 ng/L, n = 20). The 2008 values are also “projected” 46 m farther downgradient in Figure 2d, based on the time interval between September 2008 and October 2010 sampling events (2.1 years) and the horizontal groundwater velocity in the tile bed area indicated from a previous tracer test (22 m/year, Robertson et al. 2012). This projection assumes that ClO4− is highly mobile, because it is normally considered unaffected by sorption processes in sandy soils (Motzer 2001). Also, to account for the likelihood that the 2008 ClO4− values have been diluted by hydrodynamic dispersion during this 2-year period, the projected values in Figure 2d have been decreased by14% from their original concentrations, based on similar attenuation observed for Na+ (decrease of 0.31% per meter of flowpath distance, Figure 2a). The projected 2008 ClO4− values indicate a plume 40 to 70 m downgradient; however, this zone is largely devoid of ClO4− (and NO3−) except for the single high value from well 7 (Figure 1b). The loss of ClO4− is attributed to natural attenuation processes. Thus, regardless of the source of the elevated ClO4− concentrations, similar elevated values were measured in the proximal plume zone during two sampling snapshots 2 years apart, and the low concentrations of ClO4− observed in the distal, nitrate-depleted portion of the septic plume are compelling evidence that natural degradation of ClO4− is occurring in groundwater at this site.
Although Figure 1 points to a strong relationship between NO3− and ClO4−, the scatter plot of NO3−-N vs. ClO4− concentrations, in both the plume water and the background groundwater (Figure 3), shows little evidence of declining ClO4− concentrations as NO3−-N concentrations decrease, until nitrate is almost entirely removed. Considering the data shown in Figure 3 (both plume and background samples), ClO4− concentrations in groundwater with NO3−-N of 0.3 to 1 mg/L (72 ± 124 ng/L, n = 9) and 1 to 10 mg/L (196 ± 138 ng/L, n = 11) are not significantly different (p > 0.1) than groundwater with >10 mg/L NO3−-N (187 ± 202 ng/L, n = 107) even through isotopic evidence shows that most plume groundwater with NO3−-N < 10 to 20 mg/L has been highly affected by denitrification at this site (Aravena and Robertson 1998; Li 2010). Only when NO3−-N is below 0.3 mg/L are ClO4− concentrations significantly lower (11 ± 15 ng/L, n = 56, p < 0.01). This behavior indicates that ClO4− is generally not co-metabolized with NO3− during denitrification at this site and that the bacteria consortium present only degrades ClO4− after the preferred electron acceptor, NO3−, has been mostly consumed.
Figure 3. Scatter plot of ClO4− vs. NO3−-N concentrations at the Long Point site during 2008 to 2011, showing both plume values (Na+ >10 mg/L and/or acesulfame >8 µg/L) and background values, upgradient of the tile bed and overlying the plume. ClO4− values of 2 ng/L are detection limit values.
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Stream Sites: Nitrate and Perchlorate
Figure 4a shows the relationship of ClO4−, NO3−, and DO concentrations in the 21 groundwater samples collected from Tuck Creek. All of the aerobic samples (DO > 1 mg/L) had consistently elevated ClO4− in the 100 to 300 ng/L range, and NO3− was present at 2 to 5 mg/L (0.5 to 1 mg/L as N). Most of the aerobic samples from the Shoreacres Creek site had similarly elevated ClO4− of 100 to 300 ng/L (Figure 4b). These ClO4− concentrations were similar to the atmospheric deposition values measured on Long Island, New York (mean of 210 ng/L, Munster et al. 2009), which is also an urbanized area. This evidence suggests that the relatively uniform ClO4− concentrations observed in the aerobic samples from both Tuck and Shoreacres Creek likely represent background values from atmospheric deposition.
Figure 4. Scatter plots of ClO4− vs. NO3− and DO concentrations in groundwater discharging to two gaining, first-order urban streams in southern Ontario, Canada: (a) Tuck Creek and (b) Shoreacres Creek. Samples collected using a drive-point sampler inserted from 0.25 to1.0 m depth, below, or alongside, the stream bed. Detection limit for DO is approximately 0.3 mg/L. Nitrate concentrations are presented as NO3− rather than as NO3−-N for convenient axis scaling with DO.
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In contrast to the aerobic samples which had consistently high ClO4−, most sub-oxic samples (DO < 0.5 mg/L) from both Tuck Creek and Shoreacres Creek had much lower ClO4− concentrations (<20 ng/L), and were accompanied by NO3− values that were below detection (<0.1 mg/L, Figure 4). This suggests that these sub-oxic samples were likely affected by denitrification, which was possibility enhanced at some locations by increased OC availability in the stream riparian zones. Thus, the ClO4−-NO3− relationship observed at both of these stream sites was similar to the relationship observed at Long Point.