Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Constraining Nitrogen Inputs to Urban Streams from Leaking Sewers Using Inverse Modeling: Implications for DIN Retention in Urban Environments”
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and are known to leak. These conditions create ample opportunity for substantial gravity head to develop and for aerobic soil conditions in near pipe areas around the aging pipe network. This is particularly true given that this system drains a watershed with over 150 m of relief from head to mouth. Ultimately, the question remains, if this DIN is not sewerderived, how can the nitrogen mass balance be closed? The Comment focuses on an overextrapolation of a conceptual model and ignores the implications to the nutrient budget. This nutrient budget carefully characterizes potential sources, and concludes that without including sewage derived DIN, there remains a very large, unknown source of DIN. Given known, documented impairments, sewer-derived DIN is the most likely explanation. Divers et al (2013) does not use nitrate to track sewage, nor does it assume continuous water saturation in near-pipe environments. This paper estimates sewer-derived DIN inputs to urban surface waters to address substantial imbalances in watershed-scale material budgets. Suggesting nitrate is considered by the authors as tracer of sewage is a misconstruction of aims, results, and approach.
e agree that nitrate is not generally present in raw wastewater and therefore cannot be used to track raw sewage. For that reason, Divers, Elliott, et al., (2013)1 does not use nitrate to track raw sewage. Rather than use nitrate concentrations to track raw sewage, the approach measures dissolved inorganic nitrogen (DIN) concentrations in an urban stream with known sewer leakages (where DIN is the sum of nitrite, nitrate, and ammonium concentrations). A mass balance model was then inverted to constrain potential sewer-derived contributions to observed DIN loads. Therefore, estimated contributions of sewage-sourced DIN were not directly based on measurement of nitrate concentrations. This was a deliberate approach, as it is universally recognized that nitrate is highly reactive (i.e., biologically available) and thus not conservative in natural systems. Throughout the paper DIN loads are consistently referred to as “sewer-derived” or “sewersourced” to prevent any misunderstanding. Moreover this language indicates that observed nitrate in the stream arises from oxidation and processing of N-containing sewage in the environment. To imply that no oxidation occurs in-stream, particularly in this system, is erroneous. Samples collected as part of this study were taken from shallow surface waters, moreover from a restored urban stream system engineered to oxygenate water via artificial hydraulic jumps. Further, aerobic biodegradation in unsaturated zones surrounding sewer leaks is well documented.2 These conditions, conducive to converting ammonium to nitrate, would account for the low ammonium concentrations observed in Nine Mile Run (NMR). Lastly, headspace within sewers allows aeration and thereby provides aerobic conditions necessary for mineralization and then nitrification of organic N in sewage, even within the closed sewer environment.3,4 In summary, organic nitrogen originating from sewage can be transformed to nitrate in a number of well-documented environmental conditions. As for the critique of Figure 4, this conceptual model is not to be treated as a rendering of the actual, entire underground pipe and stream system. To do so ignores clear labeling in the figure caption. It is important to re-emphasize the care taken in referring to this as a conceptual model in the caption and text. This emphasis results from this model’s use in a limited context to explain an observed temporal pattern in water chemistry during extremely dry periods. Given temperate climate and locally severe problems with stormwater runoff (which precludes a portion of rainwater infiltration), the water levels fluctuate somewhere in between the two cases. Therefore, assumptions of complete oxygen limitation, at what is likely a dynamic interface, would only apply to limited portions of the year if at all. Further, discussion of directional leaks in a submerged sewer system is therefore a moot point when Figure 4 is considered as a conceptual model. This concern is simply not applicable in this system where sewers in NMR drain a watershed with high relief © XXXX American Chemical Society
Marion T. Divers* Emily M. Elliott Daniel J. Bain
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Department of Geology & Planetary Science, University of Pittsburgh, 4107 O’Hara Street, Pittsburgh, Pennsylvania, 15260, United States)
AUTHOR INFORMATION
Corresponding Author
*e-mail:
[email protected] Phone: (412) 624-9324 Fax: (412) 624-3914. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Divers, M. T.; Elliott, E. M.; Bain, D. J. Constraining nitrogen inputs to urban streams from leaking sewers using inverse modeling: implications for dissolved inorganic nitrogen (DIN) retention in urban environments. Environ. Sci. Technol. 2013, 47 (4), 1816−1823. (2) Eiswirth, M.; Hotzl, H.; Lazar, C.; Merkler, G. P. In Detection of Contaminant Transport from Damaged Sewerage Systems and Leaky Landfills, Groundwater quality: remediation and protection, Prague, Czech Republic, 1995; Kransny, K. K., Ed.; IAHS: Prague, Czech Republic, 1995. (3) Sydney, R.; Esfandi, E.; Surapaneni, S. Control concrete sewer corrosion via the crown spray process. Water Environ. Res. 1996, 68 (3), 338−347. (4) Neethling, J. B.; Mah, R. A.; Stenstrom, M. K. Causes and Control of Concrete Pipe Corrosion; Annual Report Submitted to the County Sanitation Districts of Los Angeles County, 1989.
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dx.doi.org/10.1021/es4021662 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
